Flash and Batt vs. All spray foam.
The quotes I am getting for “flash and batt” vs just spray foam are very close to each other.
Home is in Auburn Hills, MI (near Detroit).
For the 2×6 walls, my options are:
* An R21 spray foam application
* 2-inches of close cell + pack out with remainder with unfaced fiberglass.
One insulator says they prefer “flash and batt”. They claim a smidge higher R-value, and better sound proofing from the fiberglass. I believe them on the R-value. Is there validity to the sound proofing claim?
The other swears a full spray foam application is better. He will do flash and batt, but the savings are minimal (only 4%).
To be clear, both are doing a full closed cell application on the roof, I’m just talking about the walls.
Any advice on which one to pick?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
~R26.5 center wall or pay slightly more for a claimed R21 minus more thermal bridging plus higher carbon footprint?
More interesting is "should you use faced fiberglass". I might, despite the fact that latex paint might barely meet the Class III requirement.
So, strong recommendation for flash and batt over straight foam.
Hey neighbor! (I'm just outside of Clarkston, so not far from Auburn Hills).
I would normally not use spray foam in the walls at all, since it doesn't really offer much advantage over other types of insulation there. I would typically use mineral wool batts myself. If you do go with spray foam, I would do a full fill of OPEN CELL spray foam in the walls. Open cell is overfilled and then trimmed flush, so you get a full fill. Closed cell spray foam is much more difficult to trim, so they UNDERfill the stud bays to minimize the need for trimming. This means you get less total R in the cavities, and it's often enough that open cell (which is cheaper) will get to the same, or even better, R value than closed cell.
I would not bother with flash and batt in a wall. You don't mention the type of roof assembly you have, so I can't really comment on that. Note that spray foam in very many applications is an upcharge that gains you no real benefit. About the only real benefit to spray foam is that you can usually be pretty sure you're going to get a good job of air sealing as long as the installation is reasonably good. Overall insulating performance, especially in wall assemblies, is typically not any better with spray foam than with other types of insulation once thermal bridging of the studs is factored in. You can get much better wall performance with cheaper insulation in the stud bays (even plain ol' fiberglass batts), and put the savings towards extra rigid foam on the exterier -- continous insulation -- at least 1" of polyiso and ideally more. Up to 2" or so isn't really a problem for trim. That exterior CONTINUOUS insulation cuts down on the thermal bridging of the studs, and that will gain you a LOT of additional R value over anything spray foam can achieve.
Bill
I'm having an extension added to my house (NY state, where it gets very cold) and the contractors are recommending spray foam on all exterior walls, floor, and ceiling.
Are you saying that it's not worth doing this in the walls? Everything I've read said you get R=6 or so, vs R=3.5 for batt. Is that not really true?
I've edited my response. Micheal Maines' post said it all much better.
Jon Harrod also has a very good article "Why Care About Carbon" in August-September's FHB which deals with the emissions end of this.
The answer is in this article: https://www.greenbuildingadvisor.com/article/installing-closed-cell-spray-foam-between-studs-is-a-waste. Short version: unless you're only concerned about code compliance, it's not worth doing closed-cell foam in stud walls. It's also not worth doing in roofs except for specific situations, and same for floors. It's easy, which is why builders like it, and clients are often presold on the perceived benefits.
Sure, closed cell spray foam (ccSPF) will get you around R6 per inch or so. Mineral wool or high density fiberglass batts will get you around R4.3 per inch or so. But that's not really what you care about in a wall, what you want in a wall is "whole wall" performance. Since the thermal bridging hurts all types of insulating materials (except for continuous insulation on the exterior of the wall), let's just neglect that for now and deal with the other issue here: fill.
With batts, you fill the entire stud bay, which I'm going to assume to be 3.5" here (2x4s), just to keep things simple. That means "regular" density fiberglass will give you R13 in the stud bays, mineral wool R15, and a full fill of ccSPF will get you R21. That sounds impressive! Bit it's not what you think. ccSPF isn't installed full depth in a stud bay, because it's difficult to trim it flush (cured ccSPF is tough stuff), so it's installed UNDERfilled to avoid the need to trim it. It's really hard to get a perfectly even layer, so the 3" fill that is usually targeted is typically closer to 2.5" or so. That means that R6 per inch ccSPF is actually giving you about R15 or just over, very close to mineral wool batts (or high density fiberglass batts). R values don't "average out" over an irregular layer, either, the thin spots contribute more losses than the thick spots save, so half filled 3" and half filled 2.5" does NOT give you the equivalent of a perfectly even 2.75" thick layer.
You end up with very nearly the same final whole-wall R value with spray foam as you do with mineral wool batts. A full fill of open cell spray foam (ocSPF) trimmed flush with the edges of the studs will get you pretty close to R13, the same as "regular" density fiberglass batts.
In the attic, just use blown cellulose on the floor and vent the attic. Done. Excellent performance at the lowest possible cost.
The ONLY thing SPF gains you is good air sealing IF it's installed well. You can air seal the "old fashioned way" with caulk and canned foam too, for less money than a full-home spray foam job. I would air seal the old fashioned way, insulate with batts, then put the savings towards exterior rigid foam which will gain you a lot more overall R value than spray foam between the studs will.
Bill
I say the smart move is to avoid spray foam if at all possible. Spray foam is the most expensive, least green and riskiest way to insulate.
Yes foam insulation does have a higher R value per inch in most cases it costs less money to add sheet foam to the exterior or make the wall thicker and avoid the risk of uncured foam.
Walta
Does your local code require poly on the walls? I would avoid any flash and batt installation with poly. I've seen moisture issues with that set-up.
AC200,
I don't think there are any North American building codes that require poly anymore.
Malcolm,
I should have said inspector. Our code requires a vapor barrier of less than 1 perm, but many inspectors here default that to poly since it is almost universally used.