Join the MRA | Member Login
Find a Professional
Sign Up Log In Add New Topic

TOPIC: Ventilation

ventilation/vapor barrier on remodelling

Return to Ventilation

Return to Archive

Return to Forum

I'm planning to replace my shingle roof on my post and beam 4-12 pitch cathedral ceiling log home in Vermont. What's there now is 2X6 t&g decking with foil wrapped two inch insulation and asphalt shingles right on top of that. No vapor barriers or other materials, just the foam and the shingles. Bad ice dams. I'd like to increase insulation and reduce ice dams. I have been told to carefully remove shingles, spray foam in joints of insulation, add two more inches of 2 inch foil wrapped insulation and seal joints, 1/2 in. plywood decking, then standing seam. Can I get away with this or do I have to remove all insulation to get vapor barrier next to deck? I was told the foil is the vapor barrier already so long as I seal the joints. Do you agree with this? Do I also need a vent space with this configuration? Thanks, CT
Guest User

4/12/2004
It is good that you recognize the problems and are going to address them. I would strongly suggest that you not just attempt to reduce the ice dams but elliminate them. It sounds like the log home is quite valuable and bringing the roof up to spec will increase its life and your comfort while reducing heating bills. The 2" of insulation probably provides at best an R6 per inch which is R12. Modern homes should be closer to R40 in your area which means adding another 4" of foam. Adding the sprayed in place foam over the existing 2" should provide you with your interior air barrier as it has a very low perm rating. Do not shave the exterior after it opens up the cells. By adding 4", the existing shingles are far enough into the assembly that they could be left in place as they will be past the dew point, provided you had a proper foam application. This would provide protection during constrction and help reduce costs. I would consider strapping the roof vertically with 2x4 on edge screwed down through into the deck. They make deck tightscrews for this application. Make sure the strapping is over the rafters and no more than 24" on center. Then they can foam in place to the top of the strapping. You will probably have to raise any skylights. Then you are free to strapp the roof horizontal with minimum 2x2. If you are installing a vertical rib panel make sure it has large enough flutes to move air vertically over the strapping and check with the manufacturer on the required strapping spacing for proper support. I would still recommend using a reinforced underlay over the strapping directly under the roof covering. Use house wrap tape to seal around all penetratons. Then build in eaves and ridge ventilation and choose an energy star coating for best reflectance on the sun. Hope this helps.
Allan Reid
Dura-Loc Roofing Systems, Inc.

4/13/2004
Thanks for your reply, Allan. Just a few questions: First, regarding the vapor barrier. I was told I have to worry about the moist air from inside the dwelling getting up into the insulation and creating moisture, and that is why a vapor barrier should be next to the 2X6 t&g decking. Are you saying that spraying a foam insulation directly over the existing asphalt shingles will adequately seal up air flow so that no moist air will condense on the decking, which is the top side of the ceiling we can see from upstairs? Next question: are there different kinds of spray in place foams and which one do I want? Next question: What do you mean by "do not shave the exterior after it opens up the cells" ? Next question: One of the roofers looked at a 3-1/2 inch PVC plumbing vent down low on this roof and suggested we move it up higher to protect it from sliding ice and snow. This would create an 8 foot or so channel in the insulation and even if we foamed it in, I'm sure there would be a lot of heat loss there. What do you think of this? And finally: What is the purpose of the underlay over the strapping and just under the roof? Thanks for your input on this. CT
Guest User

4/15/2004
While the code calls it a vapour retarder the primary function of the interior barrier is to stop the air and the material should be a very low perm rated such as a 10 mil poly. The foam layer if not cut will serve this purpose. It is however contingent in applyoing it thick enough such that it is on the interior 2/3 of the insulation assembly and you must seal everything so that the air does not find a passage. I had the same situation on a home I purchased 18 yrs ago with the shingles nailed down through an insul board over 2x10 vertical cedar ceiling. I strapped the roof vertically with 2x4 at 24" centres, upgraded the wiring from the outside (New ceiling speakers as well) and then filled the cavity with sprayed in place. Then installed Triflex 30 reinforced underlayment and strapped it horizontally with 2x2 and had a Dura-loc tile roof installed so I speak from permenant experience. The purpose of the exterior barrier is to allow the air to flow through towards the inside while trapping any moisture. This is great for condensation and incases of severeweather acts as a secondary membrane. Whenever possible have the vent pipes moved up through the top of the roof. If you choose a smoothe roof it will shed snow and ice and can shear pipes off although in this case it sounds like it is anchored very well. I believe the foam is all the same these days and they use non hazardess foaming agents. They can ajust the agent to give denser foam and it does change the R value. Choose a reputable contractor and get referances and check them out.
Allan Reid
Dura-Loc Roofing Systems, Inc.

4/20/2004
i have a cabin in northern wi, and am in the process of trying to figure out how to eliminate this water damage every couple of years, first how can i tell if i have adaquate ventilation in a cathedral ceiling... im tearing out the drywall in my ceiling and noticed i have paperback insulation, and poly over that.would that possibly be causing moisture build-up having 2 vapor barriors and is there a number you can call to talk to a human being...its really hard to tell you every thing i need to say.......thanks again
Guest User

4/30/2004
Rosann, I have over 20 years experience in the production and installation of metal roofing. Feel free to call me at 1-800-543-8938 ext 201. Todd Miller [email protected]
Guest User

4/30/2004
Guest User

5/30/2004