Roof Ventillation or Not?

[jander3]

I need to insullate my cabin roof.  There is no attic space.  The roof is 2" x 8" rafters decked on the top with plywood and roofing, I will fill the rafter space with insullation (unbatted), cover it in plastic and then install 3/4" red oak planking on the underside of the rafters.   With this design, do you need to leave opening in the eaves for air movement? If so, what size? what location?

I'm thinking that openings are not need, but I don't know much about this topic.

[Magicman]

I'm thinking about moisture condensation and the dew point.  Surely you can't have much air movement, but in the South (which you ain't) you would have humidity.

My thoughts would be eave louvers and ridge vent.

[beenthere]

Jon
I think you should have air flow over the top of the insulation. Openings in the eaves and vent out the ridge.
Otherwise, it will be moist in that area from condensation, and will rot the wood.

[trim4u2nv]

This method is popular in chicago but its much colder up your way.  The foam baffles give a clear air path from soffit to ridge.   The drill and plug vents for the soffits will work fairly well and the copper lasts forever.

http://www.green-living-made-easy.com/attic-ventilation.html

http://www.copperlab.com/store/vents/soffit-vents.html

[Magicman]

Those insulation baffles are what I used on the vaulted ceilings in our cabin's upstairs.

[Raider Bill]

I did my whole roof  under the insulation in florida with them. Made a big differance.  Bought them at a insulation place for about $1.00 each if I remember right. Well worth it there.

[jdtuttle]

Definitely add venting. Ridge vent, soffit vents and as much airflow as possible. Condensation is very damaging & often undetected until water damage is done.
jim

[jander3]

Thanks.  Looks like insulation baffles and venting is the right path.   Also, I will  have to tell my wife that she was right.

[Magicman]

 Also, I will  have to tell my wife that she was right.

That's a good thing.   :D :D :D

[Holmes]

Insulation baffles , ridge and soffit vents will help prevent ice dams in the winter.

[ljmathias]

I did almost exactly what you describe except I used batted fiberglas under the pressed foam airflow ducts (about 1" space if I remember right) and then nailed the interior "siding" directly to the rafters (no wrap- coulda, woulda, shoulda).  Worked great on keeping the roof cool in the summer but here's the rub: in the winter, cold air just drips through the cracks and crevices, must leak around or along the open areas after the insulation settled some and then finds its way down into the house.  Doing it over, I'd put up interior sheathing under the rafters, and then the housewrap under (over?) that, then planks- wood moves, even overlapped planks crack and separate, air flows in and cold air just drops onto those below... not sure housewrap by itself is enough but would be big step up from what I did without thinking it through (Night in the Museum 2: "We're Americans!  We don't plan; we do!")

then again, you probably do a lot closer work than me and would seal everything up nice and tight...   

Lj

[jander3]

Lj

Plan? What plan? I had one once, I think back in 72.  Must have misplaced it.

Thanks for the idea on the keeping the cold air out.

[Ron Scott]

Yes, you need roof ventilation and the insulation baffles and venting is the right path. Don't block any air flow between the soffit vents and the ridge vents on the roof.

And Yes, you will have to tell the wife that she was right.  ;)

[ljmathias]

If your wife is like mine, there's no need to tell her she's right- she already knows it!   :D

If we didn't have wives to help steer us through the shoals and rocky outcroppings of life, where would we be?  Basking in the sun, enjoy surf and wind and wave on a tropical island somewhere, wishing we just had a good wife to share it all with... so take you pick: a good wife and good life, or boredom in Marquiretteville...

Lj

[Magicman]

Remember,  Happy Wife......Happy Life.    ;)   

[Stephen1]

Jander, the biggest problem I'm seeing for you will be the ridge to allow the air to escape, cause I do believe your roof is done. You will need to vent each space between the 2x8 from the bottom to the top, to allow the moisture to ascape. . I believe you will be able to buy a ridge cap to cover your whole ridge which will allow the moisture out.

[Alexis]

have you considered spraying the bottom of the plywood with urethane spray foam? it's also a vapor barrier so you wouldn't have condensation problem...

Alexis

[meddins]

I was researching cathedral ceiling ventilation and insulation a few months back and came across a couple of interesting links. They both point to manufacturers' websites so...standard disclaimers apply?!?

This one claims that dense packing cellulose between the rafters and sealing the whole thing up is superior to the traditional method of venting the soffits and leaving an airspace to the ridge.

http://www.applegateinsulation.com/Product-Info/Technical-Pages/249234.aspx

Here's a link for above-sheathing ventilation which looks like a promising way to reduce moisture and heat build up.

http://www.metroroofproducts.com/images/CMRF_19-20.pdf

Thought I'd throw those out there for your edification/confusion.  ;)

I've put a few metal roofs on and built a few houses. Never tried either of these methods but I may on my own house (under construction...for a looooong time now)...

M.E.

[Magicman]

And welcome meddins to The Forestry Forum.  Tell us a bit about yourself.

[Doug_D]

Meddins is right on the money.  When there is NO ATTIC space, you can go either way and be ok.  However, if you cannot get it sealed all the way, go with a vented roof. 

Remember when everyone would vent crawl spaces?  Just in the past several years more and more are sealed.  When I put my foundation in for my cabin, the two brick guys I talked with said they both recommended sealed.

Just an idea, but you could put a flash coat of foam in between the rafters and then batts.

Something as simple as this suff...
http://www.tigerfoam.com/

I've got foam in my house and my barn...it would be going in the cabin if I was not using SIPs.  Stuff is amazing.

[meddins]

Hey Magicman not too far from you - West Alabama. Somewhat frequent visitor to the timber frame forum but 1st time poster.

I'm building in a hot-humid climate so the topic of roof ventilation and insulation interests me. Preventing heat buildup in the first place seems crucial to reducing the load your a/c is dealing with.

I think the building science folks still have a long way to go in developing better strategies for passive cooling in hot-humid climates. Plenty of info out there for passive heating in more northern climates, but we only get 7 weeks or so of winter here so that's not my main concern...  :-\

M.E.

[ljmathias]

Looked at the site: cost is $335 for covering 200 sq ft at 1" with R 7 per inch, so for R50, you would need 7" thick foam meaning the $335 buys you 30 sq ft of coverage.  A thousand sq ft house with a cathedral ceiling might need 1500 sq ft of insulation or 50 X $335 which is over $15,000 just for the insulation: is my math off here or what?

Lj

[Thehardway]

There are a lot of variables here that determine the correct approach.  Climate, roof covering, desired R-value, heat/cooling source, etc.

Condensation only forms when a moist air mass meets a cool object and the dew-point is reached. (in other words the air temp drops and the moisture the air was holding gets deposited and collects on the solid object.

If there is no air present then there is no transport system for the moisture.

This is the principle used with SIP's, ICF's, and other solid wall/roof systems.  The lack of a physical cavity occupied by air that can move means the dew point is moved outside the actual wall or roof structure to the exterior or interior where it can dry.

If you fill the roof cavity with spray foam as Doug mentioned enough to form a vapor barrier and buffer solid surface temp to prevent the humid air coming in contact with a colder surface you have accomplished your purpose.

R- values are very deceiving and do not apply equally with all materials. It was created to rate bulk fill materials.   A roof sealed with 1" of spray foam (R-7?) under the deck will most likely outperform one with a "R-21" rated fiberglass batt when it comes to heating and cooling efficiency.  This is because it is air-tight and it performs as a integral part of the building envelope unlike fiberglass or any other bulk fill type material.  Most commercial spray foam installers will tell you that after you reach about 4.5" of thickness with a urethane type foam, the benefit gained is minimal.   Go visit a few outdoor walk in freezers and you will probably find 2-3" of urethane based foam, sandwiched in aluminum or steel panels. Facts are that "R's"
do not save energy as much as eliminating infiltration and convective heat loss/gain.

What kind of roofing material do you have on the outside? Do you do a lot of baking, cooking, canning or use heat sources inside that have a lot of moisture release such as propane? Do you have a hot tub or take long steamy showers? Do you have a lot of indoor plants?These things can also effect how you insulate and where your vapor barriers go.

You should check out the some building science websites as they will help you a lot in determining what is best for your application.

Also read this informative document:  http://www.diy-junction.com/files/Brochure-RValueMyth.pdf

[Doug_D]

LJ, The cost of that foam is very high.  Actually it is several times the cost of having someone come out with a big rig and spray the house.  I had all the walls of my 2900 sqft home sprayed for around $2k with open cell foam.  Mostly 2x4 walls, but I do have several 2x6's too.  I am pretty sure the cabin in question here is in a remote location so getting a 30' triaxle trailer would be pretty hard to do.  With the tiger foam (several other brands too) it would allow him to pack it in.

As I suggested, I would only do a "flash coat" to seal the cavity (1/4") and fill the rest with batts.  The post by Thehardway above does a good job of explaining why this is acceptable (prevent infiltration).  He's also right on the money about trying to compare R-values.  There is no comparison when you don't have to deal with infiltration loss.  I've got 1" of closed cell foam in a 500 sqft shop built inside a pretty drafty pole barn and I have no problem heating it with a simple 1500 watt baseboard heater.  I should take pictures of it and post them.  As crazy as it sounds,  the foam seems to really stiffen the structure too.  The close cell foam is so stiff a guy could just about pressure wash it if he really wanted to.

Here is the shack I built and put foam in.  It sits on a hill so we get lots of wind....
http://www.shawneeforest.com/house/default.htm

[Holmes]

One thing about spray foam insulation is it pays you back every day of the year. It always saves you money. It does not get better than that.