Fiberglass Batt Insulation Viability/Performance hit in Parallel Chord Trusses?
I know fiberglass batts are not a preferred form on insulation here on GBA. We are going to be building a house in Climate Zone 7 Colorado. It is a very rural area with difficulty finding labor. I am preapproved to be my own GC and will do extensive DIY work. My Dad owns a construction business with my brothers, and I spent 10 years working with him. We are considering 18-inch parallel chord trusses for our 12/12 cathedral ceiling that has a 26 ft span and will be reaching out to some truss manufacturers. Our structural engineer we are working with has noted some challenges with stick framing the roof due to spans, snow load, wind load and being in climate zone 7 so I am here exploring options that balance economics with performance.
I’m weighing insulation options and wondering what the performance hit would be if we did a layer of r38 cathedral with a layer of r21 for a nominal r59 at 15.75 inches deep and then a 2.25 inch vented roof with site built baffles as referenced here on GBA. We would use membrane as an interior air barrier. My concern is that the air spaces in the chords of the trusses would reduce the effective r-value of the r59 (total) batts. I’m just not sure how much they would reduce it.
How much would the r-value of the r59 batts be reduced due to gaps in the truss chords? Code minimum in our county is r38 (I believe the county amended the 2012 code to just r38 for my situation) I could live with the performance being reduced to say r52-r55, but anything below r49 would be a problem.
Would it be a bad idea to use batts sized up for steel studs (steel stud batts are wide than wood framed batts) to bridge some of the gaps in the chords by compressing things a bit horizontally? The roof will be otherwise sealed with attention to detail.
Or are the benefits of 16 inches of cellulose too much to overcome. I’d just need to find somebody who will be willing to do it which isn’t a gimmie in our area.
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Any thought on this? Thanks!
Blown in cellulose or even chopped fiberglass would certainly be the better solution. If you do go with batts, the steel stud batts would give you a little bit better fill of the gap in teh chords. If you pay careful attention to detail, you can probably fluff the batts enough to mostly eliminate the effect of the gaps, but this will be fussy work in a potential hot and nasty space. The risk is that open gaps will allow hot/cold air to circulate right down to the bottom chord of the truss, and this could reduce your total assembly R value by a lot. An exact number would be hard to calculate, but it could be a big reduction. If you can't find a blown-in installer, you should at least be able to rent a machine to DIY.