Unvented Attics: Encapsulating with Spray Polyurethane Foam
Why do we vent attic spaces? The most common reasons are to reduce the amount of moisture accumulation, to prolong shingle life, to prevent ice dams, and to reduce the amount of heat gain through the ceiling below.
In 1948, researchers discovered that moisture can move from the ground into the foundation system and then wick up through wood framing, eventually evaporating into the attic space, where it can condense on the underside of cold roof sheathing that has reached dew point conditions. It can also move as water vapor through holes that connect spaces within the building enclosure to the attic space. The idea behind venting to remove moisture is to allow it to escape out, due to the stack or wind effect. Unfortunately for homes built in humid climates, more moisture is introduced into the attic space through venting—and often the attics in those homes have less-than-ideal ventilation. This exacerbates the moisture problem and does nothing to treat the cause of the moisture intrusion to begin with.
In 1997, the Asphalt Roofing Manufacturers Association jumped on the idea of venting attics to prolong shingle life. The reasoning behind this seems logical at first: Venting the attic should flush the heat out of the attic space under the roof deck, and reducing the roof deck temperature will prolong the life of the shingle. But why do asphalt shingles get hot? First, they are made with a really dark base, then they are coated with colored crystals that overall contain a really large surface area, and then we point them at the sun so they can absorb as much solar radiation as possible. In fact, research reveals that venting the attic can reduce the shingle temperature by less than 10ºF, while changing the color of the shingles from brown to white can reduce the shingle temperature by about 54ºF. This indicates (to me at least) that the color of my asphalt shingles has more to do with prolonging their life than venting under the roof deck.
Benefits of Attic Venting
Venting an attic space will prevent ice dams by creating a cold surface on the underside of the roof sheathing, preventing the snow on the roof from melting and refreezing. But why did the underside of the roof deck get hot? Typical culprits are fireplace chimneys, underinsulated attic floors, and holes between the conditioned space below and the attic that allow convective heat transfer between the house and the attic. So why not treat the cause instead of the symptoms by air sealing the attic floor, and then add insulation to the conditioned space boundary at the attic floor? What if a membrane were installed as underlayment that would prevent the damage if the snow did melt and refreeze?
Attic venting is also supposed to reduce the cooling load on the home by flushing the heat out of the oven over our heads—which it does! Attics that are vented to code minimums are generally about 25–35°F hotter than the outside air, depending on shingle color, roofing material, and whether a radiant barrier is installed. If the attic has less than code-minimum venting, the temperature above the conditioned space boundary is usually about 56°F hotter than the outside air in the summer. Is there a better way to deal with these symptoms and build a home that performs better? Absolutely.
Codes & Spray Polyurethane Foam
Otto Bayer invented polyurethane foam in 1937, and spray-applied insulation applications were developed in the 1960s for industrial purposes and really started to crack into the residential market in the 1980s. This material is an air barrier (if installed at the correct thickness) and an insulation material, installed in one visit using a two-component product. It is not a silver bullet, though—and many applications fail to meet the code requirements for the installation of spray foams.
A couple of years ago, building codes changed the testing required for spray polyurethane foam (SPF) applications—
specifically the flame and smoke spread testing mandated by the International Code Council Evaluation Service’s Appendix X. As a result, nearly all spray foam insulation commonly used in homes must have an intumescent coating applied to the exposed surfaces. A couple of manufacturers have produced spray foams that do not require the application of an additional product, but field trials conducted by one of my insulation contractor clients have clearly demonstrated that the amount of yield (the square footage insulated multiplied by the installed thickness divided by the total weight of the installed insulation material) is much lower than expected. This requires more material and increases the labor cost of installation.
Not all code inspectors are checking for intumescent coatings on exposed surfaces, and many unscrupulous insulation contractors do not include it in their bid and only bid the code-required coating if the client asks for it. This has unleveled the playing field among insulation contractors, simply because clients and code officials are unaware of the requirements.
Spray polyurethane foam that is installed on the underside of the roof deck and exterior attic walls (such as gable end walls) can create a dramatic improvement over the traditional, vented attic if the HVAC air handler and ductwork are installed in what is now a buffered zone—even though a lower R-value may be installed.
In a traditional, vented attic, the HVAC system is trying to heat or cool the home while in a hostile environment. We typically find that the greatest temperature difference in the home is between the temperature of the air moving through the supply duct and the air surrounding the supply duct. And what do we put between those to reduce the heat loss or gain? The lowest R-value we can find in a code-approved building assembly—R-8! Some installers try to get higher R-value by burying the ducts in the insulation, only to find out later that all they’ve done is create sweating duct work as the moisture in the air moving through the insulation condenses on the exterior of the ductwork that has reached dew point.
For SPF to really work well in an encapsulated attic, it should be installed by a trained and certified installer who fills the framing cavities evenly to the required depth and gets 1 to 1½ inches covering the roof framing. The underside of the roof deck should be free from moisture (dew) and radiant-barrier coatings; and the temperature of the material should be within the manufacturer’s specifications. If the temperature of the material is off by 1°F in either direction, the foam can shrink back from the framing, or voids may be created between the insulation and the exterior sheathing.
My dad taught me during one renovation we worked on that the five most expensive words are “While we are at it...” As a consultant, I’ve learned that they are “While you are around here...” Whenever I travel, someone will always ask me to take a look at a “problem” house, opening the conversation with that phrase.
One particular home stands out because the client had specified that the home would be insulated with spray foam long before construction started. The home was built right on the Gulf coast and is exposed to a nearly constant sea breeze blowing salty, moisture-laden air at the home.
The builder gave little thought to the insulation material and went ahead and installed a full soffit vent, ridge vents, and his standard radiant-barrier roof sheathing. The insulation installer had problems getting the mix of the two components right, and sprayed the hot foam against the foil radiant barrier on a cold morning. After the HVAC system was installed, water started to seep out of the wall and the attic became extremely humid. To address the moisture buildup in the attic, the HVAC contractor cut holes in the supply trunk to supply conditioned air to that space, effectively dropping the temperature, but raising the relative humidity until it started to drip moisture off the underside of the insulation.
We looked at the insulation installed along the roof deck and discovered that there were large sections that had not adhered to the radiant barrier; we observed lots of shrinkage from the framing; and the infrared camera indicated even more concealed voids between the insulation and the roof sheathing. When we isolated the attic space and performed an infiltration test using a blower door, almost half the infiltration in the entire home came from the encapsulated attic space.
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This really underscores the value of consulting a home energy rater during the construction process. It also underscores the fact that most construction failures aren’t due to defective materials but to defective installations. If a qualified energy rater had looked at the plans and specifications for this home; had caught the contractor’s mistake of using vented soffit and installing ridge vents and radiant-barrier roof sheathing; had looked at the quality of the insulation installation before the gypsum board was installed; and had followed the RESNET standards for grading the insulation installation—these problems would probably have been prevented.
It is easier to prevent problems than it is to fix them.
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