Wall bulding insulation
I am new to this forum and apologize beforehand if this question has been asked already. I am just so confused with all of the information I have been reading and just want to know what is best for my new home build.
First off, I am in zone 5A (Massachusetts) within a mile of the ocean. I am building a 2,600-sq.-ft. ranch with no basement (slab on grade), radiant floor heating in 4″ slab, 2 x 6 walls; rear south-west exposure on a wide-open 2 acre secluded lot.
OK, my question: I am driving myself crazy with whether I should go with batt and 2″ or full 5.5″ XPS spray foam in the wall cavities. Also, I am toying with the idea of using 1″ XPS rigid board on the outside of the sheathing. What should I do?
I’m thinking (from the inside out): 1/2″ drywall directly on studs (no vapor barrier), 3.5″ batt insulation, 2″ XPS spray foam, 1/2″ OSB sheathing, 1″ XPS rigid board and then Ceder Impressions vinyl siding.
Please keep in mind that I am not rich…as a matter of fact, I will be building this house myself with help from my two brothers…maybe others 😉 Any guidance will be MUCH appreciated and I am open to any and all ideas!
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Replies
I would immediately discount any "flash and batt" approach (spray foam plus batts) because the foam is not flat enough IMO to lay batts against... you will have lotsa air pockets. You could consider a flash layer of foam followed by blown-in cellulose or maybe fiberglass.
What I'd probably do: 5-1/2" cellulose in the stud bays, 2" XPS foam outside the sheathing (assuming the 2" is enough for your climate, read Martin's blog piece of that topic).
Or: double walls ~12" total thickness, all cellulose. Read Robert's article on that topic.
Skip the vinyl siding. PVC is nasty material. Use wood.
Oh yeah... skip the OSB also, use CDX ply or 1x boards.
G M,
Forget the spray foam. You can't spray it and it is costly to hire out labor if you are on a tight budget.
Read up here. There is enough info to really confuse you. A double wall or a wall with an interior horizontal strapping layer will get you a ways toward your goal for the least coin.
Seal, caulk, glue, gasket every single layer you place as you frame. Air sealing is everything. No recessed lights. Cellulose in the attic deep as hell with raised energy heal trusses, for rafters you can trick them out higher too in a few different ways. You need to find a local builder to work with you. It would be worth the expense to be guided by an experienced green builder for the entire build of your home. We builders have a hard enough time building homes well even after decades of doing so.
Vinyl siding is not a great natural green choice, but, if you want to use it, use it. I have,and am less scared of it than an attacking grizzly.
I am at ajbuilderny at gmail for any assistance I could lend.
Strive for as close to the "5/10/20/40/60" approach as you can afford. See if the budget allows for triple paned windows R-5 (U=0.2), they cost more now but new construction is where they have the best payback (you are already buying windows, not replacement cost). If the slab is not already poured; 4" of XPS board under slab = R-20.
If the floor plan has the space consider using 12" Double stud walls or a Riversong Truss. If walls are already framed, or interior space is severally limited look at a Mooney wall with 2" of exterior XPS (that extends down in front of the slab). In either case use blown-in dense pack cellulose, a 7" Mooney wall with 2" XPS and dense-pack = a clear wall value of R-34. With trusses or rafters obtain a full depth height to allow for R-60 worth of loose fill cellulose. Read up on the Airtight Drywall Approach for your air-sealing.
In your coastal location check your codes to see if they require a rain screen. If your code doesn't require it, I would still consider it important enough to pay a bit extra considering the amount of wind driven rain and moisture you receive. Read up on window flashing details and don't skimp on materials for this aspect.
The vinyl siding is most definitely not a green choice, but if the budget has to balance energy efficiency against siding I would spend my money on the energy efficiency.
GM,
1. In general, it's risky to sandwich OSB between two foam layers, because the OSB will have a hard time drying out if it ever gets wet. If you want to use some foam, choose just one location for it -- not two. If it were my house, I would rather see rigid foam on the outside of the OSB; that's a better location for foam than between the studs, because the exterior rigid foam addresses thermal bridging.
2. If you want to install rigid exterior foam on 2x6 walls in Climate Zone 5, the minimum R-value of the foam is R-7.5. Your proposed exterior foam (1 inch of XPS) wouldn't be thick enough. To learn more about this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thank you so much for the responses...I really appreciate them! So, lets see if this is a good solution (inside-out): 1/2" drywall on a 7" Mooney wall (no vapor barrier); blown-in dense pack cellulose; 1/2 CDX plywood sheathing glued to studs; 2" exterior XPS and then wood siding. How am I doing so far? A couple of questions: this wall will have no vapor barrier correct? Stupid question alert: I'm guessing the windows and doors will be attached to the sheathing...do I make a wooden exterior box to cover the 2" XPS or is there something else I could do? The most important question: I have concerns about the cellulose (probably because I know little about it) but is it as good as comparable materials? Will it settle over time therefore diminishing it's R value? What happens if it gets wet (hope not) or insect and rodent infiltration? I like the idea of using cellulose, but need to get over the fears I have of actually using it in my home.
GM,
Q. "2" exterior XPS and then wood siding. How am I doing so far?"
A. You're doing OK, as long as you include a ventilated rainscreen gap between the rigid foam and the siding.
Q. "This wall will have no vapor barrier, correct?"
A. Not quite correct. Both the plywood sheathing and the XPS are vapor retarders. Two inches of XPS has a permeance of about 0.8 or less.
Q. I'm guessing the windows and doors will be attached to the sheathing...do I make a wooden exterior box to cover the 2" XPS or is there something else I could do?
A. For more on your options, see this article: 'Innie' Windows or 'Outie' Windows?
Q. "I have concerns about the cellulose (probably because I know little about it) but is it as good as comparable materials?"
A. Every insulation has its strengths and weaknesses.
Q. "Will it settle over time therefore diminishing it's R value?"
A. Not if it is installed by an experienced contractor familiar with the dense-packed method.
Q. What happens if it gets wet?
A. The same thing that happens with any insulation that gets wet -- it either dries out (if the wall's drying potential exceeds its wetting potential) or it stays wet and allows mold to develop (if the wall's wetting potential exceeds its drying potential).
Q. "What happens with insect and rodent infiltration?"
A. You need to seal up the holes in your wall, call an exterminator, or set out mousetraps.
Thank you again for all of the information. You mentioned having an experienced contractor doing the dense-pack method to eliminate settling, but I was hoping to insulate myself. I have used TigerFoam in the past but am now seriously considering cellulose. Again, I would really like to do this myself unless the cost factor of having a professional not a wallet buster. As a side note, what are the current thoughts on Aircrete?