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Still struggling with my bonus room…

DavidfromPNW | Posted in Energy Efficiency and Durability on

Hi all, 

Well, I am still struggling with my bonus room insulation strategy. I plan to insulate the slope down to the soffit so I don’t have to worry about the knee wall issues. From reading the “Two ways to insulate a knee wall” article on fine homebuilding  (, it says I need “rigid insulation” to seal the underside of the rafter on the slope. 

I’ve already used 1/2″ polyiso foam board to build my “site built” baffles per the instructions here : . I’ve air sealed them. I’ve run them all the way up into the attic and the air is really flowing nicely. So, given that the polyiso is basically water impermeable, doesn’t that eliminate the option of using foam board as an air seal on insulation on the inside (underside of the rafter)?  And, my bigger question, If I am also insulating and drywalling the knee wall, why do I need to air seal the underside of the rafter? 

Additionally, The floor of the bonus room is insulated all the way through so there is no open cavities. I suspect, because I insulated the floor thus limiting any airflow into the devil’s triangle, I don’t need to seal the underside of the rafters. Any insight? If you think I do, any alternatives to rigid so I don’t get into a situation where I have my batts sandwiched in between to vater impermeable products. Given my truss web design, it would need to be a flexible material. I researched Tyvek FireCurb but can’t get it in the US for some reason. Sorry for asking so many questions, I’m just trying to get this right the first time. 

Thank you, 


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  1. BarnyardBuilder | | #1

    I cant help but have a very similar project and look forward to answers

  2. GBA Editor
    Martin Holladay | | #2

    You're right -- you don't necessarily need rigid foam on the interior side of the rafters. Assuming you've decided to go with the vented approach, you need:

    1. Ventilation channels from soffits to ridge -- and it sounds like you have that. Airtight baffles are good.

    2. Adequate insulation to meet your R-value goals -- not sure whether you have this or not, but it's your business.

    3. An airtight ceiling to prevent air leakage.

    For more information on different ways to achieve these goals, see this article: "How to Build an Insulated Cathedral Ceiling."

    1. DavidfromPNW | | #3


      Thank you so much.

      1. Yes, I have ventilation channels from soffit to ridge and I have air seals them as well.

      2. Yes, I am at r49 in the rafters and using a standard banding strap from dormer to dormer to hold them up permanently. I have also put r49 in the floor joists just below it. R30 in the 2x10 floor joist and r19 on top in a cross pattern.

      3. The plan if for an airtight assembly. I've already got the ceiling drywall in which I glued and screwed, then caulked the perimeter. I've removed the super slope cans from the sloped ceiling in the stairwell and I am using all single assembly LED lights in the flat ceiling with rubber gaskets. The walls will be the challenge. The truss webs at the dormers are too oddball to accommodate anything other than polyiso, so they got 3.5 R22 poliyiso cut to size and locked in with spray foam.

      Thank you so much. I've been stressing this and it just didn't make a ton of sense. I appreciate the information.


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