Question about insulating under tin roof

[hbeane]

Not sure if I am posting this in the right place but here goes.  Been awhile since I have posted anything on forestry forum but in the past everyone has always been very helpful.
 I am building a small house 20' x 24' with a loft over over half of it.  I have cut most (not all) but most of my lumber with my woodmizer LT15.  My question is insulating under my tin so I keep the heat and the cool in. As well as the condensation under the tin. I will try to explain what I am doing.  I cut my rafters 3" x 8" and are a little less than 3' on center.  on top of my rafters I cut 1 1/4" boards to lay down.  I am going for the rustic look so I am not finishing the inside of my ceiling.  Rafters and the 1 1/4" boards will be exposed. I am not sure what to do on top of my boards. Some type of insulation I have looked at the 1 1/2" thick foam boards. However it is so expensive so I am looking for some advice on a cheaper solution but want to be certain I dont have a condensation problem.  Laying 2x4's flat making a place for insulation.  Then OSB down for my tin to lay on.  I have looked online at a foil insulation 10mm thick called prodex but I am sceptical.  I also have read where people have used rolled fiberglass insulation with a vapor barrier down but also skeptical.  Anyone have any help for me sorry I got long winded.


 

 

 

[Andries]

Insulation is one of those things that is so competitive, that you pretty much get what you pay for.
You being in W Virginia,  get a bit of both worlds: cold and hot.
If you think in terms of what's going to last long and also pay for itself over the years, it might not be so pricey.
The foam boards would be my choice, just because it's near fail proof and a proven product. I'd go with the product that has lap edges, so that you get a better heat seal.
Buy once, cry once - may be good advice for this project.

[hbeane]

Andries thanks for the advice.  I am sure you are right this is not the place to cut corners.

[Don P]

Keep an eye on craigslist for foam. I'd do at least 2 layers of 2" offsetting seams between layers, then 2x sleepers screwed thru the foam to the rafters, then either ply and metal or just metal.

An interesting video here;
https://www.youtube.com/watch?v=Ld8pzIu45F8

You are right to be skeptical of the radiant bubble wrap guys. The radiant barrier does work as a radiant barrier but not as insulation. You can do a little research yourself by googling radiant foil barriers and the Federal Trade Commission, they have some history with one another ;).

[way up north]

Im certainly no expert, but I have looked into the radiant barrier. they appear to fall apart after a year or two.
They make metal roofing with a felt like material attached to the bottom to absorb and release condensation. Might be something to look into in addition to your foam.

hope that helps

[hbeane]

Thank You to everyone that gave me some suggestions.  Gonna just suck it up and go with the foam board.  Its what my gut was telling me to do anyway. To much time and work and money to get cheap with my roof insulation.  Thanks again.

[Bruno of NH]

Foam all the way
Look on Craigslist or FB market place some times you can find a good deal

[Gearbox]

Have you looked at spray foam . you are doing what log home builders have been doing for years . I would do a min. of 3 .5 . A 2x4 on edge . 2 inches is not much for heating or cooling , its going to be costly to do after .

[coal909]

you can look into stress skin panels, super nice product easy to instal and after lining them up you just shoot foam to seal the seams up. the only VERY important part is that you make sure to run your strapping vertical not horizontal for airflow and condensation. Awsome stuff

[Bruce_Gordon]

All good options mentioned. If you want both a radiant barrier and good R-value, foil-faced foams can do the job. As mentioned, two layers laid in opposite directions and taped seams works well. It gives you a thicker-looking roof but to my eye that works well in a timber-framed house.

[Bruce_Gordon]

Forgot to mention that foam panels ordered from construction supply houses usually come wrapped in bundles of 20 (if my memory serves) so there are always a few left over (sometimes quite a few [gotta get rid of mine one of these days]). Definitely look on Craigslist like Bruno says.
Regarding moisture, as long as your foam panels are well sealed and there is air space between them and the roofing with a path for moisture to escape, there should be no problem. Moisture generally comes from within the house and the sealed foam acts as a vapor barrier. If you've got R-40 or more of sealed insulation, the conditions that might precipitate moisture condensation on your metal should not exist.

[scsmith42]

Another option would be to build some Larson I joists for the roof and use conventional fiberglass batt insulation in them. You can easily go 12" thick insulation or more.

[firefighter ontheside]

I built the exact roof system that @Don P recommended.  I have log purlins with 2x6 t&g on top.  Then I have a sheet of house wrap on the t&g.  On top of the house wrap is 2 layers of 2" poly iso with the seams staggered.  On top of that are 2x4s laying flat and running from peak to gutter board.  These are on 24" center with osb on top.  Then 30# felt and then metal.  No need to insulate in roof overhang.

[woodshax]

So... I used to own a sprayfoam insulation company....if it was my house I would do 2" of closed cell foam (at that depth it is a complete moisture barrier) and ads a lot of strength to the roof.... A bit more expensive than open cell...if you choose that then the depth needs to be what the rafters are plus plashing the rafters with foam to stop the radiant transmission through the beams

[D L Bahler]

OK so vapor permanence is going to be a major issue when insulating a roof deck, perhaps more so than it is in walls. 

I do not know for sure what sort of climate your location would be, but that will decide what you do. I would guess it's a mixed climate. The big question will be, how many days above 80% rh do you get in the summer, and how long is your heating season? How tight are your walls, etc. 

The issue with putting closed cell foam on your roof, if you have much of a heating season you will be trapping moisture underneath it with nowhere for it to go. This will become an issue especially if at any point some surface in this area falls below the dew point. However if you do like Don suggests and have 4" of foam between your decking and roofing, then you'll be fine. There's not going to be any condensation unless something goes seriously seriously wrong. With just 2" of foam, I'd be super worried if your winter gets very cold at all. But that's one of the reasons why you use 2 layers of foam staggered instead of 1 layer of 4" foam. In other news, this is the biggest reason why the Swiss and Germans like to use wood fiber insulation on the exterior. It has fairly similar r values to open cell foam, maybe a bit less, but the biggest thing is that it acts like a "smart" barrier, meaning it's semi-permeable, and its permeability increases under certain conditions. 

Typically in Indiana metal roofers have found the best solution to take care of condensation from the metal itself (which happens because wet air is penetrating from underneath and contacts the cold surface of the metal) is best taken care of with a layer of foil faced bubble wrap placed directly under the metal. But I can't say if that would be the best solution in West Virginia or not. We're not treating it like insulation, because it's too insignificant to have any real value in that regard. That being said, I don't like this idea. It to me seems a solution that will not last through the life cycle of the metal itself, and as such is just delaying the issue. They do this, because they find the older method of 1x strapping didn't do any good. In fact, it was worse than no good, because water gets to the metal,condenses, then gets trapped against the wood and either rots out the wood or rusts out the meatal (usually both) So really what I think they are doing is just using the foil wrap as an impermeable vapor barrier to keep moisture from underneath from getting to the metal. 

I'm a huge proponent of the double roof, especially with something like metal which has no vapor permeability at all. Basically what the concept is, you have your roof deck and roof covering separated by an air gap. I put 2x2's running up the rafters, then 2x2's strapped horizontally to screw the metal to. This gives you excessive airflow, which if handled properly at the eaves and ridge will allow for convection to carry a great deal of the summer heat away. But also important is that the lower level is also water proofed. Because we're assuming the metal is likely to sweat no matter how hard we try to stop it, so this way if it does it's not going to hurt anything. The water will drip down onto the deck, covered with roof underlayment, and then harmlessly run down and out of the whole assembly. It's a European technique, and they might have as much as 6" of insulation under the standoff. Using this with wood fiber insulation means vapor is free to escape from the interior, but will not condense on the roof covering itself, because it hits the air layer and convects harmlessly away. (especially if you use majrex or something like that) 

[Don P]

but will not condense on the roof covering itself, because it hits the air layer and convects harmlessly away.

If the roof is below dew point vapor will indeed condense on it. If there is wooden strapping on the deck membrane it will absorb that moisture. Is that a problem, I'm guessing that depends on local climate.

[D L Bahler]

right, sorry, i misstated. 
The intention was more that, if the moisture penetrates through the substrate and immediately comes into contact with the roof covering, it condenses AND is trapped. However, if there is an air gap, the moisture concentration is reduced because it diffuses into the air in the cavity. The dew point of this air is lower relative to the moisture penetrating through the substrate, and the moisture concentration is lower. This equates to, lower likelihood of condensation, or lower levels of condensation if it does happen, and added to that WHEN it happens, the now liquid water has more opportunity to escape and dry into the outside air. 

But you're also right about local climate concerns. In Indiana, this is a problem. In West Virginia? I don't know. Part of the state is zone 5, same as us, and part is zone 4. And I'm not sure how different things are between zones 4 and 5 in regards to moisture movement and heating/cooling cycles.

[Don P]

It involves yet another layer but is what I've done for shingles and I think would work well with metal as well. On top of vertical strapping that is over the foam put down a layer of 5/8 ply, then tarpaper, then metal or shingles. The ply won't sweat and drip or its much less prone to. In poor drying conditions it will absorb vapor until it warms up enough to dry.

[D L Bahler]

A little bit down a rabbit hole, but here is a Swiss roof section, or they might use something similar in parts of Germany and Austria also. Not sure how realistic something like this is in this scenario, but I thought it might lend some thought to the idea of insulating the roof deck to see how other people approach this idea.



 

So from bottom to top your layers are:
Gypsum board (sort of like American drywall, but stronger)
Wood fiber insulation 36 to 40 mm thick (1.42-1.57 inches)
OSB (yes, the sheathing is always on the INSIDE) (15 mm/.59 inch)
The rafters (which are like 4x91/, 24" OC) with insulation (here shown with wood fiber)
Wood fiber on the roof deck 40-140 mm thick (1.57-5.5")
underlayment
vertical roof strapping
horizontal roof strapping

Not shown is the roof covering, which in Switzerland would usually be clay tile, but might also be wood shingles or metal.

The principle idea driving this system is that the materials used are semi-permeable and can dry to the outside. If there is a vapor barrier installed, it will be between the gypsum and the insulation (actually more common, there's not any insulation between the gypsum board and the OSB, but this particular example is for MINERGIE construction, which is a high performance energy certification). They would most likely use Siga's Majrex wrap here, which is a one-way barrier, oriented toward the inside of the house.  

In a non MINERGIE situation, the rafters might be totally exposed to the house with a board decking on top of them, but everything above is basically the same no matter what sort of certification you're going for.

[woodshax]

Well, I will leave this lay as many people think they understand building science.  4" or more of of open cell foam on the roof deck is referred to as "capping the lid" and will seal in the living space. So if you have nothing in the attic worth heating then that works.  Typical "R-Value" rules of thumb are misnomers when comparing spray foam to batting or blown in because spray foam provides a uniform and continuous barrier.  You never want to insulate the roof deck and the the roof as that is what will create tons of moisture in the attic and lead to lots of condensation.  By spraying with 2" of closed cell and spraying any soffits or other venting normally found in attics you are sealing the attic and the attic becomes semi conditioned air space and will not vary more than 8 to 12 degrees above or below the conditioned temp of the living area....and there is no condensation issues.  2" is the most economical because every inch more only increases effectiveness by a few percent.  We normally on used 3 to 4 inches (2 separate passes to allow for heat dissipation) in freezers or cold rooms.  But what do I know.   

[Don P]

Lord its hard to be humble :D
We're all here to learn.

There has been lots written but check out detail 7 and its note here;
https://www.buildingscience.com/documents/reports/rr-0404-roof-design/view

[btulloh]

Excellent reference DonP.  Thanks.  

There is so much information floating around it's hard to separate the sheep from the goats.  Not to mention mis-information.  It amazes me how bad info gets repeated so often that it becomes accepted as fact.  Plus the fact that so much of this evolved in recent years and designs have been studied and improved.  You provide a valuable service to all of us by sharing these references.

[D L Bahler]

Just for the record, I'm trying to present the European perspective because it is quite different from the American one, and I think it's valuable to have different perspectives on something like this. Those who've been around here know that's what I do. I like to bring in the European and especially Swiss perspective (since it's what I know) because I think it's good for us as craftsmen to have a perspective broader than our own echo chambers might tend to produce. The world of carpentry is a big one, and even in terms of the scientific approach, there's no one way that stands out as obviously right. Every scenario is different, every job has its own unique demands, and it's good to be able to look at it from as many angles as you can.
Am I saying it's better? Maybe. I don't really know. If I am that's just my opinion and you can take it for whatever it's worth to you. I put it up because I know there are those here who will give an informed and reasoned criticism, and as a professional I'm supremely interested in finding what is the best practice for a given situation. It makes zero sense for me to argue about anything, because I'm far more interested in what's true so I have little interest in doggedly defending my opinion. My actual practices tend to be a blending of the two, and I sincerely strive to come up with what is the best solution to fit the needs of my customers and clients. That means I have to try to always look at things objectively and with an open mind. But that's hard to do, so I come to places like this, where most have a different opinion than I do, to have my ideas criticized. 

I am a bit biased against foam and plastic, I'll admit. That's because of my training I suppose coming largely from a background that adamantly opposes the use of plastic-based insulation. I buy pretty hard into the Swiss philosophy, which shouldn't come as a surprise since I am Swiss myself.

My perspective is there is room for both approaches. And which you choose depends on a number of factors. The typical "american" approach has huge benefits up front. It's super cost-effective and all that, whereas the European approach trades up front cost and performance in exchange for a superior performance decades down the road. Is this worth it? That will depend on your scenario. What are you building for? 20 years? 50 years? 100 years? I want to build the best houses I possibly can with the materials available to me, and I rely on other perspectives to drive me in what is hopefully the right direction. I hope that I can help to make others here better as well, because I like to think I'm giving back in some way. 

Am I right? Well, that's for experience to decide, and hopefully you all can judge for yourselves. I'm not interested in arguing about it. I presented what I am familiar with, and I'm happy to actually discuss things, but I'm not going to argue. 
I like discussing things with Don, because he does a very good job of being objective and stating what he sees as fact, and backing it up with citations. Do I always agree? Probably not, but I always find what he has to say to be valuable. 

[Don P]

I enjoy differing perspectives as well. My suggestions are just one path to the top of the hill, there are many. I like to provide references for further study and to show where I got such damfool notions when I can.

I wish more European materials were readily available here, and they will be in due time. I've seen video on Siga's smart wrap and, well aside from the knee jerk reaction of, "I've seen this attempted before", it does look promising. I have learned to stay back from the bleeding edge of new tech. I saw the title of one of Joe's papers and thought "oh crap he found one of Dad's houses from the '60's", it's right in his neighborhood, great idea but out on that edge, it had problems. I'm not wedded to foam, it is probably the best thing available to me now. Cellulose is just another long chain polymer, we are latecomers to the scene :D. I like the idea of an insulation/building material that is also somewhat of a hygric buffer, capable of absorbing and releasing some amount of moisture as the situation demands, part of what drew me to solid log walls, but there's a whole nuther set of issues. CLT's have been brewing in Europe for 30 years, they have code approval here as of a few years ago, not atypical for the direction of flow.

[D L Bahler]

Don; another thing you might find interesting, all of the lumber, or nearly all at least, used in Swiss construction is laminated. Now we're not talking like LVL style lamination, we're talking like glulam style. Maybe 1/2"-3/4" laminations. This is true of the studs, rafters, and pretty much every framing element. It's true of light framing, and also heavy timber and log construction. Everything is laminated. The idea being, they are making their frames out of wood that is kiln dried, engineered, and so it is stabilized and dimensionally reliable and consistent. A fascinating thing about this, is it allows them to retain their tradition of heavy timber construction while meeting modern stability and air tightness standards. 

I'm with you about solid log walls. I'm working on a system right now utilizing solid wood wall with an inboard insulation layer (I find the best solution to the problem appears to be rigid wood fiber insulation) based on the fact that it's a surprisingly cost effective way to create a pretty solid energy efficient house. The big thing comes back to the way wood-based systems handle moisture when done properly. The walls are based on the Swiss "Blockständer" construction, which uses upright timbers to bear the loads, but heavy timber infill to brace and enclose the structure. 

Anyway I'm sorry if this thread has been hijacked. I'm neck deep in some R&D right now for a couple building systems me and my partner are working on developing so I am a bit anxious to hash these things out! We're working on designing the solid wood system, and also an adaptation of conventional light framing using some European ideas. 

[Don P]

 :D thread drift, never happens here.

They are also getting better distribution of defects which gives a significant boost to allowable strength values. Their softwoods have lower design values so that is a good thing. Dad had switched to glulam and laminated roof decking by the late 60's because of shrinkage problems. Never looked back. There is research going on currently aimed at making residential CLT's on idled furniture presses, neat stuff. I'm bouncing ideas with them when a thought occurs, feel free to pm if I can help. A nearby log home company tried glulam logs 15-20 years ago, neat concept but couldn't sell them, people didn't consider them "real" and there was a healthy price increase. I'm sure there are niches but the market here is not the same as the market there, keep a few eggs stashed back.

[frwinks]

Here is the detail I came up with (with some help from guys @ the cold climate housing research center) just about 10 years ago now.  The whole system was very user friendly with off the shelf materials, 1/3 the cost of panels (at time of construction ;D) and has been performing as expected season after season.