Hot or Cold Roof? Metal or Asphalt?
My home is a 1985 passive solar timber frame with wrap and strap construction located in southeastern MA. The roof deck is 1×6 TandG followed by a plastic sheet ->3in polyiso ->1in polyiso ->plywood-> shingles(no paper). The shingles are ready for replacement and there is known rot at the sheathing around the vent stack which was never sealed(spray Foamed). In replacing some siding (2in polyiso w/vented rain space and clapboard) I noticed that there is no tape or other air sealing. My original plan was to have the roofer install a snap lock standing seam over an underlayment on top of the shingles. I dont want to miss an opportunity while I am up there. As an aside.. During a frost I can see where the sheathing nails are(melt first) and on the gable ends it melts first as well. I assume from air leaks?
Should the roof receive more polyiso that is taped? (I assume what is up there is not) Can this go over the current sheathing or shingles?
Should there be strapping under the metal roof and vents or is it ok as a hot roof as it is currently?
I am having the roofer also extend the gable ends out 1 foot so that I can increase the polyiso under the siding in the future. Also there will be a photovoltaic array on the metal roof.
Thank You for any help I will be happy to provide more info if requested.
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Replies
T. White,
Your existing roof is evidently showing several signs of heat loss, air leakage, and damage. At a minimum, your roofer needs to repair the existing sheathing rot.
Four inches of polyiso has an R-value of only R-26. Code requirements for roof insulation now require at least R-38 in your climate zone -- at least 6 inches of polyiso. So your roof could use at least 2 more inches of foam.
My own advice: strip the shingles, repair the sheathing rot, and use canned foam and high quality tape to fix any obvious air leaks. Then add 2 more inches of polyiso with taped seams, followed by 2x4s installed from soffit to ridge for ventilation and a new layer of plywood on top.
question for Martin and others:
There seems to be quite a few posts all asking nearly the same questions. Whether its roof retrofits or new construction; hot roof vs cold roof, vent vs non vent, exterior insulation vs none......etc etc etc, all seem to be the main topics with very similar results. One thing I have yet to see a concern on in any of these recommendations is added weights. It seems the go to answer is add more insulation, add vent channels, add more sheathing, etc. All of these add weight. Chances are, the roof can take the extra few founds a square foot added to it. However on the other hand, there has been projects I have been a part of where the roof is clearly already maxed out to get the most span out of the least amount of material.
Has anyone found/considered the structural integrity of the roof to ever be a concern? I've seen code designed roofs fail under snow loads or even wind loads. I am not completely convinced that existing roofs should always be assumed to be capable of having (it seems in most cases) an entire 2nd layer of roof applied to it. Just some thoughts.......
Jesse,
Good point. Many older homes have undersized rafters. If major upgrades to the roof system are planned, it's always a good idea to consult an engineer to verify whether the existing rafters need beefing up.
Since T. White's home was built in 1985, I imagine that the rafters can take an additional 2 inches of polyiso plus another layer of plywood. But you never know, so it never hurts to check with an engineer.
All good points thank you.
Unfortunately it does not look like we can afford to add increased insulation... Payoff looks to be over 35 years and we have other issues to address. So for this situation what is suggested? Should we add the 2x4's over the existing roof and create a ventilation space?
Much Thanks
T
T. White,
Q. "Should we add the 2x4's over the existing roof and create a ventilation space?"
A. Yes.