Straight answers about insulation

Open cell or closed cell spray foam?

They are two different materials that happen to come out of the same kind of gun. Here is how they compare, according to the U.S. Department of Energy's Building America guide on spray foam [1].

What we tell people

For most jobs around here, closed cell is the one we recommend: rim joists, crawl space and basement walls, pole barns and metal buildings, and the underside of a roof deck. It insulates, seals air and stops vapor in one layer, and it does not soak up water. For foam sprayed on sloped roof rafters, the DOE guide rates closed cell Preferred in all climates, and open cell only Acceptable in cold climates, with vapor retarders [1].

Open cell has two real advantages: it costs less, and it is better at soaking up sound. That makes it a fit for interior walls and floors between rooms, where noise matters and water does not. If it goes against a roof or an outside wall in our climate, the DOE guide says interior air sealing and a vapor retarder are critical [1].

Doesn't open cell let you find a roof leak?

This is the question we get more than any other. The idea is that water runs through open cell, so you will see the stain on the ceiling and know where the roof is leaking.

Here is the problem. Open cell soaks water up. The DOE guide says open cell foam will also absorb and hold liquid water, and the amount can be significant; up to one-third by volume [1]. A slow leak can soak into the foam and spread out before anything shows, and a wet patch of foam does not point you to the hole.

Closed cell is hydrophobic and will not absorb water [1]. It bonds to the underside of the roof deck and keeps the water where it came in. In our experience, a leak over closed cell usually means replacing a sheet or two of roof decking around the leak, not the whole roof.

Stop with the stupid comments about open cell being better under a roof deck than closed cell because if there is a leak the leak either dries faster or you can see it sooner. Not true. There is no compelling evidence one way or the other.

Joseph Lstiburek, Ph.D., P.E., Building Science Corporation, who has worked on spray foam under roof decks since the early 1980s [2]

His bigger point is the one we agree with most: roofs leak, no matter what insulation is under them. Keep the roof in good shape and fix leaks when they happen. Pick the insulation for how it performs the other 364 days of the year.

What is flash and batt?

A thin coat of closed cell foam goes on first (the flash) to seal the air leaks and handle moisture. Then fiberglass or rock wool batts fill the rest of the cavity (the batt). You get most of the air sealing of spray foam for less money than filling the whole cavity with foam. The DOE guide lists flash and batt as one of the hybrid ways to use spray foam [1].

The one thing that matters: the foam layer has to be thick enough for the climate, so the inside face of the foam stays warm enough that the wall does not sweat. The DOE guide says the same of closed cell on roofs: it must be thick enough to prevent dewpoint condensation on its interior surface [1]. Matt Risinger and Spray Jones both have good videos on flash and batt below.

Does spray foam make a building stronger?

Closed cell does. In 1992 the plastics industry had the NAHB Research Center, the research arm of the National Association of Home Builders (today Home Innovation Research Labs), test spray polyurethane foam in wood frame wall panels. Compared with panels without foam or sheathing, the foam filled panels resisted 300% more racking force with T1-11 wood siding and 200% more with vinyl siding [3]. The industry paid for that test, so here is the government's version: the DOE guide says the racking strength of frame walls is substantially increased when ccSPF is used in the frame cavities, and that roof structure and wind resistance are enhanced with closed cell on the roof deck [1].

That is one more reason we like closed cell in pole barns, shops and metal buildings that catch the wind.

What about a basement or crawl space that can flood?

FEMA rates building materials by how well they survive a flood. In its Technical Bulletin 2, closed cell plastic foams and sprayed polyurethane foam are Class 5, highly resistant to floodwater damage. Fiberglass and mineral wool are Class 3. Cellulose and open cell foams are Class 2. Only Class 4 and Class 5 materials are acceptable below the flood elevation in a flood zone [4].

If your crawl space or basement ever takes on water, closed cell is the insulation that comes through it.

How much insulation should my attic have?

Ohio sits in climate zones 4 and 5, and ENERGY STAR gives the same attic recommendation for both: if your attic is uninsulated, add R-60. If you already have 3 to 4 inches, add R-49 [5]. A lot of older homes around here have about 4 inches. We measure what is there before we price anything.

Do you remove old insulation?

Yes. When the insulation in an attic is matted, dirty, wet or packed down, or a crawl space has batts lying in the dirt and torn plastic, we pull it out and haul it away before anything new goes in. Topping up bad insulation just buries the problem. We also do removal by itself when that is all you need. Here is an attic we cleared down to the ceiling.

Blown in fiberglass or cellulose?

Both work, and we install both. The difference is how much depth it takes. Settled, per inch [6]:

So fiberglass needs more inches to reach the same R-value, and it weighs far less doing it. We will tell you which one we would put in your attic and why. We also install fiberglass and rock wool batts in open walls, floors and crawl spaces.

Sources

1. U.S. Department of Energy, Building America. Which Spray Foam Is Right For You? Appropriate applications for open-cell and closed-cell foam insulation. Consortium for Advanced Residential Buildings, last updated April 3, 2009.
2. Joseph Lstiburek. Are You Doing Something Stupid? 3 SPF Topics That Might Have You Rethinking Your Decisions. Spray Foam Professional, Spring 2014. Building Science Corporation.
3. NAHB Research Center for The Society of the Plastics Industry, 1992. NAHB Building Wall Test summary, as published by Tiger Foam. Industry-sponsored testing.
4. FEMA. Technical Bulletin 2: Flood Damage-Resistant Materials Requirements, August 2008, Tables 1 and 2.
5. ENERGY STAR. Recommended Home Insulation R-Values, based on the 2021 International Energy Conservation Code.
6. U.S. Department of Energy, Building America Solution Center. Loose-Fill Insulation.
7. Robert Naini. Foam & Sound. Spray Foam Magazine, 2018.

Have a question that is not here? Call or text 740-590-5352, or tell us about the job.