Ridged foam between rafters
Hello all! I’m in the planning stages of a new small home and would like some advice on my insulation strategies. First I’ll give some info on the idea. The structure will be built off site by an Amish crew and transported a short distance. My goal is to create a simple and affordable home that meets high performance standards that are so important for health and durability. The home will consider the 4 control layers and have an ERV and mini split system.
climate zone 5
footprint 18’x40′
foundation concrete piers or helical piles(if I can find a contractor in central Ohio)
14” engineered floor trusses with zip underneath
2×6 walls fiberglass batts zip sheeting
interior walls will likely be tongue and groove pine
single slope roof 2/12 pitch
2×10 rafters
steel siding
steel roofing
first question
Is it safe to fill or mostly fill the floor with blown in cellulose?
I would probably add some ridged foam underneath some of the plumbing.
second question
in an effort to keep the methods simple and more closely aligned with the crews standard processes is it safe to add 7” of xps between the rafters? there are so many combinations of insulation/exterior insulation that can be used but the simplest in my mind currently is ridged foam between the rafters. a quote for CCSF was around $6800 just for the roof! If I go with foam does it need a facing? air gap? vapor barrier behind the plank ceiling?
all the other design details are up for discussion if anyone would like to chime in.
I’m familiar with build science enough the have gotten a .67 ACH50 on my first build (standard ranch with basement) but the problem with building science is you will never sleep well again is so true! (unless you have an ERV and an air quality monitor then you will sleep like a baby!! HAhA)
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Replies
It is safe to fill your floor system with cellulose; the trick is to make sure it is completely airtight. We've taped all the seams in the floor sheathing in the past which works, you also need to be sure the underside and vertical surfaces (rim joists) are sealed as well.
https://www.greenbuildingadvisor.com/article/insulating-a-wood-framed-floor-assembly
Your plan to use the cut and cobble method of insulating your roof is not such a good idea as outlined in this article with nearly 200 comments and questions.
https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation
Thank you for the reply! Both very good articles.
On an elevated floor do the joist bays need to be fully filled or can there be an air space below the subfloor as long as the assembly is air sealed well? Of course the band joists need to be fully insulated