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Community and Q&A

Insulating Rooms in Attic

tbanwart | Posted in General Questions on

We are building a home in SE Kansas (Zone 4A) with a bonus room over the garage and a little bit of bonus space of the adjacent part of the attic over the house. About 1/2 the house attic has no bonus space. I’ve been reading a lot of articles and comments on how to insulated cathedral ceilings, all about attic venting, etc and am still faced with a few questions. I would consider putting foam on top of all my roof sheathing but about 1/2 my roof is not over any bonus space and part of it is a large overhang in a vaulted living room that hangs over an outdoor deck which would be a waste of foam on top but would be needed since they share the same roofline. In the half of the house with no bonus room, my plan was to flash n cellulose fill the ceiling which would be separated by an insulated wall / air barrier in attic from the other half of the house which contains the bonus rooms. Because of numerous hips and details, I don’t think venting is a good option unless mushroom vents will work ok and there is open attic at the top portion of the attic.

Therefore, here are my questions and I refer to page:

1) Can I do unvented rafter bays by flashing with 1″ or more of closed cell foam, then packed cellulose (or other type), then 2″ of rigid foam under the rafters to stop thermal bridging?  This would be variation of assembly #5 in the link above.  Is this permissible or would this cause any moisture issues. I’ve looked and saw something like this reference in this article, but wanted to confirm.

2) Alternatively, could I use open cell foam to fill the bays and still rigid foam over these with 2″ foam, and if so, what type? We are out of city so codes are not enforced but I believe code calls for R49 ceiling so depending on my rafter bay depth, we may have to do something different or settle for less in this area.

3) In bonus truss, Is it permissible to go unvented from soffit area with the above mentioned insulation method, then insulate the top cord (ceiling) of the bonus room the same way? (see attached drawing)  In this case, would I need a ridge vent at the peak over this area?   The roof is hip style so I’m guessing I’ll probably need to add in some mushroom vents in the open attic above the conditioned room in the attic. 

4)  The decision is not yet made to use roof trusses or cut rafters / I -joist on site.  If using rafters, it seems fairly easy to apply 2″ rigid foam across the bottom of the rafters and then fill with blown cellulose.  However, if using trusses, how would one accomplish something similar as the you can’t just fix rigid insulation along the bottom of the joist bay as easily because of the truss design.  The foam can’t lay flat w/o cutting it and piecing wherever a wood strut is tying the top/bottom cord of truss.  I guess I’m not sure of a good way to insulate between trusses and getting it sealed up fairly well.  Any suggestions here would be appreciated.

5) Could use rigid foam on top of sheathing like Assembly #3 to get an r-18 to 21 on top of the roof, but I have 2 issues:  Due to having to keep rooflines same elevations, I’d be insulating a few large overhangs that go out over a porch needlessly as it is the same roof line that takes it over the insulated space inside.  Secondly, again, I need to add more insulation inside how do you properly finish out insulating the inside / bottom side of deck if using bonus roof trusses?  All I can think of is stuffing batt insulation in there unless somehow we could get celluose to stick or be held in place.

I’m just trying to see if I can build a non-vented roof in this attic space or if there would be a better way to do this. I’m open to a vented assembly but only about 70% of my rafter bay’s have soffits below and the ridge vent would be even less space with the hips. In addition, venting 1.5″ would reduce my overall thickness of insulation so I really feel non-vented wins out here. I attached a sample detail sketch up – pardon my drawing capabilities.

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  1. Expert Member
    BILL WICHERS | | #1

    1- Not really. You'd need a lot more "flash" compared to "batt" than you might expect here, probably 40+% of the total R value as spray foam. Yes, it's doable, but it's probably not ideal. Adding rigid foam underneat runs into the "double vapor barrier" problem and I wouldn't recommend that.

    2- Open cell spray foam carries additional moisture risks in attics. Closed cell is safer. Personally, I would avoid spray foam in this assembly which I'll get to later. Even if codes aren't enforced, remember that many of those codes are based on good reasoning, so it's still worth following most codes. DO NOT go against code on anything safety related (structure, etc.) regardless of code being enforced or not. Energy codes are less of an issue here, but I'd still try to at least hit code minimum R values, which usually isn't difficult to do.

    3- I would leave a vent channel in these areas by using some 1/2" polyiso or 1/4" waferboard (cheaper version of OSB) , then insulate under that with batts or dense pack cellulose. You could then use polyiso underneath the rafters and not worry about it since the vent channel would be able to draw off any moisture that might find a sneaky way to get above the polyiso.

    4- Raised heel trusses might solve some problems for you here. You could build a vent channel up past your bonus space, then just fill in the space under that with loose fill cellulose to get up to whatever R value you need. You could use polyiso under the trusses for some support and a thermal break if you don't want to completely fill any larger spaces you may have.

    5- This would probably get complicated with a complex roof. You're probably looking at a lot of installation labor here.

    I would see if using trusses would simplify some of your insulating issues by providing additional space. Open trusses (no web like I joists) have much less issue with thermal bridging too, so you could skip polyiso on the underside and still have similar performance levels. I would try to see if there is a way to keep as much of your roof vented as possible since this tends to be both the cheapest and the most robust roof design, and avoids a lot of the complexities with unvented roofs that are necassary to avoid moisture issues. You may end up with just a few areas that you have to do unvented, and I'd use closed cell spray foam in these areas only and then only if you can't find a practical way to tie those areas in to the rest of the vented roof. Remember that air baffles can be site built quickly with inexpensive materials and a finish nailer, so they're not very complex as long as you have access to the areas where they need to be installed.


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