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You are here: Experts > Home/Garden > Home Improvement/Repair > Roofing > Adding roof top insulation on Low Slope
Roofing - Adding roof top insulation on Low Slope
Expert: Brad Zacharia - 10/29/2009
Question
Hello Brad - I live Michigan zone 5 in a 1 1/2 story home with a shed dormer low slope roof on the back. That section has pot vents but no soffit vents. I want to remove the vents, fill the space between ceiling and roof deck with blown-in insulation. I would leave the existing BUR on top of the wood roof deck but add 2 1/2" of polyiso and 1/2" of Celotex fiberboard then top it off with either EPDM or PVC membrane. I'm concerned about water vapor coming up through the house and having no where to go once it gets into the insulated space. With the PVC system I'm concerned about condensation accumulating under the membrane (if mechanically fastened) in the summertime due to temperature differentials, and water vapor accumulating under the roof membrane caused by air pressure from the outside because they want to vent the membrane. With the EPDM system it would be adhered so the air pressure is not a factor but condensation could still happen I think. Could you please give me some guidance here? Thanks a lot.
Answer
Why would water vapor come up through the house? Why is your house so full of water vapor? How would it get through the insulation? The temps on both sides of the membrane will be almost the same. If air can migrate under the membrane then it can migrate back out again (just like ventilation). I don't like the idea of insulating all the space between the ceiling and the roof. You will not be letting any heat out.
Brad
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