[franksandbeans]

I have a cape cod style home in Sacramento california (read: hot). I've insulated the kneewalls with rigid foam board and am moving upwards toward the attic. How do I insulate the sloped ceiling between the kneewall and the attic floor? Its basically just a cavity between the underside of the roof deck and the drywall of the room. There's some compressed batt insulation in there now......but inside the room that sloped part of the ceiling gets 90+ degrees so I don't think that insulation is doing much. The rafters are only 2x4, so theres not a lot of space. I have limited access from below and above . I was thinking of sliding a piece of rigid foam in there and then filling the cavity with cellulose. I would leave a direct path for the soffit vents to the ridge vent. How important is ventilation in my climate? Any advice?

 

[Bud9051]

Your shingles will be reaching 150° plus and 3.5" of insulation will fall short at keeping that heat out of the house, especially when you leave 1" (minimum for ventilation) open below the roof.

If the fiberglass insulation is currently stuffed into the rafter cavity on the slope is is limiting your ventilation while still allowing moisture to flow into those spaces. IMO, either go "hot roof" (sealed with no ventilation), or remove the drywall below those sloped rafters and pad them out to allow more insulation.

A third option would be to add 4" of rigid insulation above the roof deck and new shingles. Works best if you already need new shingles.

A "hot roof" option might work as a hybrid, leaving the current upper vents in place for the top and converting the lower side attics to sealed eliminating the soffit venting. Many more details needed so just tossing out ideas. Pictures would help.

Bud

Forgot to say, welcome to the forum.

 

[franksandbeans]

This is the rafter cavity looking down from the attic. The opening is 3.5 x 22

This is looking up from the kneewall crawlspace

And this is with a 1.5" polyiso form board slid into the cavity

I know that without the board being sealed properly in the cavity I'm losing a lot of the insulative value. That's why I was considering closing these off and filling them with cellulose (with the boards inside).

Thats more in line with a hot roof plan. The kneewall crawlspaces can easily be closed off (foam boarding the underside of the roof deck) and the same can be done in the attic.

As far as venting is concerned....these cavities were all mostly filled with batts beforehand with no dedicated air channel from the soffits to the ridge vent, so I'm not sure I'll be losing much venting if I fully close them off.

 

[Bud9051]

The article in the link below talks about dense packing with cellulose. Not exactly your application, but similar.

I have always preferred the traditional vented attic approach but with a cape the limited space on the slopes just doesn't provide enough room for insulation and ventilation. Especially with 2x4 rafters.
http://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls

Bud

 

[Gary in WA]

Welcome to the forums!

With cellulose, you may need to thicken the rafters to prevent condensation; http://www.nationalfiber.com/docs/HygroscopicPropertiesofCelluloseInsulationinSuperInsulated

And with SPF, you may need a continuous no perm vapor barrier under the shingles...

Gary