Spray Foam Art

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Edelmar Easley

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Aug 3, 2024, 4:21:12 PM8/3/24

to gentroldistbiss

William,
The short answer is that one-component spray foam (also known as canned spray foam or moisture-cured spray foam) is not a perfect air barrier. It cuts down significantly on air leakage, but it can't be considered airtight.

The density of cured one-component spray foam varies widely, depending on the brand you use and the moisture content of the air and surrounding materials on the day the foam was installed. In general, the density of cured one-component spray foam tends to be less than the density of closed-cell two-component spray foam.

To expand on Jon R's comment regarding movement capability (and possibly related to Martin's comment regarding conditions during application), in my somewhat limited experience with canned foam I've found it can sometimes be extremely brittle once cured and have somewhat poor adhesion to surfaces.

Thanks, all very helpful. OK, so we'll consider canned spray foam as insulation only, and use tape at windows. We're thinking about using 3M All Weather Tape on sheathing joints. Do you think this would be an acceptable alternative to the European tapes for window sealing? Or should we spring for the good stuff?

This is a very typical trade practice of overfilling a hole and then trimming it down. Anecdotally this makes sense as it seems to "open the pores" and the "skin" formed seems to better at "air sealing"

I agree that anecdotally that makes sense, but I don't know of any studies done on it. Most air leaks happen at the gaps and cracks at the perimeter of assemblies, not in the middle of the wall, and foam often pulls away from framing, so to me it's a moot point when talking about spray foam for air sealing.

If you are looking for foam to insulate your home or building, HandiFoam's Quick Cure FR (HFO) 605bf (board foot) closed-cell polyurethane foam kit is the one you're looking for. The P12056 spray foam kit can cover approximately 605 square feet at a 1-inch thickness of expanded foam, making it an ideal option for home or building insulation.

The FR HFO (closed cell) low pressure spray polyurethane foam kit is a two-component system designed to fill and insulate large voids and surfaces. HandiFoam"s excellent adhesion properties create a continuous air barrier which completes the building envelope, resulting in improved indoor air quality and lower heating and cooling costs.

Air sealing / insulating attic, metal buildings, sealing can lights, sealing duct work joints, retrofitting windows, RV's, refrigerated trailers, cold storage insulation, sealing silos & storage bins, asbestos abatement and encapsulation, industrial piping, post mold remediation, mobile home skirt sealing.

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1. Assuming you are not required to fully fill the cavity, you can calculate the difference in energy loss through the roof with the foam at 4" and 5.5" over the course of a year, compare that to the cost difference between 4" and 5.5", and decide for yourself, based on whatever criteria you deem appropriate if it's worth it. My understanding is that shaving closed-cell foam is really hard, so realistically this is going to be more like 5" of foam since you won't be able to fully fill the cavity.

2. Where I work, if you are insulating an existing cavity that is not sufficiently deep to achieve the new construction code minimums, the code requires you to fully fill the cavity. This makes the question of whether or not it's worth it moot, because it's a requirement either way! Because foam is expensive (and not particularly green), a common approach is to do the minimum thickness of foam required in your climate zone for moisture control, and then insulate the remainder of the cavity with fiberglass or cellulose (aka "flash and batt").

Good advice, thanks! I previously moved on from flash and batt as an option due to the depth of the rafter bay (5.5"). Based on the linked article and elsewhere, I wasn't sure there was enough space for the batt to be as effective as simply more foam. But I'm going to review that some more.

2. The assembly will have very slightly better performance than 4" of foam (similar total R-value in the insulation, but increasing the thickness will reduce the effects of the thermal bridging at the rafters marginally. This may not be significant, but it's a freebie). It will be slightly worse performance than 5" of foam. But the differences are probably fairly marginal in all scenarios.

The calculations Paul Wiedefeld is demonstrating in their comment are how you would quantify the value of increased insulation and balance the value of more insulation vs diminishing returns, and if you're excited about learning to really optimize this thing, give it a go! But if not, you're not going to go too far wrong with any code-compliant way to fully fill your 2x6 cavity, whether that ends up being 5" of foam (close enough to fully filled), or a flash-and-batt approach.

In your #2 benefit, when you mention increasing the batt thickness, do you mean beyond the 3.5"? I'm interested in how batt insulation in general improves upon thermal loss. My understanding of thermal bridging mostly focuses on direct surface contact of air temperature to conductive materials. Once an insulated assembly gets say 1/2" drywall sheathing, would batting do more for thermal loss versus 4 or 5" of spray foam? I definitely plan on filling the bay with material.

Closed-cell spray foam has a greater R-value per inch than fiberglass batts, so for a given thickness, you'll always get more R-value from the foam than from the batt. However, due to other factors like thermal bridging and whether or not you can fully fill the cavity (depending on if the contractor is willing to shave closed-cell or not), the greater R-value of the closed-cell may be diminished to the point where it is not worth the cost beyond the minimal amount to make the assembly safe from condensation (more on that later) and you're better off filling the rest of the cavity with something cheap, like fiberglass or cellulose.

In #2, what I was trying to say is that when you compare 4" of foam to 2" foam + 3.5" batt, for a total of 5.5" of insulation, at the rafters, instead of your thermal bridge being 4" of wood, its now 5.5" of wood, which has a greater R-value. This was just to say that it is advantageous to fully fill the cavity, regardless of what you fill it with.

Regarding the flash-and-batt, the advantage is that you can potentially save money and have a similar performance. The affect of the thermal bridging of the rafters is such that the advantage of the greater R/in of the spray foam is diminished.

The reason that climate zone affects the ratio of closed-cell foam to batt insulation is due to the colder weather creating a greater risk of condensation. The important part is not just the r-value of the foam, but also that it is an air barrier and a vapor retarder. It needs to be thick enough for your climate zone that it prevents condensation. The language in the building code describing these rules is fairly convoluted, but it can be found in section IRC R806.5.

The table in R806.5 will tell you that in zone 5, the minimum R-value of your spray foam is R-20. It can be reasonably interpreted that that is R-20 for an R-49 roof, and the important thing in a renovation situation like yours is the ratio. To be safe from condensation, you need 41% (20/49) of your total R-value to be in the sprayfoam. 2" spray foam + 3.5" fiberglass results in about 47% of your r-value being in the spray foam layer, so you are safe from condensation risks.

You need to watch this video. I've watched a bunch from this guy and he's really informative. One of the things he doesn't mention in this video but in others is the dramatic ability of CC foam in help sealing the building. He also addresses why CC foam R values are not relatable to batts etc. because of this.

FWIW I'll be using CC in my renovation. 2" on the basement walls and 3" in the rafters. On the roof deck I'll be using 2" of Polyiso as well to meet code and address thermal bridging. Spray foam is expensive and not exactly green. I'll use it, but within reason, including ROI.

Thanks for posting that video for added context. It was actually one of the resources that lead me to this question. He doesn't mention thermal loss (via thermal bridging) which is something that the 2018 thread I posted brings up by a commenter. My circumstances prevent me from furring things out or adding layers above the roof sheathing to combat thermal issues. Not sure how to exactly factor in the thermal loss. Advice elsewhere suggested to expect no more than 40% loss on conventional wood framed roof deck. In my case 5" nominal rafters 24" with skip sheathing to plywood sheathing. But 40% is a lot and perhaps suggests maximizing insulation depth. But this contradicts the advice in the video, as I understand it.

The diminishing returns argument is false. The reason it's false is because it is related to percentages and not to occupant comfort. They are using the hidden assumption that because the percentage increase in retaining BTU is getting less and less with more insulation it also means that a person will not feel that difference in temperature. You definitely will. Those smaller BTU gains with each increment of insulation doesn't make how it will feel go away. You will feel it as cold and drafty until you get the insulation value close to what is prescribed in standard codes for building. That is a fact. The argument is completely bogus.

As has been said "There are lies, damn lies, and there are statistics". They are using diminishing return statistics to deceive both themselves and you. Their argument agrees with BTU statistics but the intuition of the statistics disagrees with the reality of how a house with insufficient insulation will feel to you. In reality the person will just turn up the heat (or cold) and use more energy. The insulation minimum requirements in the code books are there for a reason.