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Old 08-05-2010, 10:58 AM  
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Default Proper way to Insulate Finished Cape Cod attic

I am in the process of remodeling the "attic" area of my 1 1/2 story typical cape cod home located in Chicago. It was built sometime in the 50's and originally the inulsation just went down the rafters from the ceiling to the knee wall and down to the floor. (No soffit vents just three roof vents)

Question is: would it be better to run the insulation from the ceiling down the rafters to where the rafters meet the outer floor joists (dead space behind knee wall) AND along the knee wall as well? Or should I follow the same method that was originally used? Does it matter?

Thanks in advance for helping (baby on way - gotta get this done!!)

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Old 08-15-2010, 04:57 PM  
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Here's a typical "Cape Cod style house" where the attic space is used as a second floor on the house, but the floor space of the attic is less than that of the main floor because of the construction of knee walls in the attic. You also have a ceiling in that attic space, giving you dead space above the ceiling as well.

I really don't see a lot of benefit in insulating the roof rafters behind the knee walls. The only potential benefit there may be in doing that is that it would cause any ice daming to form lower down on the roof, and closer to the eves. That would minimize the potential for water to get into your house if ice dams were to form.

But, if ice daming hasn't been a problem until now, (and you should have Ice and Water Shield that would prevent water penetration into your house) then there wouldn't be any benefit to be had there either.

If the dead space behind the knee walls is open to any soffit vents you have under your eves, (and you say you have three roof vents in the dead space above the ceiling), then it's important that you provide ventilation of the roof to keep it dry and prevent any wood rot.

I don't know what those styrofoam channels are called, but the idea is that you use the unavoidable heat loss through the roof insulation to drive a convective air current in those channels. That convective air current evaporates any condensation that might form on the underside of the roof sheathing and thereby keeps the roof dry. Dry wood is happy wood.

But, for there to be a convective air current in those channels, you have to have a way for cold air to get into those channels at the bottom (typically through soffit vents) and warmer air to get out at the top (typically through roof, gable or ridge vents).

So, if the kneewall dead space is open to the outside somehow, I'd install those styrofoam channels between your rafters and just insulate your knee walls as before. Then cover with 6 mil vapour barrier and drywall.

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Old 08-16-2010, 09:23 AM  
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let me clarify what nestor said about attic venting. The goal, to avoid ice dams and prolong the life of the roof, is that the attic should remain at the same temperature as the outside.

I finished an attic space in a cape, adding dormers and knee walls.
The insulation should be on the knee walls and the floor inside the attic space behind the knee walls. There should unobstructed be soffitt vents allowing air into the area behind the knee walls.

If you have sloped ceilings (roof rafters), that will hold finished drywall, they should be insulated but with an unobstructed airspace between the underside of the roof sheathing and the insulation. The foam channels can help here too.

If you have a flat ceiling you will have an "attic" above that space. there should be airflow through that space as well. gable (end) vents or a ridge vent would be appropriate.

Your goal is to have air movement from the soffit vents to the ridge or gable vents.

In some cases all this venting is NOT possible. In that case you can completely close in the area if you use the correct insulation. That would be a closed cell spray foam insulation. When doing that you would insulate the rafters all the way to the soffits. The space behind the knee walls becomes conditioned space.

When I did my cape attic finishing project I went with the method of having airspace between the insulation and the roof deck. It was a bit of a pain in the butt. At that time the foam technique wasn't widely known or used.

Good luck
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Old 08-22-2010, 09:21 AM  
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This is a current hot topic for me. I'm dealing with the same scenario, only the attic was previously finished, but very poorly done. I have added an extra room up there, finishing off the last of the attic space. I had one gable vent and a couple roof vents, one over the bathroom. Since I really have no effective venting, I'm choosing to insulate everything. In the bathroom (which is gutted now), and the new room, I'm leaning towards polystyrene boards cut to fit between the rafters with a radiant barrier and 1" gap between the roof and radiant barrier. Then I would have a layer of the same rigid foam over the rafters with the drywall over that. Spray foam is too expensive at this point, and this seems to be a good DIY alternative. I'll probably do the knee walls the same way, or with batts then covered in 2" polystyrene.

The existing insulation up there was R13 fiberglass batts over the foam channels mentioned above, but again, there are no soffit vents. The batts also stopped about 2-3 feet from the top of the rafters, so the drywall is hot to the touch above that point, on hot days, and presumably a lot of heat is lost in the winter. The knee walls are not insulated. In order to properly fix the main room, a rather large bedroom, I'd have to tear down all of the drywall, which really isn't feasible at this point, so I'm not sure what I'm going to do there.

I've been struggling with this problem since I bought this house 8 months ago, and I've been reading everything I can find on the topic. I've gotten some good advice on these boards. I think the ideal, 'money is no object' answer is to pay someone to come in and gut the whole place and use closed cell spray foam. When you're trying to do it as cheaply as possible, and planning to do everything yourself, it's a tough scenario to be in. It'll be nice to have that space back some day.
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Old 08-23-2010, 08:43 PM  
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Use some foam board vertically at the exterior wall, above and below the knee wall on the attic side (or house wrap), to prevent wind-washing of the insulation:
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