Cape Cod knee wall/roof insulation questions

Old 09-28-14, 02:47 PM
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Cape Cod knee wall/roof insulation questions

I have a large Cape Cod style home (2nd floor knee walls are >4 feet high). About 2/3 of the house I can access the knee walls from the soffit space (4 feet horizontal and vertical space). After last year's hard winter I decided we definitely had insulation problems, so I ripped the soffits off and found:
- floor joists were only haphazardly blocked with squares of fiberglass puffy wall insulation
- Air space allowed air from soffits to blow down behind brick into first floor wall
- Only wall insulation was between-stud fiberlgass put on behind walls -- no vapor barrier or external board at all. Basically when the wind blew, the house was uninsulated!
- No venting from lower space into upper attic; rafters above knee walls filled with fiberglass roll insulation
- Two gables had cellulose blown in under floor between them and soffits...which settled onto soffits...leaving them uninsulated

For a 1992 constructed house, IMHO the contractor should lose their license.

Anyways, what I have almost completed:
-Foam board on all knee walls and gables, sealed with expanding foam at all joints/air leaks
- Foam board cut for floor joists and foam sealed.
- Foam board cut and expanding foam used to eliminate air leak into 1st floor walls
- Fiberglass insulation stapled up under gable floors, boxed in by foam board and sealed with expanding foam

Two questions to which I urgently need answers:
1) Venting to the upper attic -- I have tried and can slide plastic vents between the insulation and the roof plywood to connect upper/lower attic. Should I:
a) Install vents on a subset of rafter spaces and seal off the rest at the bottom with foam board sealed with expanding foam, or
B) Install the vents between each rafter and seal around them on the bottom end to prevent air leak into the insulation?

From what I am reading I am learning that I probably should suck it up and do B, as it will create a better thermal barrier between the living space and roof. We have had an ice dam or two in the past several years cause water leakage.

2) Do I need to cover the outside of the foam board on the knee walls with Tyvek? I don't see any vapor barrier used in this house.

Thank you in advance. I'm in there now and need to do it right the first (ONLY) time!
Old 09-28-14, 03:39 PM
Join Date: Oct 2008
Location: New England
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I live is a cape so understand some of the terms, but not entirely clear on where you are sealing. I'll try to comment here quickly and add a related link if you haven't found it.
1. No need to add Tyvek on the outside of the foam board inside a kneewall. But I would tape all seams.
2. Your local codes will determine if that rigid needs to have a fire rated barrier.
3. How thick is the rigid? In a cold climate the inside surface of the rigid needs to remain above the dew point, roughly half the total r-value. R-5 on the outside with r-13 in the wall would allow moisture penetrating the fiberglass wall to condense on the inside surface of the rigid.
4. It sounds like you air sealed between the kneewall space and the floor joists cavity under the living space, good.
5. As for venting, if I understood it correctly, yes you should install the baffles from kneewall space up to attic space.
6. You will then want sufficient soffit venting (low) and ridge or gable vents (high).

The first link looks like they combined a bunch of articles into one paper, so many topics covered, but good reading.
The second is specifically about kneewalls.

Two Ways to Insulate Attic Kneewalls - Fine Homebuilding Article


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