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Sloped ceiling insulation

user-7005570 | Posted in Energy Efficiency and Durability on

I have a story and a half home that we are remodeling. We also added a garage with bonus rooms above. We are climate zone 6 in Gillette WY. We taped all OSB seams, are adding 2” of rockboard insulation with 3/4 rain screen and Lp smartside lap siding to the exterior and used ThermalBuck to fur out our windows. We will also be using Ecoseal Plus to air seal interior framing.

To the point:
I have read extensively about how to insulate sloped ceiling spaces( We will have an unvented roof assembly. I understand the two methods of insulating attics with knee walls. Keep the attic cold or bring it indoors. Here is my question. Can I keep the attic outdoors by insulating the knee wall and floor below with dense pack and then foam the sloped portion of the ceiling inside of the knee wall until I reach the flat ceiling at which point I would go with a blow in? In this case the first and third segments of the roof would have no insulation in the rafter bay with only insulation touching the roof in the middle third of the rafter bay. I’m trying to minimize the amount of spray foam needed to have a well insulated room and a healthy roof.




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  1. GBA Editor
    Martin Holladay | | #1

    It's possible to do as you propose. That said, it is rarely executed well, and the usual result is air leakage somewhere -- air leakage that results in unusual snow melt patterns on the roof and unsatisfying performance.

    Every transition is an opportunity for air leaks. In your proposed approach, the air barrier follows a zig-zag path, and each zig and zag is tricky.

    In short, my advice is that it's much better to just insulate the sloped roofline.

    If you ignore my advice, here are the potential problem areas:

    1. Many builders do a lousy job of installing air-sealed blocking between the joists under the kneewall bottom plate. This is fussy work if done well. The cost of the work can easily eat up your imagined savings.

    2. Many builders also do a lousy job of installing air-sealed blocking between the rafters above the kneewall top plate. Same story.

    3. Remember to include enough R-value in the section of your sloped roofline that you intend to insulate. Almost all spray foam contractors try to convince customers to skimp on R-value and install less insulation than required by code, and the results are just what you'd expect: low R-values result in increased heat loss in winter, potential snow melt on the roof, and higher than expected energy bills.

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