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Couple of Cape Cod roof insulating questions

user-7478489 | Posted in Energy Efficiency and Durability on


This coming weekend I’m going to tackle my blown cellulose insulation project on a 1910 balloon framed cape cod/1.5 story house. It has a 10/12 hip roof, 2 dormers (1 in front, 1 in back), 6′ knee walls, and essentially no insulation. I plan on using blown cellulose for 2 very important reasons — none of the rafters are 24 o.c. and the knee walls were essentially framed out of pallet wood, toothpicks, whatever scrap wood this person could get their hands on. With that said, insulating the knee wall is pretty much impossible aside from spray foaming which is out of my budget, and using fiberglass batts on the roof deck would be a major PITA since the rafters are not on center. 

I’ve done a whole mess of research on this and have decided that insulating the vented roof deck is going to be my best bet. For the flat ceiling I plan on removing my turbine vents and blowing insulation from there. Shouldn’t be too difficult. However, I’m kind of hung up on what the best way to insulate the sloped ceiling/rafters would be with blown insulation. An insulation contractor in town said they typically blow the sloped ceiling from the attic side of the knee wall then quickly cover the rafter bay with blocking before it falls back down on them. Also, I’m not sure what the best way to insulate the attic side rafter bays would be. Would insulation netting be the best way to go, or are there any other products out there that would simplify this?
Not all rafter bays have have a direct path from the soffit to the turbine vents due to the hip roof, is it important that these bays also have an air gap?

In regards to the 2 dormers, is there a best practice for retrofitting insulation in the dormer “attics”? I can’t tell if that would be better done from the roof or by drilling a hole through drywall and blowing the the whole dormer roof chock-full of insulation. 

Lastly, do dropped soffits need to be insulated if they fall within the thermal envelope? Or is air sealing more important when it comes to dropped soffits?

I’d really appreciate any feedback or advice. I’ve already done quite a bit of air sealing, but my upstairs is pretty much the same temperature as the outdoor ambient temp (St. Louis = hot and humid summers). The $250 utility bills for a 1,000 sq ft house aren’t very fun either.

Any help appreciated.


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  1. GBA Editor
    Brian Pontolilo | | #1


    I'm afraid that your plan has a bunch of flaws, from the fact that your rafters don't likely have the depth for a vented, insulated roof line to the fact that you have a hip roof, which means a lot of it isn't vented and shouldn't be insulated with cellulose alone. If you haven't already, here's an article you should read:

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