Installing Closed-Cell Spray Foam Between Studs is a Waste
Installing Closed-Cell Spray Foam Between Studs is a Waste
Why open-cell foam makes more sense than closed-cell foam between studs
Open-cell spray foam has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about R-3.7 per inch, while closed-cell spray foam has an R-value that may be as high as R-6.5 per inch. If you want to install spray foam in a stud wall, and price is no object, then it would seem to make sense to specify closed-cell spray foam, right?
Dense spray foam is installed differently
Builders and homeowners are often surprised to learn that there isn’t much difference in whole-wall R-value between a stud wall insulated with open-cell spray foam and closed-cell spray foam.
To understand why, we need to start by discussing the “trimmability” of cured spray foam.
Closed-cell spray foam is so dense that it is difficult to trim. That’s why installers of closed-cell spray foam never fill a framing cavity completely. In a 2x4 wall, the installer will usually stop at a maximum depth of about 3 inches instead of 3.5 inches, leaving the typical bumpy surface of cured foam. This type of installation doesn’t need to be trimmed.
Open-cell spray foam isn’t as dense, so it’s easy to trim. Installers of open-cell spray foam will fill a 3.5-inch-deep cavity completely, allowing the foam to expand until it is proud of the studs. Once cured, the soft foam is easily trimmed flush with the studs (see illustration below).
With open-cell spray foam, the sides of the studs (above) are not as exposed as they are in a wall insulated with closed-cell spray foam (below), and this fact reduces the thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. penalty.
With closed-cell spray foam, the exposed sides of the studs (the portions that extend inwards beyond the 3 inches of foam) make the thermal bridging penalty worse.
To calculate the whole-wall R-value of a wall, we have to divide the wall into areas with distinct R-values. For example, a 2x4 wall without any windows can be divided into two areas: insulated stud bays (areas with a relatively high R-value) and wood framing (areas with an R-value that is typically lower than insulated stud bays).
A typical wood-framed wall has a “framing factor” of 25%. That means that about 25% of the wall area consists of studs, plates, and headers. The remaining 75% of the wall consists of either stud bays filled with insulation or openings for windows or doors.
To calculate the whole-wall R-value of a wall, you first need to calculate the whole-wall U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. . (U-factor is calculated from R-value this way: U=1/R.) Let’s call the U-factor of the insulation UI, and let's call the U-factor of the framing UF. Here’s how we calculate whole-wall U-factor for a wall without any windows or doors:
Whole-wall U-factor =
(UI * percentage area devoted to insulation) + (UF * percentage area devoted to framing)
When stud bays are partially filled with closed-cell spray foam, the R-value of the studs is reduced compared to a wall that is totally filled with open-cell spray foam. For example, if a 2x4 wall has 3 inches of insulation, the R-value of the studs is based on a stud depth of 3 inches, not 3.5 inches (because the protruding portions of the studs are basically “indoors,” not part of the insulated wall assembly). As a result, the heat loss due to thermal bridging through the framing is greater in a wall with closed-cell spray foam than it would be in a wall with open-cell spray foam.
We’ll look at an example in the table below. This table compares a 2x4 wall insulated with 3.5 inches of open-cell spray foam to a 2x4 wall insulated with 3.0 inches of closed-cell spray foam.
You can see that there isn't much of a difference in the whole-wall R-values of these two wall types. Perhaps you're wondering, “Hmm ... Is it really worth a $3,000 upcharge for an R-value improvement of less than R-1?”
Now let’s compare a 2x6 wall insulated with 5.5 inches of open-cell spray foam to a 2x6 wall insulated with 5.0 inches of closed-cell spray foam.
In this case, the R-value improvement amounts to just R-1.7 — more than R-0.7, to be sure, but still only a minor improvement. The improvement in R-value is so small that most builders will start looking for a more cost-effective insulation upgrade.
In spite of the fact that closed-cell spray foam has a significantly higher R-value per inch, the R-value improvement associated with upgrading to closed-cell foam is trivial, for two reasons:
- With both wall types, much of the heat loss is attributable to thermal bridging, and
- The R-value of the open-cell wall benefits from the fact that the stud bays can be filled completely (thereby improving the R-value of the framing).
If you install an insulation with a high R-value per inch between your studs, you don’t really get full value for your investment. Closed-cell spray foam is expensive, and the incremental cost is mostly money down the drain. It's also worth considering an even more important issue: most brands of closed-cell spray foam are more injurious to the atmosphere than open-cell spray foams (since most closed-cell spray foam is manufactured with a blowing agent with a high global warming potential).
A bigger bang for your buck with continuous exterior insulation
At this point, we need to consider the use of exterior rigid foam. The calculations for adding thicker continuous insulation on the exterior side of the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. are much more favorable to incremental investments than the calculations for insulation installed between studs.
If you install an insulation with a high R-value as continuous insulation on the exterior side of the sheathing — for example, rigid foam or mineral wool — all of the insulation’s R-value contributes to the whole-wall R-value (except, of course, for areas taken up by windows and doors).
I'm aware that the whole-wall R-value calculations in the tables above are simplified versions of actual whole-wall R-value calculations. I haven't included the R-value of the exterior OSB sheathing, the interior drywall, or the associated air films. Moreover, the tables don't reflect the entire range of framing factors at different buildings.
That said, the tables are useful. They demonstrate the calculation method and do a good job of estimating the incremental R-value attributable to an upgrade from open-cell foam to closed-cell foam. While it's true that actual whole-wall R-values will usually be higher than the values shown in the table (due to the R-values of the sheathing, drywall, and air films), both types of wall (open-cell and closed-cell) benefit equally from these additional R-values.
What about cathedral ceilings?
As the analysis in this article shows, thermal bridging effects (which are always undesirable) are proportionally greater when framing bays are partially filled with insulation than when framing bays are filled completely. While I've focused (so far) on walls, the same analysis applies to cathedral ceiling assemblies.
If rafter bays are completely filled with fluffy insulation (except for a ventilation channel directly below the sheathing, of course), thermal bridging through the rafters will be less significant than when 4 inches of each rafter protrudes inward beyond the depth of a skimpy application of spray foam.
Author’s note: I’d like to thank GBA reader Dana Dorsett for inspiring this article. His repeated comments on the topic I’ve outlined here helped sharpen my understanding of this issue.
Martin Holladay’s previous blog: “A Visit to a LEED Platinum Office Building.”
- Image #1: Icynene Corporation
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