Flat roof attic insulation
![](https://secure.gravatar.com/avatar/6ac92a8e40873c233240beac2d03d643?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fn.png&r=g)
Hi,
My home was built in the late 1890s and has a flat roof. It has aluminium siding and it has two floors and basement. On the second floor, we have sheetrock ceiling. There is an older plaster and lath ceiling on top of the sheetrock. There is about a foot of space between the ceiling sheetrock and plaster ceiling. In the attic, there’s no insulation at all. On the edges, you can see the stud cavities that run down to the second floor. You can only see down to the second floor and not the first floor. So not sure if it is called balloon framing.
I wanted to know how I can insulate the attic. It is nearly impossible to air seal since there are open cavities and there’s open space between the two ceilings. In the attic, you can see there are wholes in the plaster that fell on the sheetrock ceiling in some places.
Any suggestions on how I can start to insulate the attic?
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
Do you know if you have insulation on top of the roof sheathing, as in rigid foam, or spray closed cell foam on top of the roof? If not, probably the easiest is to remove existing roofing, install rigid foam and re-roof, or you could remove all ceiling drywall and and plaster so you can insulate from underneath, at that point, with a foot of attic space you need to insulate with closed cell against the roof decking (how much depends on your climate zone), and maybe some permeable insulation.
That still leaves the walls, "On the edges, you can see the stud cavities that run down to the second floor". Are walls insulated?
Balloon framing usually has wall cavites that extend unobstructed from the top of the foundation into the attic. That has a significant stack-effect drive, and it can pull a LOT of moisture into a cold attic in winter. It's also a fire hazard, essentially a flue for spreading fire upward between floors.
Addressing those issues is necessary prior to insulating the attic, unless the attic is being insulated above the roof deck.
In most cases it's worth blocking off the wall cavities at both the attic floor and basement, and filling them with insulation too. But in some instances that's risky, so we need to know:
What is your location or climate zone?
What type of siding?
Is there plank sheathing between the studs & siding? If yes, is there tar-paper or rosin paper between the sheathing & siding?
Is there any knob & tube wiring in any of the stud bays?
Thank you for your replies.
Armando: The walls are not insulated. And the roof sheathing is not insulated.
Dana: From the attic you can only see down to the second flloor and not the first floor. So not sure if it is considered balloon framing. I'm in zone 4 climate. In NJ. I have aluminum siding. There is plank sheathing between the studs and the siding. There does seem to be some sort of paper on the inside side of the house after the plank. And. no, there's no knob and tube wiring.
Thank you for your help.
It's probably going to be pretty safe to insulate the wall cavities, given the description. Aluminum siding meets the code definition of "vented cladding", and provides quite a bit of drying capacity toward the exterior. The places where it may be an issue with antiques is window flashing, which was mostly non-existent at the turn of the last century. If there are deep roof overhangs protecting the windows from bulk water intrusion that too is a mitigating factor.
Have an insulation pro look it over, and voice your concerns about window flashing- don't assume they'll bring it up un-prompted. But it's probably worth insulating the walls first. If they insulate the walls from up in the the attic means they can be dense-packed without drilling, and the insulation will stop where it's blocked by window headers. That will block the vast majority of the air transported moisture from entering the attic. Window flashing doesn't matter nearly as much if the cavities at the sides and below the window are still open, which gives minor bulk water incursions space to dry into, if it's determined that it would be insulate those cavities without improving the flashing first.
Michael,
Follow Dana's advice -- in most cases, you'll want to install dense-packed cellulose in your walls. That will stop most of the air movement up your stud cavities (and lower your energy bill).
You have two air spaces in your roof assembly: (1) The air space between your drywall ceiling and your plaster ceiling, and (2) the air space between your plaster ceiling and your roof sheathing (that is, your attic).
You didn't tell us how high your attic is, but it is evidently high enough for human access. (That's a good thing.)
There are lots of ways to proceed with your ceiling insulation. If this were my house, I would rip down the drywall ceiling and restore the ceiling height to its former glory -- who wants a low ceiling? You want the ceiling height to be where it used to be (at the plaster level).
Armando explained, correctly, that the best way to insulate a flat roof is with exterior rigid foam. But if your attic is high enough, you may be able to install insulation on your attic floor. For more information, see this article: Insulating Low-Slope Residential Roofs.
Thank you Martin, Dana and Armando.
Martin, the article was very informative. I do have a sloped roof. The front of the attic is about 3 feet and gradually it lowers to 1.5 feet in the back. I forgot to mention that the attic does vent to the outside on the roof. Hope this doesn't change any information you provided.
Michael,
The advice in the article I linked to -- Insulating Low-Slope Residential Roofs -- applies to attics like yours. Ideally, you would need a higher attic (one higher than 18 inches at the low side) if you want to take a "vented attic" approach (as the article notes).
You didn't tell us where you live, so we don't know how much insulation you'll need.