Question - Do I need a vapor barrier if I use spray foam insulation?

Answered by: Gloria Murphy  |  Category: General  |  Last Updated: 17-06-2022  |  Views: 1135  |  Total Questions: 13

A vapour barrier is not necessary with closed-cell foam but with open-cell spray foam such as Icynene®, it is sometimes required. Any air that migrates though a building envelope will carry water vapour. As Icynene® spray foam creates a seamless air-seal, it controls air leakage and the moisture in the air. Since spray foam is an air barrier and insulation, it stops the air flow and moisture movement. Open cell spray foam is required by code to have a vapor barrier installed. To be considered a vapor barrier, a product must have a perm rating of 1. 0 or less for American building codes. Spray foam insulation is a great product. Homes insulated with it can be some of the most efficient and comfortable homes built. Since spray foam is both insulation and air barrier, proper alignment of insulation and air barrier is guaranteed. Open cell spray foam is not a moisture vapor barrier, and will allow moisture to pass. So, if you're considering spraying foam to the roof deck of your new or existing home, you want open cell. If there's a roof leak, water will pass through the foam, and you will quickly be able to find your problem. Open-cell foam is described as being vapor permeable. By contrast, closed-cell foam is said to be vapor impermeable. in extreme cold climates (Zones 6 and higher), a vapor retarder is used to restrict diffused moisture flow.

A: Tom Silva replies: Spray foam is great insulation, but it's fairly demanding to apply; the kit instructions have to be followed exactly. In fact, study them before you buy your kit so you know what you're in for. As with spray-foam kits, protect yourself and the floor from the dripping globs of canned foam.

Open-cell spray foam has an R-value 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.

Because it expands to fill gaps and cracks, spray foam is best used to air-seal and insulate the rim joist that extends around the house at the top of the foundation. This is where most energy-wasting air leakage takes place in a crawl space or basement.

“Normally, what we teach is that all foam applications should be installed in 3/4-inch to 3-inch-thick lifts. If foam is applied thick and quick — say, 4 inches thick — you gain yield. You can cover more wall with less foam. But the foam will be less dense.

If the basement wall is insulated on the outside, then the vapor barrier is superfluous; otherwise, is it dangerous, as it will trap moisture against a moisture-sensitive element, as I indicated. Should I include a polyethylene vapor barrier? No. Basement wall systems should never include any polyethylene.

Not every crawl space is made the same, so the size of the area to be insulated is the main contributor to the cost of the spray foam insulation project. The average cost to insulate a crawl space with spray foam is between $1, 200 and $2, 500.

The best insulation material for crawl spaces is rigid foam insulation board. Instead of installing the foam insulation between floor joists in the crawl space, the foam board is installed against the crawl space foundation walls.

As a product, spray foam insulation does not attract mold. And unlike wood or metal, spray foam does not rot, rust or deteriorate. This means that an area properly sprayed with foam insulation will never become a food source or a hospitable environment for a mold colony.

Closed cell foam has a higher R-value than open cell foam, usually about 6. 0 per inch. This higher rating makes closed cell foam better at keeping heat in or out of a structure. Open cell foams have an R-value of around 3. 5 per inch.

Insulating with spray foam stops the moisture-laden air from reaching the surface, keeping it from reaching the dew point and then the water vapor in the air does not turn into a liquid. The foam prevents the cold water from meeting the warm air resulting in a lack of condensation.

And there's also a pretty reasonable answer to will spray foam will rot your roof: No spray foam will not rot your roof. But water can. Open-cell spray foam not only insulates to improve temperature consistency, but it also seals cracks and crevices that allow air to leak into and out of your home.

But the best solution is actually to insulate both the attic floor and the underside of the roof. In this way you prevent the heat from the rooms below escaping to the attic rooms above. At the same time, the areas below the roof benefit from increased comfort both during the winter and the summer.

Open- or closed-cell spray foam is applied to the underside of the roof sheathing, and additional fiberglass or cellulose insulation is blown in as a cost-saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck.