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Proper Over-Deck Insulating and Vapor Barrier-ing (with pictures)

Started by Mattjohndeere2, February 08, 2022, 10:31:33 PM

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Mattjohndeere2

Hey all - I know there have been some threads on this over the years, but I wanted to open one with detailed pictures and have a discussion on what might be the best way ahead for the house I'm working on. 

The picture attached below is pretty much what I am planning on moving ahead with. Note the question on vapor barrier (or some sort of moisture proof/resistant material) above the insulation, that is what I would like to focus on, however the design as a whole is up for discussion if there are better ways to do this.

For this design it will be a tin roof. Under the tin roof, and on top of the purlins, around here people have been using rolls of that bubble foil insulation, which helps prevent condensation, so I plan on using that, theoretically I shouldn't have any moisture in the cold roof area between the insulation and tin - which leads me to the question of, is it worth it to have a vapor barrier atop the insulation, as a safety margin in case of condensation events? Would doing that cause ill effects (moisture entrapment in the insulation layers)? 

Picture attached below. The three scenarios I was looking at are 1) As drawn, 2) 1/2" layer of foil faced polyiso as the last layer of insulation, or 3) Ice/water or other thin vapor barrier on top of insulation.

Thanks in advance for the feedback. This forum is the greatest forum to have ever existed, and I am glad to be a part of it. 


 

Don P

I'm in a mixed climate with far more heating than cooling. Heat is a "drive" trying to leave the building. Moisture piggybacks on the warm air. If the warm moist air meets something at dew point temperature or below, the moisture will condense back to liquid and wet whatever that surface is.

buildingscience.com is a good resource for this stuff.

You'll see a number of ways and schools of thought.

What I've done sort of similar is rafters, 2x6 T&G decking, vapor barrier, foam, sleepers (forming vent channels) 5/8 ply (min for roof screws, ply has higher withdrawal), tarpaper, metal sealed to the deck (foam closure strips top and bottom of ribbed sheets or standing seam on the deck).

This hopefully slows moisture migration into the assembly due to vapor drive from indoors, any that gets in there will be vented out the vent channel. The moisture laden air does not contact the underside of the cold metal.



 

Then I got down and realized I wasn't happy with the connection from side to side so ran strapping from side to side over the ridge to help it "hang" and to keep the sides attached to one another if the gable windows were to ever blow out and inflate from the inside. Looking at the toeboards and tie in on that. I was younger then, be safer.



Mattjohndeere2

DonP, thanks. Funny, I was just on the buildingscience site, what a great resource!  You had linked an article in a previous thread, and I also found another one targeting what we're doing here (almost), and explaining as you did the point of the vapor barrier atop the roof deck.

It would seem that with this kind of design, in at-least my climate zone (cold, 6), what matters is having a class II vapor retarder atop the roof deck. Adding another barrier atop the insulation, mostly for water shedding purposes in case of severe condensation events, seems like it could work for my climate zone, and would not cause ill effects, since the rigid polyiso is also acting as an impermeable barrier itself. I could go either way at this point, probably the best way is the 1/2" foil faced as the last layer, with taped seams, it would be the best of both worlds (add a little R value and allow condensation shedding. Whatdya think?

https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned

Mattjohndeere2

I do like how your sealing the metal roof to plywood and basically creating a sealed unit. Plywood is expensive though. If I were doing standing seam it would be a must anyways, though I want to stick with the normal ribbed 29ga metal roofing, so strapping with a well designed cold roof seems to fit the bill. 

Some guys around here are doing sprayfoam atop the deck, and plywood right to the sprayfoam (no cold roof). They seem to be having good luck with it, though not many of those roofs from what I know have been in place for long periods of time yet.

bannerd

They key is airflow, I have not done my roof yet but we put felt paper down on the TG paneling and will be using Larsen trusses at 24" OC.  We will be putting rockwool down for insulation and then strapping with a metal roof.  The air gap is going to be around 2-4" which should give plenty of room to remove hot air/moisture.  Last house we were in was way to tight and there was a lot of mold in the attic.  You would never know it was there, we ended up with a small attic fan to keep it from growing.  Not sure how I feel about spray foam to be honest, just seems to be too tight and you end up with some sort of mechanical solution.

Air flow is really important.

Joe Hillmann

I plan to do something similar for my roof



 

The roof boards will be exposed on the inside.  It will take me a while after the boards are on until I can get the roof finished and I don't want them to get stained.  I have access to used rubber roofing.  I intend to temporally cover the roof with the rubber and leave it there as my underlayment.  Above the Styrofoam I plan to put some type of radiant barrier that will also act to catch condensation on the back of the tin.

A horizontal 2x4 will be put on top of each rafter(2 feet on center)to give an air space for hot air to flow from the bottom to the ridge. And a horizontal 2x4 will also be put top to act as strapping for the metal roof(pole barn steel).  Long screws through the 2x4's into the rafters below is how I intend to anchor everything together.

I don't know how useful the radiant barrier will be.  My main reason for it is to prevent heat gain in the summer, and keep any condensation from the underside of the tin getting the Styrofoam wet.  Any help in keeping heat in in the winter is an added bonus.

However with the rise in the cost of Styrofoam the last two years I may consider putting 2x10's on top of the roof boards and putting fiberglass in the cavities. Then 2x4 strapping on top to attach the tin.

Mattjohndeere2

Quote from: bannerd on February 09, 2022, 09:26:31 AM
They key is airflow, I have not done my roof yet but we put felt paper down on the TG paneling and will be using Larsen trusses at 24" OC.  We will be putting rockwool down for insulation and then strapping with a metal roof.  The air gap is going to be around 2-4" which should give plenty of room to remove hot air/moisture.  Last house we were in was way to tight and there was a lot of mold in the attic.  You would never know it was there, we ended up with a small attic fan to keep it from growing.  Not sure how I feel about spray foam to be honest, just seems to be too tight and you end up with some sort of mechanical solution.

Air flow is really important.
Thanks Banner. What is the cost and advantage of using Rock wool above something like Polyiso or EPS?

Mattjohndeere2

Quote from: Joe Hillmann on February 09, 2022, 06:31:22 PM
I plan to do something similar for my roof



 

The roof boards will be exposed on the inside.  It will take me a while after the boards are on until I can get the roof finished and I don't want them to get stained.  I have access to used rubber roofing.  I intend to temporally cover the roof with the rubber and leave it there as my underlayment.  Above the Styrofoam I plan to put some type of radiant barrier that will also act to catch condensation on the back of the tin.

A horizontal 2x4 will be put on top of each rafter(2 feet on center)to give an air space for hot air to flow from the bottom to the ridge. And a horizontal 2x4 will also be put top to act as strapping for the metal roof(pole barn steel).  Long screws through the 2x4's into the rafters below is how I intend to anchor everything together.

I don't know how useful the radiant barrier will be.  My main reason for it is to prevent heat gain in the summer, and keep any condensation from the underside of the tin getting the Styrofoam wet.  Any help in keeping heat in in the winter is an added bonus.

However with the rise in the cost of Styrofoam the last two years I may consider putting 2x10's on top of the roof boards and putting fiberglass in the cavities. Then 2x4 strapping on top to attach the tin.
Joe, very nice. Identical designs we have. The Polyiso, brand new is pricey, but the surplus places are about half the cost of new, if they have it. Around here they are pretty much out of it, I missed out on buying some yesterday, just called the guy a little too late, someone else had already snatched it up.
I thought a bit more about the radiant barrier today, it seems that since the polyiso is supposed to be quite low in permeability to moisture that with a vapor barrier underneath, and a radiant barrier on top (essentially doing vapor retarding also), that there shouldn't be any problems in the polyiso layers, unless they had abnormally high amounts of absorbed moisture to begin with. Then the venting takes care of keeping the environment between tin and radiant barrier following that of outside the tin. I'm no expert at all...hopefully I am making logical sense.

Joe Hillmann

Quote from: Mattjohndeere2 on February 09, 2022, 07:13:56 PM
Quote from: Joe Hillmann on February 09, 2022, 06:31:22 PM
I plan to do something similar for my roof



 

The roof boards will be exposed on the inside.  It will take me a while after the boards are on until I can get the roof finished and I don't want them to get stained.  I have access to used rubber roofing.  I intend to temporally cover the roof with the rubber and leave it there as my underlayment.  Above the Styrofoam I plan to put some type of radiant barrier that will also act to catch condensation on the back of the tin.

A horizontal 2x4 will be put on top of each rafter(2 feet on center)to give an air space for hot air to flow from the bottom to the ridge. And a horizontal 2x4 will also be put top to act as strapping for the metal roof(pole barn steel).  Long screws through the 2x4's into the rafters below is how I intend to anchor everything together.

I don't know how useful the radiant barrier will be.  My main reason for it is to prevent heat gain in the summer, and keep any condensation from the underside of the tin getting the Styrofoam wet.  Any help in keeping heat in in the winter is an added bonus.

However with the rise in the cost of Styrofoam the last two years I may consider putting 2x10's on top of the roof boards and putting fiberglass in the cavities. Then 2x4 strapping on top to attach the tin.
Joe, very nice. Identical designs we have. The Polyiso, brand new is pricey, but the surplus places are about half the cost of new, if they have it. Around here they are pretty much out of it, I missed out on buying some yesterday, just called the guy a little too late, someone else had already snatched it up.
I thought a bit more about the radiant barrier today, it seems that since the polyiso is supposed to be quite low in permeability to moisture that with a vapor barrier underneath, and a radiant barrier on top (essentially doing vapor retarding also), that there shouldn't be any problems in the polyiso layers, unless they had abnormally high amounts of absorbed moisture to begin with. Then the venting takes care of keeping the environment between tin and radiant barrier following that of outside the tin. I'm no expert at all...hopefully I am making logical sense.
When I first started planing on my roof with exposed rafters and boards I pretty much came up with this design.  But felt it was very convoluted.  But the more I looked into how others had done it everything about it is normal.  Except for the rubber roof and the set of vertical 2x4 to make an air space.
I originally was going to use second had insulation from flat roofs.  Two summers ago I was going to buy 4'x4' x4" for $7 each.  But money was a bit tight so I figured I could hold off.  Now those same second hand sheets are $15-$25 each.  I screwed up not buying them then.  If the price stays high the rubber roof may be the final roof and I will have no insulation for a few years.  So I plan to build eaves with the roof boards that I can cut off when I put the insulation and tin roof on. 
The whole cabin is being built on a shoestring budget so the extra price of secondhand insulation would put me back a year if not for the rubber roof as a short term solution.

Don P

Think of double vapor barriers as a bag. Moisture will get in, that I can guarantee. Design and detail to dry.

Joe Hillmann

Quote from: Don P on February 09, 2022, 10:01:22 PM
Think of double vapor barriers as a bag. Moisture will get in, that I can guarantee. Design and detail to dry.
That may be true.  How does it work on homes that have a vapor barrier under the sheetrock as well as a foil faced and taped sheeting or styrofoam on the outside?
In my design, both the top and bottom will be open to venting in the eaves or ridge and any moisture that gets trapped should be in the Styrofoam only.

I had thought putting the radiant barrier so it is on top of the vertical 2x4 and below the horizontal 2x4's.  That way there would be a 1.5 inch air gap above and below the barrier so it should work as a conduction break as well as a radiant barrier. It would also allow air to flow between the two vapor barriers and for moisture to work its way out better. 

I decided against that route because the added difficulty and the higher possibility of ripping it when installing the metal roof.  But I may have to take another look.  It would prevent your concern of being a plastic bag.

I wonder if there is any sort of radiant barrier that is full of small slits or some other way to let vapor through but still mostly act to reflect radiation?

Mattjohndeere2

Quote from: Joe Hillmann on February 10, 2022, 01:53:40 PM
Quote from: Don P on February 09, 2022, 10:01:22 PM
Think of double vapor barriers as a bag. Moisture will get in, that I can guarantee. Design and detail to dry.
That may be true.  How does it work on homes that have a vapor barrier under the sheetrock as well as a foil faced and taped sheeting or styrofoam on the outside?
In my design, both the top and bottom will be open to venting in the eaves or ridge and any moisture that gets trapped should be in the Styrofoam only.

I had thought putting the radiant barrier so it is on top of the vertical 2x4 and below the horizontal 2x4's.  That way there would be a 1.5 inch air gap above and below the barrier so it should work as a conduction break as well as a radiant barrier. It would also allow air to flow between the two vapor barriers and for moisture to work its way out better.  

I decided against that route because the added difficulty and the higher possibility of ripping it when installing the metal roof.  But I may have to take another look.  It would prevent your concern of being a plastic bag.

I wonder if there is any sort of radiant barrier that is full of small slits or some other way to let vapor through but still mostly act to reflect radiation?
This has taken me down a rabbit hole of learning about vapor barrier'ing and vapor permeability, and when/where it is appropriate to completely block moisture from moving, and when to allow it to transition in or out of certain layers on the house. Joe, you have a great point about foil insulation outside a wall, with batt insulation inside, behind the sheetrock. 
I see where DonP is coming from, if we seal up that insulation and don't allow some sort of permeability out of it, over time any amount of moisture in there will cause problems. In theory, it would seem that sealing it up tight to not allow moisture to move in or out would be a good thing, but in practice, that would most likely end poorly. Like DonP mentioned, moisture most certainly will get in, and if it does not have a "fast" way of getting out, then it'll fester and create mold and reduce the insulation effectiveness.

Looking at why Tyvec is appropriate on walls can help to understand this. For reference, Tyvec is something like ~50 perms, ice/water barrier is ~0.05 perms. BIG difference there, and there is reason for that. In a normal batt insulated wall, the interior paper layer on the batt insulation is considered a Class II barrier (0.1-1 perm). Now, if Tyvec on the outside of the wall were also a good barrier (lets say 1 perm), then you'd have a similar "bag" situation like DonP referenced. Moisture would not be able to move, i.e. the wall would not be able to dry to the outside, and would over time cause issues. So hence Tyvec's permeability. ~50 perms allows moisture to pass through, fast enough that moisture can't stay long enough to cause problems, yet slow enough to inhibit large gains of moisture in short periods of time.

Vapor barrier location is somewhat dependent on region also. I won't get too far into that because I'm still learning - but cold climates, like ours, the rule of thumb is the barrier is at the interior of the house, and allows walls/roofs to dry to the outside. 

So being much more schooled than I was a week ago - apply all this science to our roof scenario, it makes sense to have a good barrier at the interior (i.e. on top of the roof decking), and then everything above that should be allowed to dry to the outside. In DonP's roofs, he has a cold roof, with plywood on top. What he's doing is creating a unit out of the tin + plywood, giving more control to moisture movement. Plywood is semi-permeable (~10 perms)  when it comes to moisture movement, so it slows the moisture movement from tin to cold roof space. I'm sure the plywood also gives a small R value, which slows thermal increase/decrease, which probably helps control condensation somewhat.

What I'm going to do is different, but should net a similar effect - instead of plywood, the bubble foil insulation. Its perm rating is lower, but should provide some thermal control for the tin heating/cooling, and give the cold roof time to handle any condensation. I renig on my usage of foil faced for the last layer of insulation. It seems the best approach is nothing atop the insulation layer. Something like Tyvec atop the insulation may be appropriate, it would allow moisture movement, and could also create an air movement barrier. 

DonP, thanks for your input, it's much appreciated, I hope the science here makes sense. 

Joe, I think, based on what I've learned, that the radiant barrier is un-necessary as far as moisture goes. What I think could be a good thing to do, is have something directly underneath the tin, such as the bubble insulation. It will somewhat help with your radiation concerns, and from a moisture perspective, prevent moisture reaching your purlins and insulation.

Reference link below to understanding permeability of materials
https://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials

bannerd

Quote from: Mattjohndeere2 on February 09, 2022, 07:07:23 PM
Quote from: bannerd on February 09, 2022, 09:26:31 AM
They key is airflow, I have not done my roof yet but we put felt paper down on the TG paneling and will be using Larsen trusses at 24" OC.  We will be putting rockwool down for insulation and then strapping with a metal roof.  The air gap is going to be around 2-4" which should give plenty of room to remove hot air/moisture.  Last house we were in was way to tight and there was a lot of mold in the attic.  You would never know it was there, we ended up with a small attic fan to keep it from growing.  Not sure how I feel about spray foam to be honest, just seems to be too tight and you end up with some sort of mechanical solution.

Air flow is really important.
Thanks Banner. What is the cost and advantage of using Rock wool above something like Polyiso or EPS?
Rockwool is expensive BUT it is a lot different than eps paneling. I attended several natural home building classes in vermont for the last 5-6 years and there is just so much chatter about a breathable home against a non breathable home.  The designs here is basically like a SIP panel which if I remember correctly requires a mechanical vent if you use spray foam to seal or thermal tape.  The issue is inside the home, air becomes stagnate and you want to move that out otherwise mold can occur or poor air quality.  So generally wood absorbs and then transfers.  Felt paper is an awesome way to absorb that as well and transfer.  Rockwool also absorbs and transfers.  By the time it hits the air gap/vent in the roof the temp change is nominal.  The goal is how slow can the process be?  You don't want your R value or heat /cooling to just leave a building.. you want it to happen slow.  Rapid heat or cooling is where issues start to happen in terms of vapor.

I'm still learning but in my own mind I strongly believe fresh air that can be brought slowly into a home even when it's -30F or 100F outside is just better quality of living.  I've also learned that a roof is the most important part, the farther you can get water away from the building the better.


Mattjohndeere2

Quote from: bannerd on February 11, 2022, 11:11:21 AM
Quote from: Mattjohndeere2 on February 09, 2022, 07:07:23 PM
Quote from: bannerd on February 09, 2022, 09:26:31 AM
They key is airflow, I have not done my roof yet but we put felt paper down on the TG paneling and will be using Larsen trusses at 24" OC.  We will be putting rockwool down for insulation and then strapping with a metal roof.  The air gap is going to be around 2-4" which should give plenty of room to remove hot air/moisture.  Last house we were in was way to tight and there was a lot of mold in the attic.  You would never know it was there, we ended up with a small attic fan to keep it from growing.  Not sure how I feel about spray foam to be honest, just seems to be too tight and you end up with some sort of mechanical solution.

Air flow is really important.
Thanks Banner. What is the cost and advantage of using Rock wool above something like Polyiso or EPS?
Rockwool is expensive BUT it is a lot different than eps paneling. I attended several natural home building classes in vermont for the last 5-6 years and there is just so much chatter about a breathable home against a non breathable home.  The designs here is basically like a SIP panel which if I remember correctly requires a mechanical vent if you use spray foam to seal or thermal tape.  The issue is inside the home, air becomes stagnate and you want to move that out otherwise mold can occur or poor air quality.  So generally wood absorbs and then transfers.  Felt paper is an awesome way to absorb that as well and transfer.  Rockwool also absorbs and transfers.  By the time it hits the air gap/vent in the roof the temp change is nominal.  The goal is how slow can the process be?  You don't want your R value or heat /cooling to just leave a building.. you want it to happen slow.  Rapid heat or cooling is where issues start to happen in terms of vapor.

I'm still learning but in my own mind I strongly believe fresh air that can be brought slowly into a home even when it's -30F or 100F outside is just better quality of living.  I've also learned that a roof is the most important part, the farther you can get water away from the building the better.
Thanks banner. That is a very interesting perspective, comparing rockwool to non-permeable rigid insulation. I see where the breath-ability comes into play. You have me reconsidering my wall insulation. I think for my roof, for now i'll keep it to rigid foam, it's just a little cheaper and logistically easier/faster to do than framing out for rockwool installation. It takes a good bit of rockwool to get R49 values also. 
So based on those classes you took, is there a solution that is valued above others for insulation practices? And is there any data about amount of air exchange over time, if you compared rockwool to fiberglass to sprayfoam?
By the way, hows your house coming along? Last I saw on your thread you had some roof decking on.

Joe Hillmann

Quote from: Mattjohndeere2 on February 11, 2022, 09:51:46 AM


Joe, I think, based on what I've learned, that the radiant barrier is un-necessary as far as moisture goes. What I think could be a good thing to do, is have something directly underneath the tin, such as the bubble insulation. It will somewhat help with your radiation concerns, and from a moisture perspective, prevent moisture reaching your purlins and insulation.

I do think some type of moisture control should be directly below the tin.  I have only seen it two times in my life, but if the humidity is high enough(like 100%) and the air temperature is low enough(just above freezing) it can actually rain under a tin roof.
It is very rare but I would like to prevent that water from having any chance of working its way into my insulation.  And give it a path out through the eaves as liquid or ridge as vapor.  Since I intend to use second hand foam I will be doing at least 2 layers of 4 inch foam and maybe 3 layers of 3 inch foam(it depends on what I can get my hands on).  The spaces between the layers would be able to hold on to a lot of water through capillary action.
There is another downside of using Styrofoam insulation that kind of has me on the fence of using it.  How it behaves in a fire.  The amount of time you have to get out of burning house with styrofoam is much shorter than a burning house with any other type of foam.  Which is an extra concern for me because the kids bedrooms will be in the loft.  I don't intend to my house to ever burn down but my thought is a larger cavity with and air space to act as a chimney would allow any fire that starts in the roof to grow very quickly.
For fire reasons (and cost) I have though of going with fiberglass insulation and 2x12 spacers above the rafters for insulation in the roof as well.
But I can put those decisions off for years if necessary because of the temporary rubber roof.

Don P

If almost every morning is rare. It is a relative humidity and dew point equation, the conditions are not extreme. Think about how often you see dew or fog, the metal does a much better job of collecting and channeling that water repeatedly to the same places day after day. If you are pulling that damp nightime air along the underside of the metal, what is keeping that same condensation you see on the top every morning from forming on the underside.

Figure out how to avoid wetting and dry as quickly as possible if wetting happens. What the method I was showing does is it allows the dew to form on the top side but because the high humidity pre dawn air cannot get to the underside of the metal, it does not sweat on that face. If any does occur it is absorbed and dispersed within the tarpaper and then the roof sheathing as heat from the sun drives the vapor towards the vent channel. I can see any number of ways to look at the problem and solutions though, this seems to work.

The sheathing under the metal does one more thing I think. In the 60's-70's we were using 2" foil faced polyiso foam over a heavy timber T&G roof deck (times have changed!) shingles were laid directly on the foil and nailed with 3" roofers. The shingles died from the backside, the foil was cooking them. It shortened their life, but not enough that Dad didn't have a bunch of them out there before the problem showed. Luckily most of his were on pretty heavily wooded lots but the exposed ones cooked.  The vented channel above the foil gives the radiant barrier a working gap and a vent channel to wisk that heat away quickly. The sheathing helps keep the finish on the metal or the shingles from overheating over the foil.

And of course one can avoid the issues of condensation on metal by avoiding a metal roof. Like everything it is a compromise, is it the most favorable one.

I'm not pushing one way or another, I'm not that bright, the guys behind me will let you know how mine turned out, just more grist for the mill.

Joe Hillmann

Quote from: Don P on February 12, 2022, 10:37:43 PM
If almost every morning is rare. It is a relative humidity and dew point equation, the conditions are not extreme. Think about how often you see dew or fog, the metal does a much better job of collecting and channeling that water repeatedly to the same places day after day. If you are pulling that damp nightime air along the underside of the metal, what is keeping that same condensation you see on the top every morning from forming on the underside.

Figure out how to avoid wetting and dry as quickly as possible if wetting happens. What the method I was showing does is it allows the dew to form on the top side but because the high humidity pre dawn air cannot get to the underside of the metal, it does not sweat on that face. If any does occur it is absorbed and dispersed within the tarpaper and then the roof sheathing as heat from the sun drives the vapor towards the vent channel. I can see any number of ways to look at the problem and solutions though, this seems to work.

The sheathing under the metal does one more thing I think. In the 60's-70's we were using 2" foil faced polyiso foam over a heavy timber T&G roof deck (times have changed!) shingles were laid directly on the foil and nailed with 3" roofers. The shingles died from the backside, the foil was cooking them. It shortened their life, but not enough that Dad didn't have a bunch of them out there before the problem showed. Luckily most of his were on pretty heavily wooded lots but the exposed ones cooked.  The vented channel above the foil gives the radiant barrier a working gap and a vent channel to wisk that heat away quickly. The sheathing helps keep the finish on the metal or the shingles from overheating over the foil.

And of course one can avoid the issues of condensation on metal by avoiding a metal roof. Like everything it is a compromise, is it the most favorable one.

I'm not pushing one way or another, I'm not that bright, the guys behind me will let you know how mine turned out, just more grist for the mill.
I want to avoid the hot roof(where insulation is directly against the roof with no air flow)  I know it kills shingle roofs.  Supposedly it isn't a problem on tin roofs.  I don't know if I believe that. But I also want the air gap to keep the loft area cooler in the summer.  And a radiant barrier only works if there is an air gap on at least one side of it.  Other wise the radiated heat gets absorbed by what ever the barrier is touching, and then the heat passes through the barrier as conduction into whatever is on the other side of the barrier.

In my case I am going with a steel roof because of cost: no need for sheeting under the steel, and the steel is (or was) cheaper than a good shingle.  I am not sure on prices since things have changed a lot in the last 2 years.

I also thing the steel will last longer(even if it is all beat up and ugly after a couple decades and hail storms) than a shingle roof. 

I like the slight extra fire resistance of a metal roof as well.  Of course with Styrofoam under the steel it may not make much difference.

bannerd

Quote from: Mattjohndeere2 on February 11, 2022, 09:56:54 PM
Quote from: bannerd on February 11, 2022, 11:11:21 AM
Quote from: Mattjohndeere2 on February 09, 2022, 07:07:23 PM
Quote from: bannerd on February 09, 2022, 09:26:31 AM
They key is airflow, I have not done my roof yet but we put felt paper down on the TG paneling and will be using Larsen trusses at 24" OC.  We will be putting rockwool down for insulation and then strapping with a metal roof.  The air gap is going to be around 2-4" which should give plenty of room to remove hot air/moisture.  Last house we were in was way to tight and there was a lot of mold in the attic.  You would never know it was there, we ended up with a small attic fan to keep it from growing.  Not sure how I feel about spray foam to be honest, just seems to be too tight and you end up with some sort of mechanical solution.

Air flow is really important.
Thanks Banner. What is the cost and advantage of using Rock wool above something like Polyiso or EPS?
Rockwool is expensive BUT it is a lot different than eps paneling. I attended several natural home building classes in vermont for the last 5-6 years and there is just so much chatter about a breathable home against a non breathable home.  The designs here is basically like a SIP panel which if I remember correctly requires a mechanical vent if you use spray foam to seal or thermal tape.  The issue is inside the home, air becomes stagnate and you want to move that out otherwise mold can occur or poor air quality.  So generally wood absorbs and then transfers.  Felt paper is an awesome way to absorb that as well and transfer.  Rockwool also absorbs and transfers.  By the time it hits the air gap/vent in the roof the temp change is nominal.  The goal is how slow can the process be?  You don't want your R value or heat /cooling to just leave a building.. you want it to happen slow.  Rapid heat or cooling is where issues start to happen in terms of vapor.

I'm still learning but in my own mind I strongly believe fresh air that can be brought slowly into a home even when it's -30F or 100F outside is just better quality of living.  I've also learned that a roof is the most important part, the farther you can get water away from the building the better.
Thanks banner. That is a very interesting perspective, comparing rockwool to non-permeable rigid insulation. I see where the breath-ability comes into play. You have me reconsidering my wall insulation. I think for my roof, for now i'll keep it to rigid foam, it's just a little cheaper and logistically easier/faster to do than framing out for rockwool installation. It takes a good bit of rockwool to get R49 values also.
So based on those classes you took, is there a solution that is valued above others for insulation practices? And is there any data about amount of air exchange over time, if you compared rockwool to fiberglass to sprayfoam?
By the way, hows your house coming along? Last I saw on your thread you had some roof decking on.
I'm not sure to be honest, I do know a lot of products are coming out for walls that are geared towards breath-ability.  Dimple sheet seems to be popular now and dimple house wrap.  Wood seems to be a more common siding these days and a lot of products are by design giving channels for air flow/water but they do not complete what it should be.  The classes were more geared towards natural products such as COB/LSC (Light straw clay), rammed earth, etc etc. 
ASTM E2392/E2392M-10(2016) - Standard Guide for Design of Earthen Wall Building Systems
I will be doing 13" LSC walls in my house but since the roof needs to be done fast, I can't mess around there so rockwool it is.  There are people who are building entire houses from insulated blocks from nexcem, another system that is made from clay/portland/woodchips.  My basement is done in this manner and so far I like the system.  I keep a small heater down there and I can get it up to 80+ from a wall mounted propane heater (1500sqft).
House is coming along but the new diesel prices are killing us.  I'm hoping to get a metal roof on this year.. it was a rush this fall to lay tar paper which is all we have on there.  Anyways the roof is on and the larsens are on the walls, can't wait to move into it and get out of our trailer.  
Roof is main thing, wood siding, electrical conduit, LSC infill, Mass heater.. I don't even want to think about it... ha

Mattjohndeere2

Quote from: bannerd on February 14, 2022, 07:14:51 AM
Quote from: Mattjohndeere2 on February 11, 2022, 09:56:54 PM

Thanks banner. That is a very interesting perspective, comparing rockwool to non-permeable rigid insulation. I see where the breath-ability comes into play. You have me reconsidering my wall insulation. I think for my roof, for now i'll keep it to rigid foam, it's just a little cheaper and logistically easier/faster to do than framing out for rockwool installation. It takes a good bit of rockwool to get R49 values also.
So based on those classes you took, is there a solution that is valued above others for insulation practices? And is there any data about amount of air exchange over time, if you compared rockwool to fiberglass to sprayfoam?
By the way, hows your house coming along? Last I saw on your thread you had some roof decking on.
I'm not sure to be honest, I do know a lot of products are coming out for walls that are geared towards breath-ability.  Dimple sheet seems to be popular now and dimple house wrap.  Wood seems to be a more common siding these days and a lot of products are by design giving channels for air flow/water but they do not complete what it should be.  The classes were more geared towards natural products such as COB/LSC (Light straw clay), rammed earth, etc etc.  
ASTM E2392/E2392M-10(2016) - Standard Guide for Design of Earthen Wall Building Systems
I will be doing 13" LSC walls in my house but since the roof needs to be done fast, I can't mess around there so rockwool it is.  There are people who are building entire houses from insulated blocks from nexcem, another system that is made from clay/portland/woodchips.  My basement is done in this manner and so far I like the system.  I keep a small heater down there and I can get it up to 80+ from a wall mounted propane heater (1500sqft).
House is coming along but the new diesel prices are killing us.  I'm hoping to get a metal roof on this year.. it was a rush this fall to lay tar paper which is all we have on there.  Anyways the roof is on and the larsens are on the walls, can't wait to move into it and get out of our trailer.  
Roof is main thing, wood siding, electrical conduit, LSC infill, Mass heater.. I don't even want to think about it... ha
I hear ya on the trailer thing - wife and I are doing the same thing.
As far as code compliance goes, I assume they are good with the practices of making a breathable house, with materials such as straw and whatnot?  I'm assuming up by you, you also have to meet the <3 air changes per hour blower door test? I agree with modern codes that we should try and save energy in our buildings, however I disagree with the strictness of the requirements (such as being below 3 ACH), which introduce new costs and complexities (HRV/ERV ventilation systems). I certainly don't want unhealthy air in the house, but building a house so tight and then having to mechanically ventilate is an oxymoron, it doesn't make logical sense to me. This not making sense is magnified by seeing all the 150 year old farm houses that are still standing, built without any code intervention and pure common sense - they may not be efficient, but they are breathable and probably healthier living than even having mechanically controlled ventilation. How many modern day built houses will we see in 150 years? I am inclined to think strict codes is reducing the effective service life of houses. A brand new house up to codes with proper insulation and vapor control does its job very well, but what happens in 30 years when the insulation degrades, or the vapor control layer begins to fail to do its job? That "perfectly" constructed house is now operating outside of the expected environmental conditions, and requires another round of "perfecting" before it rots away.
I think you are in the same thinking as I am Banner, I would like to build a house that will last for another 150 years. Building a tight house with less permeable materials and less "drying to the outside" type of techniques provides a narrower window for error, it can only handle the environmental factors it was designed for, but outside that, it may not perform well at all. A breathable house, built correctly, has a wider margin for error with respect to handling differing environments (cold, hot, humid, dry). Codes is good, when it comes to structural requirements, however I think there is somewhat of an illusion of "good code practices" when things like environmental impact or energy savings drive regulations and requirements. 
You do have me rethinking my whole insulation plan now that we've gotten on this topic Banner. I'd love to build a house that was breathable enough to not need a mechanical ventilation system, however I am not sure that is possible, while still meeting the requirements that codes has set forth (i.e. blower door test). I really appreciate the candid discussion here, maybe this summer I can lend you a hand with your house for a weekend.

bannerd

I think you'll find the right path, most new construction around here ends with tyvek on the outside of the home.

Nbpete08

Great discussion here guys. It's been fun/insightful to read. Has anyone seen documentation on the longevity of SIPS? Are they great insulation until a certain point? I imagine their "permeability" is not good. Or do guys turn their nose up at SIPS when trying to build a breathable long lasting structure?

canopy

One question looking at those pictures if you don't mind. What techniques are used to anchor the purlins and strapping that are sitting on foam?

Ljohnsaw

John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Don P

On the one in my pics the 4x rafters are on 4' centers so long struc screws through sleeper, foam deck and into the rafter. The "odd" sleepers had a carefully selected screw length to bite but not penetrate the 2x roof deck/ceiling. the straps over the top run from sleeper to sleeper tying them together over the struc ridge.

Joe Hillmann

Quote from: canopy on March 08, 2022, 07:09:42 PM
One question looking at those pictures if you don't mind. What techniques are used to anchor the purlins and strapping that are sitting on foam?
16 inch long screws through the foam, roof boards and into the rafters below with large washer heads.  The Styrofoam is squeezed between the purloins and roof boards.  If a person is concerned about the Styrofoam compressing over time you can cut out spots in the Styrofoam where the screws will be and place some type of space in there made of wood or metal.  That way even if the Styrofoam compresses nothing will loosen up.
With current prices and world events I assume it will be a few years before I can afford to insulate and put the steel roof on my house so I will have time to decide the finer details of it.

In my build the rafters will be every 2 feet and smallest ones will have 3 inch wide flats on top and the biggest ones will have 8 inch wide flats on top so I won't have to be super careful to hit  a 1 1/2 inch wider rafter.

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