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l8rg8r

09:22PM | 05/18/04
Member Since: 05/18/04
5 lifetime posts
Bvbasement
I need to insulate overhead in a 720sf unfinished living space addition. It must be done from the inside, as it is already roofed. It has a flat roof, with 1/2" roof sheathing nailed directly on top of the 2 x 10 ceiling joists. The spaces are 9" deep on 16" centers. It is divided into a bedroom, a living room, a kitchen and two 3/4 bathrooms. The insulation was already purchased some time ago and cannot be returned or exchanged. It is in bundles of individual unfaced fiberglass bats, about 6 feet long. There is no drywall up, all bare framing. I was planning on stapling 4-6 mil plastic sheeting on the living space side of the ceiling joists as a vapor barrier, once I figured out how to keep the bats in place while I did so. Any suggestions on the best way to do this and get the best results?

Also, there are 2 x 10 cross-braces about 1/2 way down the run of the ceiling joists, with vent three 2-1/2" holes drilled through each of them. There are also three 2-1/2" vent holes drilled in the overhang freeze blocks to the outside and the freeze blocks at the house. Should these be left open, or closed off? Thanks.


Piffin

06:35AM | 05/22/04
Member Since: 11/06/02
1284 lifetime posts
Staple screen over the holes to stop insects from moving in with you.

You do not mention the depth of the fibreglass batts or your climate but I doubt they will be adequate. r30 is the minimum standard now and more in many places. If you have nine inch thich insulation, that may meet the requirement but then you will not have room for ventilation.

Other than that, what you propose is good. There are wire snap in keepers for loose FG batts in joist spaces.

Here is how you could improve on things though,

after installing the batts, nail up inch thick thermax or other foil faced foam and tape the seams. then run 1x3 or 1x4 straping across it perpendicular to the joists, then your rock. The foam is R-7 or thereabouts added to the ceiling and it provides some resistance to vapour transmission.

Excellence is its own reward!


l8rg8r

06:08PM | 05/22/04
Member Since: 05/18/04
5 lifetime posts
Thanks. We are in the middle of the Sacramento Valley (CA) with very warm summers and normal winters. I had already stapled some screen over the outside vents for bugs, but hadn't thought about adding the foam board inside. If I ran the batts perpendicular to the joists, I think the expanded batts would have enough "breathing room. If not, I will add some furring strips. I think the batts that are already here are just under 9" and rated R30, which was the required, but with the (almost) flat roof, some extra insulation for summer sure wouldn't hurt.

l8rg8r

06:11PM | 05/22/04
Member Since: 05/18/04
5 lifetime posts
Make that: If I ran the foam board (NOT batts) perpendicular to the joists, I think the expanded batts would have enough "breathing" room.

BV002023

04:38PM | 09/05/13
HELP!! We live in a 1962 condo townhouse with a flat roof in Burbank CA. Our west-facing bedroom has a gyprock ceiling and above it there is not more than 10" of space between the gyprock and the roofing underlay. Having seen the conditions when I installed a ceiling fan, any insulation installed when the structure was built is now collapsed and ineffective. As a result, the room is intolerably hot in the months of August, September and October each year. There are 2" X braces partway down the space between the joists at 16" centers, plus some wiring conduits running parallel with and across the joists. Can I install a 'blown foam' or other material to get me a maximum of insulation in those spaces with minimal destruction of the ceiling? Any suggestions?
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