Roof Leaks and Spray Foam

If you’ve looked at spray-foam insulation for your attic lately, you’ve surely heard the pros and cons about open cell versus closed cell foam. For attic installations I always recommend open-cell foam for one overwhelming reason – roof rot. Every roof will develop a leak at some point in its life. How a roof assembly handles the incoming water can mean the difference between a $200 repair job and a $2,000 spot replacement.

To highlight how pervasive this situation can be, think back to all of the roofs replaced after Hurricanes Katrina, Gustav and Ike. How many times did you hear about contractors uncovering rotten wood from small or unknown roof leaks? These small leaks can exist for years without detection – primarily because the water seeps through the deck and dries to the interior of the attic. Because roof leaks are inevitable, it makes sense to create unvented attics with materials that have the same ability to dry out.

Open and closed cell foam differ broadly in two ways – flexibility and impermeability. Closed cell foam is rigid and resists water, open cell foam is flexible and allows water to pass through. When a roof leak develops, the impermeable closed cell foam traps water and holds it against the roof decking. Over time this leads to a large, rotten section of roof decking that will have to be replaced. Open cell foam, on the other hand, allows any water from a roof leak to weep through the foam and dry out in the space below. After the roof leak is repaired, the open cell foam will dry out and regain its thermal properties. Open cell foam’s permeability allows leak detection and repair with minimal or no damage to the insulation.

In addition to these differences between open and closed cell foam, open cell foam is usually less expensive than closed cell foam. For more in-depth information, go to Energy and Comfort Solutions.

This blog was written for Paul LaGrange’s BuildWrite website and was originally posted on March 20, 2009.

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