Flat Roof Under Pitched Roof: How to Insulate
LESLIE: Alright. Now we’ve got Monica on the line with a roofing question. Tell us what’s going on.
MONICA: Yes. My daughter had just purchased a home and originally, apparently the house had a flat roof. And then they put a peak roof over the top of it. I was just wondering if on that flat roof – if you should go into that space and insulate over that flat roof.
TOM: It’s a good question. So, first of all, the idea of covering a flat roof with a pitched roof is not unusual. Folks do that for a number of reasons, both architecturally because they like the appearance of the pitched roof and because they’re just tired of dealing with flat roofs’ leaks and other problems that happen more frequently as a result of them, including flat roofs’ effects on home efficiency.
Now, typically, you would have insulated inside that ceiling under the flat roof. So, therefore, there’s no reason you cannot add additional insulation over that. You could do something as easy as laying fiberglass batts right on top of that roof: side-by-side unfaced fiberglass. It would add additional insulation to that space.
MONICA: Oh, that was what I was wondering. That would help keep some of the heat from getting on the tarring of the flat roof and help cool the lower level, right?
TOM: Well, yeah. I mean it would – more importantly, it’s going to keep the heat or the coolness inside the house. So, yeah, it will help separate some of that heat. And there should also be ventilation in that new pitched roof. That will be necessary for the insulation to work well.
MONICA: I think that – the house is actually a three-layer house. Both roofs, apparently, were flat and they peaked both of them.
TOM: What you can do in that situation is you can add a ridge vent to the peak and you can add some roof vents lower on the roof and that will improve the ventilation dramatically.
Monica, thanks so much for calling us at 888-MONEY-PIT.