Installing Closed-Cell Spray Foam Between Studs is a Waste

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Installing Closed-Cell Spray Foam Between Studs is a Waste

Why open-cell foam makes more sense than closed-cell foam between studs

Posted on Mar 17 2017 by Martin Holladay

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?

Not necessarily.

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.

Whole-wall R-values

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.”

Click here to follow Martin Holladay on Twitter.

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Image Credits:

  1. Image #1: Icynene Corporation

Mar 17, 2017 9:40 AM ET

Education should be mandatory!
by Armando Cobo

Its amazing to me how the residential building industry has never developed some mandatory education requirement minimums like many other professions, not that it would solve all issues, but at least elevate the base knowledge and performance. We, as builders, berate the Real Estate industry for "doing nothing" to collect commissions, or make fun of "sleazy" used car salesmen, and yet we seem to do the same or worst.
What blows my mind is that most of the trade industries within the whole residential building industry have always fought against any minimum mandatory requirements for education, and states and municipalities turn their blind eye to the issue.

Mar 17, 2017 9:41 AM ET

One minor critique
by Antonio Oliver

While I agree that "both types of wall (open-cell and closed-cell) benefit equally" from the additional R value provided by sheathing and drywall, I disagree on air films. In your analysis, the closed-cell insulated wall has a half inch of confined air in the stud bays (between the drywall and insulation) that the open-cell insulated wall does not. Would you agree?

I also think the analysis for a ceiling gets tricky depending on the structure used for framing--TJI beams (with thin OSB chords), 2X dimensional lumber, or roof trusses (with open spaces between chords which could be filled by insulation to mitigate thermal bridging).

Mar 17, 2017 10:04 AM ET

Edited Mar 17, 2017 1:03 PM ET.

Response to Antonio Oliver
by Martin Holladay

Thanks for your comments. You're right that a 1/2-inch air space has R-value. According to ASHRAE Fundamentals, a 1/2-inch air space (when heat flow is in a horizontal direction) has an R-value of R-0.9 (assuming that the surfaces facing the air space have an emittance of 0.82, that the mean temperature is 50 degrees F, and that the delta-T is 30 F degrees; the R-value will vary slightly under different circumstances).

You're also right that the shape and conductivity of a rafter affects the thermal bridging penalty.

That said, the conclusions reached in my article are not seriously affected by these points.

-- Martin Holladay

Mar 17, 2017 12:09 PM ET

Yer welcome, Martin! (... to keep on singin' my song! :-) )
by Dana Dorsett

Also note: The difference in environmental hit is large, even if using low-impact HFO blowing agents for the closed cell foam. The 3" of 2lb foam is 1lb of polymer per square foot of cavity area, whereas 3.5" of half-pound foam is only 0.146 lbs per square foot- call it 0.017lbs to account for any excess that's trimmed & discarded. That's 700% (7x) as much polymer, for a paltry net performance gain on the order of 10%.

With ceilings typical framing fractions are on the order of 7% rather than 16" o.c. studwalls ~ 25%, and it can be even less with TGIs, etc. The thermal bridging performace hit isn't quite as large as with walls, but it's still real enough. Even without thermal bridging factored in (call it zero) it's still ~2x the amount of polymer per R to use 2lb foam instead of half-pound.

Bottom line: If you're going to use closed cell foam, using only the minimum amount needed for dew point control and using lower impact fiber (or open cell foam) for the rest is going to be nicer to the planet and usually quite a bit less expensive too.

Mar 17, 2017 6:49 PM ET

Controlling the Dew Point
by Christopher Welles

Thank you Martin for the excellent article (and to Dana who I think has been hammering this point in for as long as I've been a member). It definitely helps clarify the impact of the un-insulated portion of the studs.

This is part of why I personally like the idea of flash-and-batt more. The closed cell portion appears to provide a nice way to address condensation issues, and it feels like it's an extra safety measure against water intrusion. The batt addresses the portion of the studs that would not be insulated by closed-cell foam.

With that approach, it does seem that thermal bridging through the studs makes them a potential condensation point, so that would seem to require exterior continuous insulation. Yes, it does start to look increasingly complex and costly. We'll see what I actually end up building with.

In cathedral ceilings, my current instinct would be to go way overboard on the amount of closed cell foam at least towards the ridge, given the way relative humidity spikes there. Either that, or skip the closed cell entirely there and do a vapor diffusion ridge vent (at least for a simple roof line). Roofs are complicated.

Mar 18, 2017 5:25 AM ET

Response to Christopher Welles
by Martin Holladay

While the flash-and-batt method that you advocate will work, I don't think that the analysis presented here is a very strong argument in favor of flash-and-batt.

To me, this analysis supports the conclusion that walls benefit from a continuous layer of exterior insulation (rigid foam or mineral wool).

-- Martin Holladay

Mar 18, 2017 3:45 PM ET

Edited Mar 18, 2017 3:46 PM ET.

Can't say I disagree
by Christopher Welles

I can't say I disagree with that. Continuous exterior insulation seems like an inescapable necessity. I wasn't trying to debate that flash-and-batt is superior in general. I guess the "more" I was referring to was versus just using ccSPF.

A couple of the reasons I'm leaning towards flash-and-batt + some exterior insulation are:
- Every additional inch of wall depth adds to my much too high NJ property taxes.
- Having a layer of ccSPF just seems more robust.
a. I don't just have to rely on the tape and zip panels to provide a robust air barrier.
b. It would seem mitigate some types of potential bulk water intrusion failures.

These are things that more resonate with me personally rather than a general argument as to its superiority, so I don't expect it to convince anyone. The biggest doubts I have personally are related to cost and ability to find someone able and willing to do the work. My next-door neighbor had trouble finding anyone comfortable working with Roxul ComfortBoard when he built last year. If that's the case, my own odds don't seem great.

Again, I very much enjoyed this article. Thank you.

Mar 19, 2017 3:59 PM ET

Edited Mar 19, 2017 4:00 PM ET.

closed cell followed by open cell
by Antonio Oliver

If I read Dana correctly, he's advocating using closed cell followed by open cell foam. Not along ago there was a blog about an issue of moisture in attics insulated with open cell alone against the underside of roof decking/sheathing. I'm assuming that you, Martin, will not be advocating open cell foam alone for that application now. Just one more way that this issue is a bit trickier for ceiling applications.

Mar 19, 2017 4:16 PM ET

by Charlie Sullivan


It sounds like taxes are based on the exerior footprint of the house. That's an unfortunate incentive for thinner walls. Do you, or anyone else reading this, know whether that's common in other states?

Mar 20, 2017 12:43 AM ET

by Malcolm Taylor

I've never heard of property taxes being primarily driven by house size. They almost always go by appraised value, with the size of the dwelling being one factor. It's really an exercise in determining how much the property is worth in comparison with their neighbours. The total assessed value of an area then gets multiplied by a mill rate so that the total taxes necessary are raised.

Even if it were entirely based on the building footprint, increasing the depth of the walls by an inch would for the average house would equate to about 12 sq. ft - or 0.8%

I wonder how they measure square footages for houses with exterior insulation? traditionally here they have used the framing dimensions from the plans and ignored siding, trim etc.

Mar 20, 2017 5:56 AM ET

Response to Christopher Welles (Comment #7)
by Martin Holladay

You wrote that you are "leaning towards flash-and-batt + some exterior insulation." If you do that, make sure that your exterior insulation is mineral wool. Otherwise, you'll be encapsulating your OSB sheathing (or plywood sheathing) between two vapor-impermeable layers.

-- Martin Holladay

Mar 20, 2017 6:02 AM ET

Response to Charlie and Malcolm (Comments #9 and #10)
by Martin Holladay

Charlie and Malcolm,
Thick walls can, indeed, have expensive tax consequences. Local practices for measuring the area of a house vary from town to town, and it may be worth bringing the topic up with your local tax assessor if your house has thick walls. I've heard of a few owners of energy-efficient homes who managed to convince the local assessor not to include the entire area of a thick wall.

In towns that use the area of the foundation as a guide, it may reduce your taxes to switch from double-stud walls to Larsen trusses (since Larsen trusses overhang the foundation).

-- Martin Holladay

Mar 20, 2017 6:14 AM ET

Edited Mar 20, 2017 6:17 AM ET.

Response to Antonio Oliver (Comment #8)
by Martin Holladay

You're right that spraying open-cell spray foam on the underside of roof sheathing can be risky. (GBA readers who are unfamiliar with this discussion might want to read High Humidity in Unvented Conditioned Attics.)

Dana Dorsett's suggested method -- spraying closed-cell spray foam against the underside of the roof sheathing, followed by a layer of open-cell spray foam on the interior side of the cured closed-cell foam -- is generally considered safe, especially if the thickness of the closed-cell spray foam layer meets the minimum R-value rules described in this article: Combining Exterior Rigid Foam With Fluffy Insulation.

(Although the article I linked to discusses rigid insulation and fluffy insulation, the R-value ratios in that article also apply to the minimum thickness of a layer of closed-cell spray foam.)

-- Martin Holladay

Mar 20, 2017 6:35 PM ET

I'm not really advocating ANY closed cell foam (response to #8)
by Dana Dorsett

Using closed cell foam on the under side of the roof deck is a band-aid fix to a design screw-up, not a go-to solution. It can usually be designed out. High-R properly vented cathedralized ceilings are cheaper and more resilient, if it's new construction with reasonably simple roof lines.

But where/when that band-aid becomes necessary (often the case in retrofits, or where the architect went nuts on new construction creating 1001 different roof planes to scratch some bizarre aesthetic itch) using the LEAST amount of closed cell foam required for getting the job done is being much nicer to the planet (and usually your wallet.)

Even OPEN cell foam isn't the greenest or most sustainable solution. Where possible it's nicer to the planet to use ultra-low impact cellulose rather than open cell foam, if not cellulose, rock wool, and use caulks & tapes for air sealing. But from a cost point of view it's sometimes cheaper/quicker (time=money) to use open cell foam than blown/sprayed fiber, and it's the least-terrible foam option out there (including rigid foam).

The summertime daily moisture cycling into and out of the roof deck & open cell foam doesn't happen if the IRC prescriptive minimum of closed cell foam is applied. The vapor retardency of the close cell foam keeps moisture from moving into and out of the roof deck at a high rate, and the R-value of the closed cell layer keeps the overnight cool edge of the open cell foam layer warm enough to take on much moisture.

Mar 21, 2017 10:09 PM ET

Good things are happening
by Andy Kosick

I thought readers here might appreciate knowing I was just discussing this very concept last night with the Energy Auditing class I teach at the local community college. So there's sixteen more souls with this seed planted in their brain. Also, it's worth noting that many of them are in the residential construction program at the college to meet the educational requirements to get a builders license in Michigan. I understand that requirement may be under threat in the legislature, but for now, good things are happening. Sometimes I have to remind myself of this.

Mar 22, 2017 10:55 AM ET

Practical observation
by Rick Wertheim

This is a very interesting analysis on the benefits of open cell foam v. closed cell on a cost basis. We are affordable green builders and stopped using open cell foam as cavity insulation the day we finished our very first home with the product. The trimmed out overspray filled an ENTIRE 10 yard dumpster. I'm not sure if anyone has done the environmental calculations on that material finding its way into a landfill forever.
Thus, it made our decision easy: We use densepack cellulose and exterior rigid on all of our building envelopes.
Some more food for thought....

Mar 22, 2017 11:23 AM ET

Edited Mar 22, 2017 11:25 AM ET.

Years of Experience
by Todd Witt

In Zone 3 open cell is the best option for wood framed walls and ceilings. Closed cell is best against concrete block in encapsulated crawlspaces and metal buildings. In zones where an interior vapor barrier is required open cell might not be the best solution. I don't do work or have experience in those areas so I will only comment on Zone 2 and Zone 3.
Flash and batt - check your closed cell manufacturers ESR reports and you will see that the closed cell flash required to be an air barrier is at least 1.3 inches. Most flashers in our area spray about 1/4" of an inch or less and then cram the batt in. Flash and batt in my opinion is the choice of unprofessional installers.
Closed cell in our area is normally installed at about 2" of depth in walls and 3" in attics. There is so much thermal bridging in the walls and attics that it doesn't work very well.
Icynene open cell foam installed in attic assemblies is not risky. Every case study you mentioned in previous issues resulted from interior moisture issues unrelated to the foam or improper foam installation. I can guarantee you out of thousands of homes insulated and HVAC systems designed we have not had 1 moisture issue.
Note the Icynene open cell foam does not hold moisture in walls or attic assemblies. Not all open cell foams are created the same.
Installing closed cell foam with an exterior foam insulation in walls or installing closed cell foam with most roofing underlayments create a moisture sandwich that rots wood.
Closed cell has blowing agents that off gas over time versus low VOC Icynene products.
Almost any amateur foam installer can install closed cell but it takes a professional to install open cell foam properly. I recommend running from any company that preaches only the virtues of closed cell foam.

Mar 22, 2017 4:06 PM ET

How much rigid foam ends up in the dumpster? (@ Rick Wertheim)
by Dana Dorsett

Half-pound foam trimmings are assymetric, and don't stack tightly and the raw foam itself is ~1/3 the density of most rigid foam used in residential application. The density of the trimmings tossed in the dumpster is lower still. You can end up with a lot more polymer in a much smaller volume with rigid foam.

Cellulose is clearly greener than open cell foam as cavity fill (and has some thermal mass in higher R assmblies) though I can understand why sometimes people will go to open cell foam on a cost basis. And using rigid foam for dew point control is also greener than sprayed closed cell, but in new construction why use any foam at all? It can be designed out.

More food for thought:

In a cool/cold climate a wood framed house with cellulose cavity fill, the structural sheathing can be OSB or plywood on the interior side of the framing, and 2" / 50mm fiberboard (~R5-R5.3) on the exterior under rainscreened siding. This works over an enormous range of whole-wall values and climates, since the interior OSB is a smart vapor retarder, and the fiberboard is vapor permeable. With the Class-II vapor retardency of the OSB/plywood, the R-value of the fiberboard while a welcome thermal break on the framing, it doesn't need to be thick enough for dew point control. In US zones 5 & lower it doesn't even need the low vapor retardency of the interior side wood sheathing to meet code, only for structure. (In much warmer or mor humid climates you might still need an exterior side vapor retarder, since the fiberboard may too vapor permeable to avoid problems in air conditioned buildings.)

This approach has been successful in a number of temperate and cool climate European high-performance houses, including this UK Level 6 standard house in Norwich, Norfolk England:

Some of the insulation details can be observed in the Grand Designs television episode, which is available on Netflix, if you're interested:

(I think it's the 2nd season, 6th episode in the Netflix listings- could be rong, offen am...)

UK Level 6 construction is all about sustainability, and it's fair to say that fiberboard & cellulose are far more sustainable than foam insulation. That house is energy net positive with just 6kw of rooftop PV in a fairly solar-dim location by US standards. It looked like 2x6 or 2x8 framing (I haven't found a good description online) and 2" fiberboard on the exterior. One nit to pick is that the cellulose was blown in ~4" holes (on the interior OSB side) with a large hose, not with dense-packing hoses, and thus probably not at a density sufficient to eliminate settling over time.

And yes, while Norwich is a pretty temperate climate- roughly a US Marine 4C climate though cooler in summer- it would have a 12 month heating season for a code-min house: . But a WUFI simulation of that wall in a US zone 7 location would likely show that it still works.

The amount of foam you really NEED in a new home design is pretty small (if any), and going without doesn't necessarily have to be more expensive or lower performance.

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