Insulation for 2×8 exterior walls
We are building a home with 2×8 exterior walls in zone 6 (Wisconsin). Due to cost, exterior insulation was not an option so the plan is: vertical corrugated metal siding, coravent, LP Weather Logic, 2×8 studs, Intello vapor barrier, plaster.
because there’s no external insulation, which will have more thermal bridging, we are trying to decide what insulation would be best incase of slight condensation. I know dense packed cellulose has the ability to absorb and release moisture but after many calls, we have found no one who uses it in our area (fiberglass batts or spray foam seems to be the options). So, this leads us to possibly doing it ourselves with either rockwool or a product like lambs wool from Havelock Wool.
Wool seems to be similar to dense packed cellulose in that it absorbs and releases moisture, but maybe it would hold moisture too much in a thicker wall? Also wanting to avoid moisture gathering at bottom of walls and not sure if this would be more of an issue with rockwool as it doesn’t absorb moisture? Would appreciate any thoughts and insight.
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Replies
You mention 2x8 studs but don't describe what you mean by 4x8 walls. 4x8 what?
If you're planning on using 2x8 studs, have you considered building as a staggered double stud wall using 2x4s instead? A staggered wall like that could use common R15 mineral wool batts for 2x4 walls, and would have far less thermal bridging than the 2x8 studs would have.
The "4x8" you mentioned is confusing. If you use 4x8 timbers for framing, you're going to have a lot more thermal bridging than normal size framing would have, making thermal breaks more important.
Bill
Mike and Bill,
I think the 4"x8" is a typo, like the later " No exterior siding".
Yes, so sorry. I meant 2x8 wall construction and no exterior insulation.
We did consider the 2 2x4 walls that were varied but were told it complicated some of the top plates construction where the trusses connect and although our contractors are willing to try, I don’t know that we want to be the first house. This is why we settled on the 2x8 deep walls as it was much more straight forward for the builders while still providing more insulation than a 2x6 wall.
Does anyone have thoughts on the rockwool versus sheep’s wool?
Any fluffy insulation will work just well, it really doesn't matter.
There are definitely some additional green benefits to cellulose or wood fiber though.
Using a double top plate over the staggered double stud wall might solve those truss connection problems, since the alignment issue (getting trusses lined up over studs) is no longer as much of a concern.
I don't see a problem using any type of batt here, although personally I don't like using natural fiber products in walls where they might see moisture. Natural fiber stuff can be "food" for things like mold and critters. Synthetic stuff like fiberglass and mineral wool can't be used as food -- nothing eats glass or rock. In practice though, there probably isn't a whole lot of difference. Mice can and will tunnel in fiberglass, and they can tunnel in mineral wool too (but seem to not like to most of the time), so keeping critters out of the assembly is important regardless of the type of insulation you choose.
Bill
Ah, that makes much more sense now!
Emel, as Bill says, you'll get better performance from a double-stud (or staggered-stud) wall, but using 2x8 studs is easier for builders without a wide range of experience and will perform pretty well.
Both mineral wool and sheep's wool make excellent insulation. Sheep's wool costs more but it's nicer to work with and has a much lower embodied carbon emissions. z
It may not be available in your area yet, but wood fiber batts are worth considering as well: https://www.timberhp.com/products/timberbatt. They're a little more finicky to install than soft batts like fiberglass or sheep's wool, but not that different than installing mineral wool batts. They provide similar moisture-buffering ability to dense-packed cellulose.
Wool seems to be similar to dense packed cellulose in that it absorbs https://casinoschileonline.com/
Another reason to consider a double stud configuration in a 2x8 wall is this. A lot of the dimension lumber these days over 2x6 is Douglas Fir. You surely do not need this quality of lumber for stud walls.
Doug