Still struggling with my bonus room…
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 (https://www.finehomebuilding.com/2012/09/06/two-ways-to-insulate-attic-kneewalls), 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 : https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs . 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.
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