Flat Roof Loose Pack Cellulose
I have a really old house with a flat roof with 2 inches of foam on the exterior. I’m currently working to do some air sealing on the interior, but was looking after that to explore increasing the insulation. At the highest point of the roof space, there’s nearly 4 feet from the ceiling joists to the roof trusses. At the lowest point it’s maybe 6 inches. There’s currently maybe 3 or 4 inches of loose cellulose currently in there. The perimeter has wall vents, but from what I’ve been reading that isn’t really enough to count the roof as being vented. I’ve spent some time in the small attic space doing electrical retrofitting and I have not seen any mold or rotting.
So my question is this: It seems like dense packing cellulose would be a bad idea. Would adding loose packed cellulose also be a bad idea? There isn’t a roof replacement in the near future so adding more exterior foam isn’t an option at the moment.
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You didn't mention where the house is located - it matters. If I understand correctly, there is 2" of rigid foam on top of the roof, but the space under the roof is vented by perimeter vents? If so, the insulation above the roof isn't doing any meaningful insulating. The perimeter vents will help to "ventilate" that attic space between the ceiling and the underside of the roof, but the effectiveness is hard to predict. You did mention a few inches of cellulose between the roof rafters. That's a start. How deep are the ceiling rafters and roof trusses. As in, how far is it from the top of the ceiling surface to the underside of the roof surface? A few more details will help
It is in Chicago. It's a traditional two flat grey stone. I don't know that the perimeter vents do much. They are quite small, mostly at the higher side of the building, and seems to have had some of them blocked over by past tuckpointing.
The ceiling rafters are about 6 inches deep. It varies from several feed to the underside of the ceiling to the roof to maybe 12 inches (the depth of rafter and roof truss) at the low end.
ev22,
You roof probably doesn't show signs of moisture problems because it has so little insulation it is kept warm and fairly well ventilated. If you add cellulose things will almost certainly change for the worst. This article does a good job of describing what's in play and your options:
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs?oly_enc_id=7565D0080934G5L
Yeah, it's been articles like that one that has made me more unsure about what should be done. From my reading, my roof does not have adequate ventilation. Perimeter vents might be the wrong term. There's basically a few slits through the brick periodically. So my read is that I should either make the roof airtight and have insulation on the exterior (which is cost prohibitive for me) or make better venting to add insulation to the interior. Adding more cellulose reduces the warmth and possibility for ventilation; which also seems like a problem. So I wasn't sure if there was a possible middle route of increasing at least some energy efficiency without causing other major problems. I am slowly doing projects on the various systems of the house, and roofing/insulation is pretty new to me.
ev22,
That's certainly the problem. It will be fine until it isn't - and how much change causes that to occur is hard to predict.
One thing that would definitely improve efficiency and comfort would be to remove the loose fill cellulose (or mound it up at the high end) and air-seal the ceiling as well as you can. Then maybe put a layer of batts over the ceiling joists, wait a year to see how things are, then add another.
Why batts and not loose fill cellulose? For some reason I was thinking the cellulose was better than batts in terms of some mitigation of moisture condensating though at the cost of a shorter insulation life
If your roof slopes from 48" at the highest point to 6" at the lowest point, it seems like it might not be a flat roof. (I'm assuming that your ceiling is flat).
If you know the distance over which that change in height occurs, you can work at the slope in terms of X:12, which might affect what solutions are most appropriate.