Home>Discussions>INSULATION & HVAC>attic insulation and wall Q-new idea?
4 posts / 0 new
Last post
attic insulation and wall Q-new idea?

Cliff notes:

Can I pin up plastic sheeting to the underside of my Rafter joists and than blow in cellulose (and use my old fiberglass batts)- will the plastic act as a vapor/moisture barrier and avoid moisture/insulation issues, or should I put rigid non permeable foam panels between my rafter joists and spray foam the joints-than put up old fiberglass batts and cellulose.
Just using the fiberglass because I'm pulling it out of vaulted ceilings and doing a "tight pack" cellulose fill and I will have a ton left over.

Longer version below.

Just bought a 1985 home with vaulted ceilings and about an R-3 of insulation in the attic-if you don't count the massive air leaks around the return air vents and anything else that goes into the attic space.
The attic floor is 2"X6", and not well insulated with blow in fiberglass- at least where there is insulation.

SO, this is the plan thus far:

Take 1" R-max foil face rigid foam insulation and adhere it to the bottom of the roof sheathing and seal with expanding foam.
Use the fiberglass batts (that are going to come out of the vaulted ceilings) and batt under the R-max rigid foam insulation, I think this will work well, but....

Could I instead place thick plastic sheeting on the roof joists (inner attic side) as a vapor barrier and than blow in cellulose up to the roof sheathing and also use insulation batts? Would the plastic sheeting keep out moisture and keep the cellulose/ used batts dry?
I believe the cellulose and fiberglass batts will keep the cold outside air insulated from the warm/moist attic air, and the plastic sheeting will keep the inner moisture aware from the cellulose and used fiberglass batts

Also, I will be pulling the fiberglass batts out of the vaulted ceiling and "tight pack" cellulose to improve R value and use the tight pack method as a vapor barrier/retardant.

As far as walls-I've gutted my master bath down to the studs-r-11 fiberglass batts-except along the top plate, where there is no insulation for the entire span of the wall from the wall plate to the floor sheathing above.
The plan is to install 2" R-max rigid foam in between the studs (I know, there's going to be thermal bridging) and spray foam around the edges of the rigid insulation for a vapor barrier and than plastic sheeting vapor barrier under the dry wall.
This will create a1.5" air space between the rigid foam and the plastic sheeting/drywall combo.

Any suggestions on my plastic sheeting with cellulose between rafter joists-seems like it would insulate well and have a nice vapor barrier while keeping costs minimal.
Also, will there be a moisture issue between plastic sheeting installed underneath drywall in a bathroom and rigid foam in the wall cavity?

Thanks all.

Re: attic insulation and wall Q-new idea?

***, your post is very confusing. I take it that you have a house with one room or one section that has vaulted ceilings and no attic and the rest of the house has flat ceilings with a conventional attic.

You want to remove the batt insulation from the vaulted ceilings and use it between the rafters in the attic portion of the house. Not a good idea unless you are going to fully insulate the attic and use it as a living space, complete with heat and air. Put the batts between the joists and leave the rafters alone. Otherwise you will end up damaging the roof and shortening the life of your shingles.

In the portion with the cathedral ceilings you want to put rigid foam right under the roof sheathing. I think you would be a lot better off by having a closed cell foam sprayed into these cavities. Using a rigid foam board under the sheathing, then dense pack cellulose under that can become a problem. If any vapor (moisture) penetrates into that cavity, it will not be able to escape. Eventually the cellulose will get wet and wet cellulose is not a good insulator.

You would be better off with batt insulation that is 2" shallower than the dimension of the rafter used in the cathedral ceiling so that there is a 2" air gap between the sheathing and the insulation. I.e, if your rafters are only 2x6 (I would hope they would be more like 2x10), then you would use 3.5" batts. Then put your 2" foam boards to the underside of the rafters and seal the seams with tape or foam spray. Then hang the sheetrock with really long screws, no nails, only screws about 4 to 4.5" long.

As for the walls, if you want to use the rigid foam boards, OK but leave the gap on the outside of the foam boards, not on the inside. The gap should be between the foam and the exterior sheathing, it will help the paint to last longer. The wall needs to breath from the outside.

Re: attic insulation and wall Q-new idea?

Keith, thanks for the reply.

The attic is comprised of mostly vaulted ceiling with about 200 sq ft out of 1550 sq ft being flat walkable suface.

I was considering making the attic a conditioned space by proxy of insulating it well so I could decrease my energy consumption heating and cooling the rest of the house

As far as shortening the life of the shingles- the roof has 28 yr old tiles (less heat sensitive than shingles) and the latest research indicates that roof life is minimally affected by utilizing a "hot" roof technique.

My question with removing batts from the rafters was aimed at people who have done tight pack cellulose in rafters-I have no vapor barrier, but could install plastic on top of the dry wall between the rafters before I blew in the insulation. The ceilings are finished and the insulation is R-30 thats been in there for 28 years- I would like to improve on this R-value without causing condensation on the roof sheathing.

The research I've done indicates that a tight pack cellulose prevents moisture from getting to the roof sheathing via being a vapor barrier/retarder and that the underside of the sheathing (for Portland Oregon climate) needs an R-10 value to prevent condensation from occurring.-The tight pack cellulose covers both of these requirements.
I would think, and some articles I have read agree, that "tight pack" cellulose would work well-I'm looking for any experienced advice from some one who has used this technique successfully or as a failure.

I would love to put some tuf-R rigid foam under the roof sheathing in the vaulted rafter spaces-but the ceiling is finished....and I'm not willing to tear down the drywall-these are big areas with many angles. The closed cell foam method would be my choice-but not at 2+$ per sq/ft, that is just a ridiculous price and will never recoupe from energy bill savings.

I thank you for the suggested install of rigid foam in walls, I've seen exterior sheathing sprayed with foam between the studs and than fiber glass placed at the dry wall side-I always thought this practice was back wards-although it would act as a vapor barrier.
I am installing the rigid foam in a bath room and I would rather not have the insulation collecting moisture between the drywall and rigid foam- although the argument that a vapor barrier will be under the drywall could be made. I have been given both as advice.

I have decided not to completely seal the underside of my roof and instead I'm going to use 5" of rigid foam under my attic walkway foam sealed, and than seal up all gaps and cracks and air infiltrations-than I'm going to cellulose over all other location to get a nice warm R value.

I'm still on the fence regarding the vaulted ceiling rafters and tight pack cellulose......:(

The research continues, I'll be improving my attic vent in the coming weeks and that will give me more time to research tight packing beyond what this manufactures article states:
apple gate insulation

Re: attic insulation and wall Q-new idea?

Read this article before you do anything. Using dense pack cellulose on cathedral ceilings is not recommended. It is recommended for a lot of things but not this and it is not a vapor barrier.


Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.