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Justin Fink’s Canned Spray Foam Tip

Have a spray bottle handy, so you can mist the area with water before using the spray foam

Posted on Jan 21 2014 by Martin Holladay, GBA Advisor

I've been paying attention to energy-efficiency and air-sealing tips for many years, but I still learn something new every week. This week, I learned a very useful tip from my fellow Fine Homebuilding editor, Justin Fink.

Justin wrote a great article on canned spray foam, “You Don't Know Foam,” that appeared in the current issue of Fine Homebuilding.

I've known for years that one-component spray foam (unlike two-component spray foam) is moisture-cured. I've also known that freshly installed foam pulls moisture from the air to cure, and that builders working in very dry climates (like Nevada, New Mexico, and Arizona) know that it helps to mist a little water on surfaces before using one-component canned spray foam.

I didn't realize, however, that misting water before using canned spray foam can make a significant difference in New England. Nor did I realize that misting water can double the yield from a can of spray foam.

Quotes from Justin’s article

Here is what Justin wrote:

“Polyurethane cures by reacting with the moisture in the air. If very little or no moisture is available, the light and fluffy foam will collapse into itself as it cures. A light spray of water applied to surfaces before filling voids with foam will go a long way toward getting foam to cure properly.

“This is especially important when working in areas of low humidity, but it’s also best practice when filling deep voids. In the case of deep voids, apply the foam in layers, misting lightly between them. It only takes an ounce of water to cure an entire can of polyurethane foam, so think in terms of a spray bottle, not a garden hose.

“Manufacturers of some cans of foam, both straw-dispensed and gun-dispensed, recommend misting water either into the cavity to be foamed or onto the uncured foam itself. Polyurethane is a moisture-curing resin, and the water is said to help speed up the curing process.

“I was interested to see if a light mist of water sprayed into the footing tubes before applying each layer of foam would help the foam to cure more fully and thereby eliminate the uncured pockets from the first experiment. Indeed, just two spritzes from a spray bottle before each layer of foam yielded foam that was firmer, that cured in minutes rather than hours, that expanded to roughly twice the yield of the cans in the first test, and that left hardly any voids.”

Justin's full article, “You Don't Know Foam,” is available on the Green Building Advisor web site as well as the Fine Homebuilding website.

Subscribers to the Fine Homebuilding website can read the article by clicking here.

Subscribers to the website can read the article by clicking here.

Martin Holladay’s previous blog: “Twenty Below and Off the Grid.”

Click here to follow Martin Holladay on Twitter.

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  1. Fine Homebuilding
Tue, 01/21/2014 - 09:10

Cold air is dry air
by Allison A. Bailes III, PhD, GBA Advisor

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I also saw that article last week, Martin, and it made me recall one of my first air-sealing jobs. I was working in an attic in January here in the Atlanta area, and I saw the foam from my gun get thin and wispy as it cured. It didn't collapse, but that's where I learned the misting trick.

You wrote:

I didn't realize, however, that misting water before using canned spray foam can make a difference in New England, where the air is not considered to be dry.

I believe it was your last article when you talked about how cold it gets in Vermont, and that's the key. The psychrometric chart makes it clear that cold air is dry air, so New England air is really dry in winter.

Tue, 01/21/2014 - 09:24

Edited Tue, 01/21/2014 - 09:31.

Response to Allison Bailes
by Martin Holladay, GBA Advisor

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I understand the psychrometric chart, and I certainly realize that indoor air is dryer during the winter than during the summer.

But I've been using canned spray foam for years, and it always cured just fine. So I concluded that the foam was finding all the moisture it needed to cure.

I had no idea that misting a little water would double the yield from a can of foam. That's the part that really surprised me.

So, Allison -- over the past few years, have you always misted surfaces with water before using canned spray foam? If you have, good for you. I haven't.

Tue, 01/21/2014 - 10:25

No, I haven't.
by Allison A. Bailes III, PhD, GBA Advisor

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I knew you knew that but wanted to make the point for other readers.

After learning firsthand about misting, though, I didn't apply that lesson consistently. I didn't realize the quantitative effect of adding moisture, just that it helped in winter, so I only carried the water with me when I noticed a decrease in the quality of the foam. After reading Justin's piece, I now know that I should have done it more consistently.

Also, since I'm not in the contracting business anymore, I haven't used my foam gun in a while. In fact, it may not even work anymore since I haven't cleared it of foam.

Tue, 01/21/2014 - 20:28

Carefully with the H20
by kye ford

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I would be cautIous with the water. A couple winters ago on a very cold day I spray foamed in all the windows on a new apartment building. The inside of the building was heated and there was condensation present on the tops of the window jambs. Long story short the spray foam went bananas and ended up bowing several of the window frames. Now I always make sure windows are nice and dry.

Wed, 01/22/2014 - 22:16

Edited Wed, 01/22/2014 - 22:18.

Access denied
by Derek Roff

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Both of the links for the complete Justin Fink article, that appear at the end of Martin's blog posting, take me to pages that allow access to members only. That is not unreasonable, but I suggest noting it with the links. On the other hand, if the links were supposed to allow access to anyone reading this blog posting, then something was amiss when I tried.

Wed, 01/22/2014 - 22:38

Response to Derek Roff
by Martin Holladay, GBA Advisor

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The reason I wrote this freely available blog was to share information from Justin Fink's article with people who might not have access to Fine Homebuilding.

It's well worth subscribing to Fine Homebuilding magazine, in my opinion. Even when I was an indigent hippie, long before I imagined that I might be an editor at the magazine, I subscribed.

Fortunately, GBA offers free 10-day trial subscriptions to our site. This free trial will grant users access to all GBA Pro pages. Details can be found here:

If you've used up your free 10 days, access is available for $14.95 per month.

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