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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 - 11/3/2009
Question
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
---------- FOLLOW-UP ----------
QUESTION: Hey Brad-
Thanks for taking my questions. I wasn't clear but, what I'm really trying to accomplish is to add insulation to my roof. That's why I propose the "system" I asked you about. What do you think of it and are my concerns valid?
I don't understand why you are asking me questions about water vapor. Water vapor is in a home due to cooking, showers, breathing, and vapor pressure based on weather conditions. That's why we have air space between insulation and roof deck (and to counter ice damming. The blown in insulation is permeable. How can you ask: "How would it (water vapor) get through the insulation?
Certainly air can migrate under the roof membrane but the problem is that air has moisture in it and that moisture can build up. During the winter, there is a vapor pressure drive which forces water vapor into the roof system. This drive will cause water to accumulate under the membrane.
You say you "don't like the idea of insulating all the space between the ceiling and roof"...why not? Wouldn't this be considered a warm roof assembly? Isn't that the idea of insulating since heat goes to cold....to "not let any heat out". Why would I want to let heat out in the winter time? In the summer, with the added polyiso insulation I would have a lot more insulation than I have now. Not trying to be a pain here I just want to get it right. Do you think the system will work?
Thanks
Dan
Answer
I was talking about water vapor because you brought it up first. You specifically made the statement, "I'm concerned about water vapor coming up through the house and having no where to go once it gets into the insulated space."
So to address your concerns about water vapor I need to talk about water vapor.
Water vapor is not just in a home. It is all around you. It is the water in the air which is measured as "humidity".
Insulation works by trapping small parcels of air in the insulation. Yes, insulation is not airtight and air can slowly migrate through the insulation but it is a much smaller rate than if you had no insulation. So again, how does air from the house get into the attic? Do you have holes in the ceiling? How would it get through the insulation since one of the ways insulation works is by blocking air movement.
You said, "Certainly air can migrate under the roof membrane but the problem is that air has moisture in it and that moisture can build up. During the winter, there is a vapor pressure drive which forces water vapor into the roof system. This drive will cause water to accumulate under the membrane."
Where did you get all that?
What happens is that all air has moisture. When you cool that air down you get to what's called the "dew point". That is the temperature at which that parcel of air with that quantity of humidity will make the moisture come out of the air as water. The lower the humidity in the air the more you have to cool down the air to make it happen. This is called condensation. It's what happens when you put an ice cold drink on the counter on a humid day. The condensation on the outside of the glass is not the water that was in the glass. It's what came out of the air.
If you get humid air inside the attic, and then cool it down overnight, you wind up with condensation depositing on the wood in the attic. If you don't keep a good airflow in the attic the humid air will be trapped in there. You want to bring in dryer air that will not deposit water or dryer air that will dry up the moisture from the other air that was there earlier.
The reason I don't like insulation all the space between the roof and the ceiling is that you have left no place for air to move. In the summer you will get hot just under the roof and you can't bring in cooler air. The hotter everything is above the ceiling the hotter the house will be and more you have to run your A/C. Picture the difference between holding a heat barrier on your head in the sun and holding a parasol so it can breathe underneath. You will feel cooler with the parasol.
Insulation does not stop heat. It slows down the rate at which heat will pass through. You can’t keep pipes from freezing by insulating them. All you’re doing is slowing down the rate at which it WILL freeze. In the summertime, if you have the top of the insulation very hot that heat will push down into the house. You want to pass air over it to keep it cool. In the wintertime you might get an advantage but most people have bigger problems keeping their house cool than heating it.
To finalize, your system will work but is way overkill and I think you're setting yourself up for moisture problems in the summer when the air is much more humid.
You can get minimum heat loss with Iso board on top of insulation or get the same results with just more insulation. You also want the barrier to be the smallest envelope possible so that means stopping the heat/cold loss right at the ceiling. Setting up another barrier farther out is a waste. Put your money in increasing the barrier closer in to the living space.
Brad
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