open-cell spray foam

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High Humidity in Spray Foam Attics

This is why you need to think of these spaces as conditioned attics

Posted on Oct 26 2016 by Allison A. Bailes III, PhD

I recently investigated an attic with spray foam insulation where we observed an interesting humidity pattern. We placed data loggers near the ridge and floor of the attic as well as in the living space and outdoors.

The graph at below shows dew point data for the four locations. The really interesting part is the big difference in dew point between the highest and lowest points in the attic, shown by the red and green curves in the graph.

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Image Credits:

  1. Energy Vanguard

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High Humidity in Unvented Conditioned Attics

What’s going on in these damp attics insulated with open-cell spray foam?

Posted on Aug 26 2016 by Martin Holladay

If you convert your vented unconditioned attic to an unvented conditioned attic by installing open-cell spray foam on the underside of your roof 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. , you may be surprised to discover that your attic is now the most humid room in your house.

Why? We don't know. Although building scientists haven’t achieved a consensus on the answer, we do have enough information to paint a picture of what’s going on.

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Image Credits:

  1. Image #1: Fine Homebuilding
  2. Images #2 and #3: Kohta Ueno

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Air Leakage Through Spray Polyurethane Foam

How thick does a layer of spray foam insulation has to be to qualify as an air barrier?

Posted on Sep 25 2015 by Martin Holladay

Many builders use spray polyurethane foam as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., raising the question: How thick does the spray foam layer have to be to stop air flow? There's a follow-up question, of course: Is the answer different for open-cell spray foam than for closed-cell spray foam?

As with most building science questions, there is a short answer and a long answer. The short answer is that closed-cell spray foam needs to be at least 1 or 1.5 inch thick to act as an air barrier, while open-cell spray foam needs to be between 3.0 and 5.5 inches thick to act as an air barrier.

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Image Credits:

  1. Rick Duncan, Spray Polyurethane Foam Alliance

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The Return of the Vapor Diffusion Bogeyman

For years, some builders assumed that they didn’t really need to worry about outward wintertime vapor diffusion — but it turns out that they might have to worry after all

Posted on May 15 2015 by Martin Holladay

Fully aware that I am engaging in gross oversimplification, I’m going to offer a cartoon version of the History of Vapor Barriers. (I’m not a cartoonist, though, so someone else will have to make the drawings.) Here goes:

Panel 1: In the late 1940s, residential building codes in the U.S. began requiring the installation of vapor barriers on the interior side of walls and ceilings. These requirements had complicated historical origins but were not based on credible building science.

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Open-Cell Spray Foam and Damp Roof Sheathing

When open-cell spray foam insulation is installed on the underside of OSB roof sheathing, the sheathing sometimes gets damp

Posted on Jan 3 2014 by Martin Holladay

UPDATED on July 8, 2015

Now that insulation contractors have been installing spray foam insulation on the underside of roof 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. for several years, we’re beginning to accumulate anecdotes and data on successful installations and failed installations. The anecdotes and data are enough to provide a few rules of thumb for designers and builders who want to install spray foam on the underside of roof sheathing.

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Image Credits:

  1. Mark Parlee
  2. William Miller, Andre Desjarlais, and Marc LaFrance
  3. Simon Pallin, Manfred Kehrer, and William Miller

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Can Spray Foam Rot Your Roof?

In the Q&A forum, Nick from Louisiana asked why these icicles appeared after he spray foamed the underside of his roof.

Posted on Jan 14 2010 by Daniel Morrison

Ice dams are a familiar problem in New England and other parts of the country where winters are long and cold. Snow on under-insulated and under-ventilated roofs melts, pools and refreezes to form a dam. Water backs up under the shingles and much to the horror of homeowners often finds its way inside the building.

Spray foam polyurethane insulation is supposed to be a hedge against that problem. By forming an effective seal around rafters, and offering respectable R-values, foam should be blocking the migration of cold air into the roof where it can condense into water.

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Video: Green Builder Won't Compromise on the Envelope

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Pro/Con: Open-Cell or Closed-Cell Foam?

Two spray foam experts explain their preferences

Posted on Oct 29 2009 by Martin Holladay

Everyone agrees that spray polyurethane foam does a great job at reducing air leakage. But which type of insulation makes the most sense: open-cell foam or closed-cell foam? presents two spray-foam experts — Jim Coler, the owner of Coler Natural Insulation in Ionia, New York, and Neal Ganser, a spray-foam consultant from Bozeman, Montana — who explain the advantages of the two main types of spray foam insulation.

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