Insulation in side attics and dormers
I can’t seem to find consistent and clear info on what I need to do with insulating the side attics in my 1940s cape. It is the classic design with knee walls in the upstairs rooms and storage access in the side attics. Because of this I am inclined to insulate under the roof slope as opposed to the knee walls and side attic flooring. Access under the flooring is also an issue (requring the sub floor to be removed). The house is masonary and I’m not sure how the exterior wall and trusses are joined (I’ve not gotten a good loook yet). I am in central Virginia and this side of the house gets direct sun (SSW orientation).
So the big question is; how do I properly provide for the following:
1. What is the best/easiest way to provide an air barrier at the eaves? The soffits are not vented, so do I need to provide baffles and venting under the roof deck to prevent moisture problems?
2. vapor barrier – I assume I could use fiberglass bats in the trusses with the kraft backing facing the interior. But I have seen some info indicating I would also need to install EPS or similar foamboard insulation as a vapor barrier.
Any other guidance?
thanks, Jake
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Replies
If it's a 1940s cape with kneewalls, you don't likely have roof trusses but rafters.
If the attic kneewall spaces are used for storage, then you'll have to insulate along the roof slope down to the eaves and create an air barrier at the rafters. This can be done with spray foam or rigid foam cut to fit between rafters and foamed in place with a foam gun. Whether there's also room for a vent channel depends on the depth of the rafters, but this would require installing continuous soffit and ridge vents.
Jake,
Robert's advice is good. Whatever you do, don't install fiberglass batts.
To do a good job, you will need to remove enough of the subflooring to see what is going on at the perimeter of the second floor. You need to have continuity between the thermal barrier installed in your walls and the thermal barrier you are about to install in your roof plane. You also need air barrier continuity, which can be achieved with spray foam or rigid foam and caulk.
Robert and Martin, thanks for the replies. After crawling around a bit here is what I know:
Regarding thermal barrier, I think it is essentailly non-existent. It is all brick on the first floor and I am virtually positive that there isn't any insulation between the plaster walls and the brick over block exterior. In some places a gap is visible between the exterior masonry and the interior plaster (likely from the lath), but I don't see evidence of insulation and the total width of the exerior walls would indicate no insulation.
There is a a large open space in the eaves (behind the soffit/fascia) but I was not able to see whether there is any blocking or other barriers under the flooring between joists. From what I can tell it doesn't appear there is. The original owners had old rock wool batts put in the rafters which are full 8" deep, though many of the batts have fallen out or never were installed. Above the knee walls the batts are in place up to the attic. No venting currently exists, either along the ridge or under the soffit. There are gable vents at either end of the main attic. So I'm wondering where this leaves me if there is truly no thermal barrier in the first floor walls.
Jake,
Even if your house lacks wall insulation, it still makes sense to improve the insulation in your roof.
If you choose to install new insulation between your rafters, do the best job you can of filling the rafter bays right down to the level of your subfloor. Do a good job of air sealing at the perimeter of your second floor.
It's up to you to decide whether or not to install ventilation in your rafter bays. The ventilation decision depends partly on the type of insulation you will be using.
I have a very similar situation at my house which is a 1 1/2 story cape that has a flat ceiling on the upper floor. I have been trying to determine the best way to insulate the area of the roof above the knee walls since there is very little access. The house has gable vents but no soffit. Can spray foam be an effective option there or should batts be installed that lay on top of ceiling?
Jason,
If you intend to install insulation in your sloped rafter bays, you can use either dense-packed cellulose (assuming you can install site-built vent chutes between the insulation and the roof sheathing) or closed-cell spray polyurethane foam. Fiberglass batts should not be used to insulate rafter bays.
The flat ceiling above your second floor can be insulated with fiberglass batts, although almost any other insulation would perform better.
Jake, you mentioned that there is a gap between the exterior masonry and the wall. Do not insulate right up to the masonry. You need to leave a gap so the masonry can breath and dry. Good luck.
- Jim
Thanks, Martin. I just wasn't sure how that transition would work where the top of the knee walls meet the rafters if spray foam is used.
Jason,
Ideally, the insulation that you install between your rafters will extend past the kneewall, right down to the floor.
If you decide instead to insulate your kneewall, you introduce a new problem: the need to install airtight blocking between the floor joists under the kneewall. This work is difficult in a retrofit situation.
Whatever approach you use, you need to be sure that you have established both a thermal barrier and an air barrier. Spray foam insulation can perform both roles. If your thermal barrier is at the kneewall, however, air barrier details can be tricky, especially if the kneewall is insulated with fiberglass batts and if the kneewall lacks a top plate.
Martin, can you explain why fiberglass batts are not a good option for the rafter bays whereas cellulose is? Is it purely a "green" issue, or is there an actual performance issue or something else? With cellulose, how is it held in place in the bays?
thanks, Jake
Jake,
Fiberglass batts do nothing to stop air movement, and it's the movement of warm, moist air due to the stack effect that causes rot at the roof plane. Cellulose, if installed at a minimum 3.5 pcf density, is highly resistant to air movement and can absorb, redistribute and release incidental moisture.
To install dense-pack cellulose requires a closed cavity, so you'll have to seal the bottom and top of the slant ceiling - or seal the bottom, blow cellulose in the slant from above and then fill the upper flat area to a sufficient depth.
Jake,
Fiberglass batts are the worst possible insulation to use in rafter bays because, of all insulation types, fiberglass is the most permeable to air. If interior air gets into the rafter bays, the fiberglass batts do nothing to prevent the warm, moist air from contacting the cold roof sheathing. That's bad.
Dense-packed cellulose does a much better job of slowing air movement than fiberglass batts.
If you want to use cellulose in your rafter bays, you must first install site-built ventilation chutes in each rafter bay, using 1"x1" sticks in the upper corners of the bays and thin plywood or other stiff material. Then cellulose can be installed in a variety of ways:
-- blown through holes in drywall;
-- blown through holes in polyisocyanurate sheets;
-- blown through stapled air-permeable netting.
Thanks, while this means more work I'm glad I did the research to find out the best solution for my situation. And the cellulose solves the problem of insulating the space between the roof deck and the sloped interior ceiling up to the main attic space.
The key issue is, the water vapor that is created in the home by washing, cooking, breathing, sweating.
When this rises into a cold roof, you get condensation on the wood, if there is a steady amount of ventilation this will dry out and blow away, if it does not dry out you can have damp, mold and wood rot.
If its freezing outside and you do have water vapor entering the roof, a careful study of the roof will show signs of frost and small frost caps on exposed nails.
If it is not freezing, then the purchase of a weather centre will show both temperature and relative humidity, these are the figures you need to establish, if indeed you have high humidity and a risk of condensation in the roof.