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Attic conversion insulation oprions

jsh1515 | Posted in General Questions on


I’ve inherited a project from a previous homeowner …. attic space to loft conversion. On both sides of the loft there is attic space. Keep in mind that the knee walls and loft ceiling were never finished. In place of an old gable vent a window was placed. There are no soffit vents or ridge vents. The house was built in the 50s and the insulation on the floor of the attic is batt. My questions concern how to insulate this new loft. I.e. the ceiling and knees walls. Is spray foam the way to go in the cathedral ceiling with no venting? Does this need to extend into the triangle space behind the knee walls? Is it possible to spray foam just the cathedral part in the loft and backside of the knee walls. If so what do I do with the triangle space in the absence of soffit and roof vents? Cut them in? I should also note that one side of the attic would be hard to reach if spray foaming it in becomes the only option. 


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  1. Expert Member
    BILL WICHERS | | #1

    Spray foam is usually the best option in an unvented roof assembly unless you can put all the insulation on the exterior. Closed cell spray foam is least risky too. You generally want to insulate the roof, and not the kneewalls, so that "triangle space" becomes INSIDE the building envelope. You would insulate the roof all the way out to the eaves where it would tie into the exterior walls of the building and the insulation/air barrier there.


  2. charlie_sullivan | | #2

    Here's an excellent article from a few years ago:

    Note that if you use closed cell spray foam, you can choose between using an HFC gas to blow the bubbles, which has a horrendous global warming impact, literally thousands of times worse than CO2, or and HFO blowing agent, which gives you a better performance foam, the ability to apply more foam in one pass, and it gets rid of that climate impact. But cellulose would be a still greener insulation option.

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