Insulate a low-sloped roof
user-7228064
| Posted in General Questions on
My wife and I have a mid century home in San Diego. The low sloped roof has no attic. It is constructed of 2×8 tongue and groove boards over beams. The roofing is a torch down on a layer of plywood.
The house gets warm in the summer (no AC yet) and cold in the winter (okay cold for San Diego).
We would like to add insulation to the roof and provide enough space for AC ducting. We have also considered increasing the roof pitch enough to use better roofing materials.
Thanks August
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Replies
Do you happen to know the specific roof pitch? Does the roof have parapets? These details affect the workability of some suggestions.
Personally, (presuming the beams are currently exposed) I think it would be a shame to cover up the original beams and T&G boards, especially since you are presumably replacing it with boring old flat white drywall ceilings that are in every building. If nothing else, know that this stranger on the internet will judge you harshly and morn the loss of another piece of architectural history.
To that end, I advise that you put your ducts elsewhere (or use ductless mini-splits), and put a layer of closed cell spray foam over the top of the roof membrane. All the spray foam would need after that is a coating to prevent UV damage. This approach often has local energy incentives.
If the tongue and groove forms the ceiling and covers the “beams”, then I’d think this is a candidate for closed cell spray foam, which eliminates the need to try to ventilate the low pitch roof. You won’t have enough space remaining for ductwork if the rafters are only 2x8s though.
If it’s a very high ceiling, you might consider using heave round duct (this is NOT what the box stores carry), paint it and leave it exposed under the tongue and grove ceiling. Lots of loft-style places have ductwork installed in this way and used as an architectural feature.
Bill
@Aedi - I believe the house has a 6% slope and the garage is 4%. Most of the house is gabled but in the front room (living room) the slope continues up and there are windows below the ceiling. It kind of looks like the bridge of a cruise ship. These windows are glazed with obscure glass and overlook the garage roof.
I agree with you about covering the T&G and the beams. We initially wanted someone to build a new roof above our existing roof but with a steeper slope.
August,
Here are links to two relevant articles:
"How to Install Rigid Foam On Top of Roof Sheathing"
"Insulating Low-Slope Residential Roofs"
Installing rigid foam above the existing roof sheathing is done all the time with low-sloped roofs. I'm not sure where you're going to install your ducts, however. My advice is the same as Aedi's: Consider installing ductless minsiplits. That solves your duct problem.