Re: Foam insulation under shingles
There's still some debate when it comes to shingles on unvented roof assemblies. There are some manufacturers that will allow their product to be used on unvented roofs. Usually those that will allow it will likely insist their premium shingles are used ( upsell ) and written confirmation from the maunfacturer for the warranty.
Originally Posted by DIYDAN
Research has been done showing the shingles may only be exposed to anywhere from 2 - to 10 degree temperature increase to the roof sheathing. The estimated impact on the shingles might equate to only a 10% decrease of lifespan --- so a 25 year asphalt would have a predicted life of 22.5 years. Despite the research some manufacturers simply ignor changing their minds.
Btw -- SIPS used for roofs aren't vented assemblies and asphalt shingles are sometimes used --- albeit with written confirmation from the manufacturer for warranty.
Since you will be tearing off the shingles you have a couple of options..........
#1 --- you could remove the existing sheathing --- sister a larger 2 by 6 or 2 by 8 to the existing rafters --- fill the rafter bay with whichever insulation you choose --- secure a 2 by 2 cleat ( or nailer ) along the new rafter --- secure to the under side of the cleat a vent channel ( could be plywood or rigid foam or .. ) all the way from the eve line to the ridge.
Then re-sheath the roof deck and leave the appropriate opening at the ridge for a ridge vent.
#2 --- based on your original tought. This system has merit and does work.
However, to appease the shingle manufacturer gods a modification may be used.
Lay the rigid foam down on the roof deck ( ensure a water barrier is on the wood decking before the foam ) completely covering the entire deck --- ensure to seal all the seams ( both horizontal and verticle ) to ensure an air tight seal --- on top of the foam secure 2 by 4's on the flat ( sleepers ) through the foam and into the rafters or plywood sheathing --- fasten new decking onto the sleepers.
This method provides a continious thermal break for the entire roof surface providing better performace than the interuppted method of insulating between the sleepers. Also, this will allow the venting under the roof sheathing to appease the shingle concern.
The R value needed is dependant on climate conditions where you live --- R 24 may not be enough but it will help with the continious thermal break.
Condensation considerations will be dependant again on the climate wher you live and the indoor relative humidity both inside and outside. For example --- with R 24 and if you experience winter temps below 45 F for extended periods and your indoor RH is high ( 60 + % ) you could end up with vapour drive from the interior out and condensing on the cold side of the sheathing. This would require the interior RH would have to be maintained at a lower level --- 40 - 50 % .
Just some thoughts.
"" an ounce of perception -- a pound of obscure "