August, 2005

How to Install Insulation

5 Ways to do it Better

By Dave Griffin

Residential insulation options have never been better, and consumer demand is driving builders to spec thicker layers of the products available. But a high-performing insulation job depends on more than just good materials in generous quantities. For insulation to work right (not just look good on a real estate flyer) three conditions must be met: the thermal envelope must be air-sealed; the insulation needs to be put in the right place; and the insulation needs to be installed right.

Why seal the thermal envelope? Up to 40% of a house’s energy loss is from air leaks. Air leaks have nothing to do with R-value and are not typically stoppered by adding more insulation. If you want to offer performance, you have to address air leaks. Here we discuss ways some insulation materials can be used to air seal, and we touch on other methods of plugging the gaps.

Insulation placement affects energy performance of the house. For example, many builders insulate walls well but leave the foundation un-insulated, even when the basement will be heated. A seven-inch thick poured foundation wall offers less R-value than an average window, so this can be an energy-costly omission. Moving up, wall cavities are typically insulated, but what about framing members? Heat can travel right through these. We will look at ways to position insulation in walls for greater effect.

Of course insulation is useless until it’s installed and only attains its peak value if installed right. Fortunately, the best insulation manufacturers offer a wealth of information on how to apply their wares. You owe it to the end users of your product— the people who will live in its embrace—to learn as much as possible about installing it correctly. Here we offer a few tips on batt installation, as this is a commonly misapplied form of insulation.

A leaky house will be energy inefficient, too, even if well insulated. The way to foil air leaks is to create an impervious air barrier, preferably in the same place as the thermal envelope. (The thermal envelope of a house is what divides heated and cooled space from unconditioned space.) Taping and sealing at vapor barriers, flashings, and the weather-resistive barrier helps with air sealing. In the real world, however, these barriers are usually done imperfectly and later damaged by knife- and drill-wielding trades-people.

What’s an insulation contractor to do? There are a number of spray-on insulations for use in framing cavities that help to seal the thermal envelope. Damp sprayed cellulose and spray fiberglass treated with adhesive help with air sealing at a competitive price. Spray on products form-fit around framing and other obstacles, sealing off open conduits for air. Form fitting also maximizes the inherent R-value of the material and makes these insulations excellent sound attenuators. More dense spray-on and blown-in insulations stop more air than looser, fluffier applications.

Polyurethane foam-in-place insulations do a superb job of air sealing. If you use a more porous insulation, at least consider using a polyurethane spray product around window and doorframes. Special minimally expanding foam formulations designed for windows and doors won’t bow the frames but will provide an airtight barrier at these air-vulnerable locations. More aggressive expanding foams can be used to seal less delicate gaps.

Whole house foams require designated rigs, but the demand for large-scale foam insulation is growing, especially since the foams have become safer for people and the environment. Foam seals the cavity stud to stud. Wires, electric boxes, nailers, bridging—it all gets enveloped and sealed like chips in cookies. If the roof deck follows a ceiling, such as with the now-popular cathedral ceilings, the rafters can simply be foamed in. There’s no practical need for roof-deck ventilation where foam is used, since there is no scope for moist air to escape from the house to the cool roof deck where condensation would occur. Low-density foams like Icynene offer about R-3.5 per inch. Higher density R-7 polyurethane foams offer the best residential R-values in the industry, but these are expensive.

Because of the stack effect (see sidebar), attic-level and foundation-level air sealing is extremely important. The sill/foundation juncture and basement-window /foundation junctures are problematic spots that should be sealed by the carpentry crew (see image). Sometimes an outside-ventilated crawl space will open into a conditioned basement providing a wide-open air path between inside and out. A ventilated crawl space should be sealed off from conditioned areas whenever this situation is encountered.

Alternatively, crawl spaces may be treated as conditioned space and not ventilated to the outside. Instead, the soil in the crawl space is covered with a cross-laminated vapor barrier that’s overlapped and taped. The vapor barrier is sealed to rigid foam insulation at the walls. The insulation is taped or sealed at the joints and sealed to the sill plate and, if code requires, covered with drywall. With proper air and vapor sealing, radon and moisture loading from the soil is taken care of without outside ventilation, and the crawl space is warmed and cooled along with the rest of the house.

Perhaps the most pernicious air heat transfer occurs between an unconditioned attic and the living space below. The insulation contractor should seal any gaps that could allow air to flow between the conditioned space and the attic before the insulation is installed. Blown in cellulose and fiberglass (blown in or batts) will not stop airflow and will not stop convective heat loss and gain. Existing fiberglass insulation will often reveal air pathways with dust trails, like a used cigarette filter.

Your arsenal for air sealing at the attic includes caulk, expanding foam, rigid foam for blocking, and fiberglass for packing tightly around potentially hot pipes and chimneys. Common pathways to the attic, called "attic bypasses,” include spaces around pipes, ducts, chimneys and wires; spaces around light fixtures in the ceilings below; spaces above interior walls; and the cavities between joists that cross kneewalls or cantilevers between conditioned and unconditioned space.

Fiberglass batts are one of the least expensive and most versatile ways to get R-value in walls. But despite the ubiquity and seeming simplicity of fiberglass, it’s frequently misapplied. Gaps at the framing and at the backs, fronts and corners of the cavity rob fiberglass of R-value. Perhaps worse, gaps and channels allow convective currents to form within the framing cavity. This happens as air in the cavity is unevenly heated and expanded, creating mini weather systems that move heat on air. If air sealing is poor, gaps make the situation worse. Fiberglass is like a sweater: it’s a great insulator in dead air space but is easily robbed of its insulating properties by moving air.

The first rule of fiberglass, then, is to leave no gaps. The batt should friction fit the framing at the sides. When applying the batt, it should first be pushed firmly to the back of the cavity, especially at the sides where gap-formation is common. The front of the batt should then be pulled forward until it’s flush with the edges of the framing. When inspecting a job, look for shadow lines indicating seams between the batt and the framing and look for exposed shoulders on the framing indicating that the batt wasn’t pulled forward all the way.

Tables friction-fit or unfaced batts are better at getting a flush and grooveless fit with the framing. Wires and pipes should be accommodated by slicing the fiberglass. Irregular framing and electric boxes should be cut around. Compressing the fiberglass to fit around obstacles creates wrinkles and gaps. The gaps are worse than the R-value lost by the compression itself. Finally, to achieve the best R-value for a given space, use high-density batts.

Choose the right size unfaced batt for floors, then make sure they’re supported firmly against the subfloor. Importantly, 16-in. o.c. wood I-joist floors require a 16-in. commercial batt instead of the 15-in. wide batt designed to fit between thicker, solid-sawn joists. Use push-rod joist hangers every two feet to keep batts tight to the subfloor. Stapling faced batts to the joist will generally leave a gap between the floor and the insulation, markedly diminishing its effectiveness. Also, where houses are heated more than air conditioned, stapling faced batts puts the vapor barrier toward the cool side, which is incorrect. As with walls, cut batts around obstacles and weave bats through bridging.

Work with other contractors to make sure you have access to all joist bays when you need it, such as those in cantilevers and over the plates. Sometimes framers will use an extra joist as a drywall nailer, making it impossible to insulate the cavity next to the parallel rim joist. Request that the framers tack a 1X drywall nailer flat to the plate instead, so you can insulate this critical joist bay.

Wall- and floor-framing cavities that are backed by a ventilated space need to be air sealed from behind. Otherwise, moving air can rob the insulation of R-value. For example, kneewalls in finished attics should have some kind of backing to the insulation. In heating and mixed climates, use a vapor permeable material such as housewrap or polystyrene foam board, which is semi permeable to water vapor. In primarily air conditioning climates, a vapor barrier may be put to the outside of the insulation.

Cavity insulation takes care of the cavities, but what about the framing? Fifteen to 40% of wall area is backed by framing and therefore is un-insulated. Wall areas with lots of windows can have R-values up to 40% less than blank walls because of the extra framing. (This R-value reduction only measures the opaque wall areas, not the windows.) Wood has an R-value of about one per inch, so framing constitutes a pathway for heat loss (or gain). If metal framing is used, which is highly heat conductive, the pathway becomes a superhighway. Heat conduction through framing, called thermal bridging, is a hot topic in the building science community. You can cool it down significantly in the houses you insulate with rigid insulation.

Rigid insulation applied outside non-insulated sheathing or directly to the studs covers the whole wall like a coat, blocking thermal bridges. Foam-based sheathings applied directly to studs also blocks heat transfer through framing members. Foam insulation or sheathing also adds to the cavity insulation, mathematically improving the R-value here. When the rigid insulation is sealed, taped and integrated with window and door flashing, it also makes an exceptional air barrier and substitutes for a separate weather resistive barrier (like housewrap). If wall foam is integrated with rigid foam used over the foundation walls, it may additionally block airflow between the sill plate and the foundation.

To be effective against heat conduction, rigid insulation must be thick enough. Some siding companies try to pass 1/4-inch extruded polystyrene R-1 fanfold off as a significant insulation layer. By contrast, an inch-thick polyisocyanurate sheathing or insulation board can offer R-6.5. If you are going to expend the labor to apply rigid foam, you might as well offer some serious R-value.

Houses lose heat in the winter and gain unwanted heat in the summer through three mechanisms: conduction, convection and radiation. Conduction is the tendency of heat to flow across a solid from the warmer side to the cooler side. R-value is simply the way we measure how effective a material is at resisting heat conduction. A metal and a foam coffee cup illustrate how different materials conduct heat at different rates. Even if the metal and the foam cups were the same thickness, the metal would conduct heat faster. It has a lower R-value. Note that conduction doesn’t respect gravity: the heat of the coffee flows as readily out the bottom of the cup as the lid. Likewise, heat moves as readily through floors as ceilings. More insulation is recommended in attics than floors because it is less expensive to put it there, not because heat rises.

Convection describes how heat moves on air and also how heat helps air move. For example, the air in a heated house is lighter than cold outside air, so it rises (helping support the myth that heat itself rises). The rising warm air escapes forcefully through even small gaps near the top of the heated space. The lost air is replaced, equally forcefully, through gaps low on the house. This is called the stack effect, which explains why air sealing is so important in attics and at the foundation. In a cooled house, the stack effect works in reverse. The relatively heavy cooled air drops, oozing out gaps low on the house. Solar heated air near the roof deck is dragged, against its will, after the falling cool air to fill the void. Some people intentionally leave doors open into the attic in hot weather, which makes matters worse. Attics over houses are hot in the summer because roof decks are hot, not because hot air is moving up from the house. Unconditioned attics should be ventilated over air-conditioned and heated spaces but sealed off from the rest of the house.

Radiation is energy that heats materials. For example, radiation moves through the frigid near-vacuum of space to heat the earth. Radiation explains why a cat in a sunbeam is warmed, even though the air in the sunbeam is barely warmer than the air elsewhere in the room. Some radiation is visible light, and enters through windows. Infrared radiation, however, can pass through opaque materials. Radiant barriers are thin membranes that reflect infrared radiation. They are cost effectively applied to roof-sheathing panels for use in southern climates. Reflective coatings on insulation products block infrared radiation, too. Radiant barriers do not substitute for thermal insulation because they don’t stop heat conduction.