Buildability vs. durability
I’m going to be building a 28×40 (aprox 1500sf living) chalet style house in upstate NY CZ6 (design temp 6f) it will sit on a walk out basement (one narrow end walk out the rest under ground) I am going to put pex in the slab even though I don’t currently plan to ever need it warmer than 55 or so down there.
Been reading on the subject quite a lot, and think that the pretty good house concept you guys have come up with is a great idea, and should get me in a balance point between up front cost, and monthly utility bills.
So I am looking at 4″ of eps under the slab (R 15 ish), 6″ of eps outside the below grade foundation walls (R 23 ish). R40 for the walls, and R60 for the cathedral ceiling, and the best windows I can afford (hoping to go with triple pane upvc from intus, zola, or ???).
First question, I think I should increase the insulation for the above grade part of the foundation to R40, but should this go all the way down to the footer, switch to R20 4′ below grade or some other answer?
I’m looking for opinions on the best way to build an R40 wall (with a rain screen gap), and R60 cathedral ceiling with vented sheathing (I’ve read the cathedral ceiling article several times)
Initially I thought that PERSIST would be the way to go (all of the important sealing happens on the outside of the sheathing where its easy to inspect, and keep continuous), 10″ of foam on the walls would be a pita, but doable (is it as hard as i think it would be to hit a stud though 10″ of foam and a 1×4), 16″ on the roof just seems daunting.
From what I’ve been reading its safe to move 1/3 of the R value into the walls/roof inside of the sheathing, (would use cellulose) is this true? would it require an intelligent vapor barrier, or would sheetrock alone be ok? This would save some $ on foam.
The other wall choices all seem to be variations of a double stud wall or larsen truss, is any one easier to reliably build than another?
The other choices I’ve narrowed down for the roof would either be deep I-joists dense packed and vented (the roof design makes it easy to get a clear path from eve to peak in every single bay) or parallel chord truss’s dense packed and vented.
Is either one better or easier than the other? Is there a better solution I’ve overlooked?
How do you keep all that weight from making the sheetrock sag/bow?
I will be doing most of the work myself with friends/family supplying help. and contractors doing the foundation, setting the heavy stuff with a crane, and weathering in the roof.
If this is too much for a single thread let me know and I will break it up.
thanks in advance
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Replies
Trevor,
Have you investigated a "PWF"? Also investigate the "Arctic" wall which is a 2 cavity double stud wall that places the sheathing on the out side of the inner studs and can eliminate exterior foam.
I read the presentation for the "arctic" wall, but wasn't sure how to transition that to my ceiling/roof. Would one sheath the inside of the rafters or trusses with plywood, sealing to the plywood in the wall, and then build down an additional cavity?
I haven't looked into a "PWF", I've never heard of any one actually using one around here, or even seen one in person.
Trevor,
You have lots of questions.
Q. "I think I should increase the insulation for the above-grade part of the foundation to R-40, but should this go all the way down to the footer, switch to R-20 4' below grade, or some other answer?"
A. If you are going to install your basement wall insulation on the exterior, the usual approach is to choose an insulation thickness and to install the same thickness from the top of the wall (or the top of the rim joist) all the way down to the footing.
Q. "Initially I thought that PERSIST would be the way to go; ... 10" of foam on the walls would be a pita, but doable (is it as hard as i think it would be to hit a stud though 10" of foam and a 1x4); 16" on the roof just seems daunting."
A. There's always the REMOTE approach, if you are willing to forgo the conditioned attic, and instead choose a vented unconditioned attic.
Q. "The other wall choices all seem to be variations of a double stud wall or Larsen truss. Is any one easier to reliably build than another?"
A. More builders choose to build double-stud walls than Larsen truss walls. But both approaches have their fans. Remember, this is your house, so you get to build what you want.
Trevor,
I sure don't want to sound like a PWF salesman. There are over a million PWF foundations in the US. Over 40 years ago I watched my neighbor build a PWF, they still live in it and have no issues. The cost of a well insulated basement is dramatically lower with PWF.
You are correct that the details of how to connect the wall's air barrier to the ceiling need to be addressed, but this is true for any wall not just the Arctic wall. The Arctic wall creates a "natural" service cavity in the inner stud spaces, this may or may not continue into the ceiling. .
Martin, I've designed what I want to live in (floor plans, elevations siding etc, etc) and even though I know a flat ceiling is an easier/cheaper way, I'm willing to put in the extra work for cathedral, so I am looking for suggestions on chosing the path to get there, and reaching out to those with experience building high R enclosures. (standard practice around here is 2x6 with fiberglass batt, and maybe taped zip sheathing)
My reading on BSC seems to paint a normal double stud wall as risky
Jerry, You don't sound like a salesman, the more info the better.
Trevor,
Q. "I'm willing to put in the extra work for cathedral, so I am looking for suggestions on chosing the path to get there."
A. In case you haven't read it, here is an article that discusses all of the different ways to insulate a sloped roof assembly: How to Build an Insulated Cathedral Ceiling.
Get with Bruce Brownell and have him take your plan and draw up his details for it.
Google; bruce brownell adirondack alternate energy
If you are planning to do the work yourself with friends, and you want both buildability and durability, it's hard to mess up with dry stack blocks. After stacking them, you coat both surfaces with surface-bonding cement and fill the cores with concrete. Basically anyone can learn to do this and it's much easier than learning to build a complicated and non-traditional wood-framed wall assembly. The interior coat of surface-bonding cement can replace the drywall, and this wall is naturally one big air barrier. You can screw as much foam or mineral wool or whatever onto the exterior, without having to worry about hitting studs. You also gain the benefit of a structure that's invulnerable to fire and rot, and doesn't feed mold or termites. It is highly buildable and highly durable. The interior thermal mass won't give you any thermal benefit in upstate NY compared to somewhere like Arizona, but you'll still benefit from its increased durability.
I know "wood is good" and all, but IMHO it's hard to beat masonry for both buildability and durability. Most of the cost differential beyond wood frame construction is in the labor (especially with systems that use more wood like double-stud walls), so if you're supplying that yourself, it seems like a great match.