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Sealing Strategies

Because framing details, air leakage paths, and insulation quality vary from home to home, there are no boilerplate solutions. Sealing air leaks into the attic is usually the most cost-effective improvement, but it can also be the most challenging. Fiberglass batts and loose-fill cellulose do not stop air leaks, so simply covering the leakage points with insulation doesn't work. Neither does sealing cracks in the attic floor above the insulation: Since the insulation is air permeable, you have to seal the leak below the insulation.

It's important to keep in mind that the drywall ceiling is discontinuous — it's interrupted by interior partitions and framed chases that enclose leaks. While those partitions and chases may appear airtight from inside the house, viewed from the attic they are full of penetrations for electrical, mechanical, and plumbing runs. Even the long intersections between the edges of the top plates and the cut ends of the ceiling board provide air paths into the attic.


Moving attic insulation uncovers a common cause of leaks: wire holes through partition-wall top plates.


The double line of soot stains indicates upward air leaks between the drywall and a partition plate.


Fiberglass stuffed into a plumbing wall was no match for the air rising from below.


A double layer of fiberglass did not prevent air movement through an interior soffit into the attic.

The first step in the sealing process is to move the existing attic insulation to expose the air leaks. These include penetrations in the middle of a ceiling, like air supply registers and light fixtures, as well as the top plates of all partition walls.

Two-part foam. For sealing the leaks, most weatherization contractors use two-part polyurethane foam, which comes in cardboard containers in various sizes with an attached 30-foot hose and spray nozzle. The foam sticks well to dirty surfaces, and the high-pressure applicator makes it easy to spray in hard-to-reach places.


Because it sticks even to dirty surfaces, two-part urethane foam works well for sealing around attic penetrations.


A worker seals beneath an attic ledger, after first chinking the gap underneath with fiberglass.

Small penetrations, like the gaps around light fixtures, fan boxes, and duct boots, can be sprayed directly. With larger bypasses — wet walls, unblocked balloon-framed cavities, the space under an attic knee wall — the opening can first be loosely stuffed with fiberglass, then sprayed. For extremely large openings like drop soffits and large chases, it's best to fit a piece of plywood or rigid insulation board over the hole, then seal it in place with foam and screws.


Rigid foam board is ideal for sealing large openings like this oversized framing chase. After cutting the board to fit, the worker beds it in wet spray foam.


Then seals the edges.

Fire-code sealant. One place where you can't use foam — due to fire codes — is around chimneys and flues. Because of code-required clearance to combustibles, it's not uncommon to find leakage areas of 3 square feet or more around a masonry chimney. Here it's best to seal the gap with sheet metal sealed with an ASTM 136 high-temperature caulk.

Adding Insulation

Once penetrations have been sealed, the attic is ready for an additional layer of blown insulation, which fills gaps in the fiberglass batts and also covers the tops of the ceiling joists, reducing conductive heat loss.

Knee walls. Field and lab testing has confirmed that batt insulation works best when it's enclosed in an airtight space. This is rarely the case with attic knee walls, which are typically open on the attic side. So I often recommend adding a layer of rigid foam to the back of knee walls and taping the joints. This prevents air movement into the wall cavity from the attic side, and cuts conduction through the studs. Another method is to add a second layer of fiberglass horizontally across the backs of the knee-wall studs and secure it with a layer of housewrap — again, to reduce infiltration and conductive loss through the studs.

If for some reason an attic wall has not been insulated, I recommend securing housewrap across the studs and blowing the cavities with loose-fill insulation.

Some knee walls contain ductwork or pipes that make them impossible to insulate effectively. In these cases, it makes sense to move the thermal and air barrier to the outside roof slope by adding a code-approved rigid board to the underside of the rafters and blowing in cellulose. Where the budget can handle it and for spaces with limited access, an approved spray foam also works well.

Don't forget exterior walls. Although this article focuses on attics, it's worth noting that we often recommend additional blown-in insulation in the exterior walls below. I say "additional" because many of the homes I visit already have some type of wall insulation, but the scans frequently show that it's settled or insufficient. Be sure to have the insulation contractor blow in dense-pack cellulose at such spots; otherwise, cold air in the walls will find its way along floor and ceiling cavities and make the house cold and drafty.

Looking Ahead

With energy costs increasing, the demand for weatherization is also on the rise. If you're a remodeler, this might be a good time to consider providing your clients with a more comprehensive approach to thermal upgrades, in addition to the traditional home-improvement services.

After the first energy crisis, we learned that simply adding insulation is not enough — you have to address air leaks as well. Hopefully that lesson is not lost, and with a new generation of diagnostic tools on hand, there's no excuse not to get it right.

Bruce Torrey is an infrared thermographer and building consultant. His company, Building Diagnostics, provides technical support and training to builders, architects, and insulation contractors.