From the attic to the basement, insulation is like a down jacket around your house that keeps you warm in the winter (and, like a thermos, cool in the summer). In many houses, it is probably easier to insulate than it is to seal air leaks, though it’s a big job if you need to insulate exterior walls. Attics are relatively easy to insulate, and most houses can benefit from some added insulation. Regardless of the age of your house, don’t be fooled just because there’s some insulation in the attic—even if it looks like a lot. Some homes have a thin layer that does little good; others have insulation that was installed in only the areas that were easy to reach. Even newer homes that have a lot of insulation can still have significant gaps and installation problems that are worth fixing—by someone who cares enough about the details to do it right.

Be aware that there are some situations that need to be addressed before you can safely insulate a house. Roof leaks or other moisture problems should be fixed; any old knob-and-tube wiring (common in houses built before about 1940) should be replaced or deactivated; and attic venting may need to be added to meet codes, if you are insulating the attic. An energy auditor or home performance professional can help you look for and understand these issues before you proceed with work.

Attic Insulation


Attics are typically insulated first, and they usually receive the most insulation. People usually assume that attics are insulated because “heat rises,” but heat doesn’t actually rise at all. The reason attics tend to get more insulation is much less glamorous: The attic is simply the cheapest and easiest place to add insulation, in both old and new houses. In fact, in a moderately insulated two-story house, there may be two to four times as much conductive heat loss (not including air leakage or windows) through the exterior walls as through the attic. Perform air-sealing before you insulate, unless you plan to insulate with sprayed foam and complete the air sealing at the same time. Also, if you intend to vent a bath fan, install a ventilation system, change any electrical wiring, or do any other work in the attic, now is the time.

How much insulation?

Insulation is measured in R-value; the higher the R-value, the less heat loss. To some extent, there is an effect of diminishing returns in that the first 6 in. of insulation will save you more money than the next 6 in. But it doesn’t pay to skimp, either. Once you’re up in the attic installing insulation, you may as well do it right. Generally, the minimum recommended R-value for an attic is R-38. In cold northern climates, aim for between R-50 and R-60.

Fiberglass Batts​

Fiberglass batts are typically the insulation of choice for do-it-yourselfers. The material is easy to transport and install by yourself. It’s ideal for an attic where you may want to add living space later (or make other changes), because it is easy to remove and replace. But there is a price to pay for convenience. No matter how carefully you detail fiberglass batts, they rarely work as well as the factory ratings suggest. Because of the annoying, itchy quality of fiberglass, several manufacturers market encapsulated batts that are surrounded by plastic. Steer clear of these products; they’re more of a marketing gimmick than a worthwhile innovation. Encapsulated batts are more expensive and harder to install properly. Although the coverings may make the material more comfortable to work with, they also increase the difficulty of detailing the batts properly.

Blowing attic insulation​

If your roof is made with trusses, or the attic has flooring with little or no insulation under it, it’s hard to get good results with fiberglass. In those cases, your best bet is to use a loose-fill material, such as cellulose. One big advantage of blown cellulose is that it is fast and easy to install. You don’t have to worry about cutting, fitting, fluffing, or most of the other details that concern fiberglass batts. You can also cover the top of existing insulation with cellulose, which will settle into all the gaps and spaces between the batts. This improves what’s already there and adds R-value.

Also, the flame-retardant chemicals used to treat cellulose tend to discourage insects and rodents from nesting in it.

Storage areas

People often use attics for storage. Eliminate attic storage, if possible. There are plenty of good reasons to get rid of stuff (both thermal and psychological). Still, many homes lack storage space, and the attic is a natural place for those boxes of holiday decorations. If you have a partially or fully floored attic, you can set aside an area near the access hatch or stairs by making a dam out of 12-in.-thick fiberglass batts. Insulate the rest of the attic, making sure there is insulation under the floorboards as well. Once the dust has settled, you should be able to move stuff in and out without disturbing the insulation.

Fiberglass batts are the simplest and most common do-it-yourself type of attic insulation. Photo Credit: Randy O’Rourke

Cellulose insulation is widely used by contractors to effectively insulate attics. Photo Credit: John Curtis

Sloped Ceilings


Many homes have sloped ceiling areas—places where plaster or drywall is applied directly to the underside of the roof rafters. Examples include Cape-style houses, which usually have sloped ceilings between the knee wall and the flat ceiling above or rooms with partial or full cathedral ceilings. Some homes just have a narrow-sloped area for a few feet near the eaves. These enclosed cavities present more of a challenge than an open attic. Some, like the typical Cape, have fairly easy access from an attic space either above or below. Others, like a full cathedral ceiling, are more difficult. The biggest distinction between dense-packed and open-blown cellulose is that dense-pack is installed in an enclosed cavity. But it’s not enough just to be enclosed—to dense-pack, by definition, means using enough air pressure to compress the material more densely than it could ever compress from settling.
Reducing moisture risk
There are serious concerns relating to venting a cathedral- or flat-roof ceiling space. Good air sealing (which dense-packing helps) and indoor humidity control via mechanical ventilation help reduce condensation and moisture buildup. But even an excellent dense-pack job can allow some air movement. In an unvented cathedral ceiling or flat roof, this can deposit moisture at the roof deck, especially in a home with high humidity. There are two basic strategies to avoid increasing the risk of condensation and potential damage to the roof deck: using foam insulation to control condensing temperatures, and ensuring an opening from the unvented cavities into a larger, vented space. The first approach—using continuous foam insulation—is the only proven, code-approved method for an unvented roof. It can take two basic forms: rigid foam insulation added above and in contact with the roof deck, or sprayed foam added directly to the underside of the roof deck. Rigid foam is typically added during a re-roof, but usually a new sheathing surface (with a furred, vented space in heavy snow areas) is needed on top of the foam. This approach is expensive, but it’s a practical way to address a house with full cathedral ceilings if the roof is close to needing replacement. Consider interior sprayed foam insulation if you are planning extensive interior renovations (see “Renovations” for more discussion). With either foam method, once the roof deck is protected from condensation by a high enough R-value of foam, the remainder of the cavity may be insulated with blown insulation or batt (called “flash and batt”). The R-value of foam insulation depends on climate (see the table on the facing page and the map below). Both of these methods are explicitly allowed by code; both are energy efficient and low risk in any climate. The second approach is to provide a partial venting path for the closed dense-pack area. This can be achieved in homes that have sections of sloped ceilings (such as Cape-style homes) that may be difficult or impossible to vent properly. If one end of the dense-packed area is open to a vented attic space (preferably the top), any wetting effects appear to be balanced by drying toward the vented space. This approach can also be used under low-slope roofs (for example, a row house or shed dormer), where access near the low side is impossible. Experience has shown that up to one-third of the total attic area can be dense-packed without venting, provided that the remaining attic space is vented normally. Note that this method does not conform to standard code requirements but has been accepted by many local building officials. And I would consider this approach much riskier in climate zones 6 to 8. Of course, just as with an open attic floor, you have to be careful to avoid chimneys and non-IC-rated recessed lights. If you have a chimney in a key leak area, you may want to consider using a high-performance fiberglass. You could also open up the ceiling in that area to air-seal the chase with sheet metal.

Insulating Walls


If you have little or no insulation in your walls, you will definitely benefit from installing some. Conductive heat loss through uninsulated walls represents a significant heating and cooling load, and it’s usually cost-effective to add insulation. Exterior walls are fairly difficult to insulate, unless you are remodeling, installing new drywall, or re-siding the house. Retrofitting insulation almost always means blowing in insulation; the process is very much like that for sloped ceiling dense-pack outlined above. The type of siding has a big impact on how easy it is to get into the wall, and it may help you decide whether to tackle the job yourself or hire an insulation contractor.

Existing Insulation in Older Homes

Most homes built before the 1930s had no insulation at all, so if your home is from that era, you will be looking for insulation that was added later. It may be blown-in cellulose or urea-formaldehyde foam. If the walls are already insulated, it is rarely worth adding more, unless you find large areas that were never done at all, or if the walls were not insulated properly. If the installed insulation has settled and each wall bay has a large empty space at the top, it may be worth filling them in. Also look out for “back-plastering.” Although unusual, backplastered walls are built with an extra layer of lath and plaster in the middle of the stud space, to add an extra airspace and reduce heat loss through the wall. Those walls are difficult to insulate due to the narrow space, and it’s probably better to leave them alone unless you do major remodeling or re-siding. Homes that were built prior to 1960 may have minimal insulation; some have insulation that is sandwiched between layers of paper. It is possible to add cellulose insulation to walls with those thin insulation layers, but only with a fill tube. If the wall has thicker batts, it will probably be almost impossible to get the fill tube into the wall cavity without bunching the insulation; in that case, it won’t be worth trying to add more.

What’s in there now?

How do you know what’s in the walls already? If you have done any remodeling on the house, you may already know. If not, there are a several ways to find out. It’s important to know what is in there, because that will have a big impact on how easy—or useful—it will be to insulate the walls. Remember that there may be different types of insulation (or none at all) in different parts of the house. If some rooms have been remodeled, or built at a different time, they will likely be insulated differently. Sometimes people start to insulate a house and don’t finish, so look in several places to be fairly certain of what is there. If the insulation is spotty or inconsistent, an infrared camera can identify all the areas that are uninsulated. An energy auditor or home performance professional can help with an infrared scan of your house if it’s appropriate.

Insulating Floors


Some floors need insulation, and some do not. A basement with a furnace or boiler is typically warm enough that insulating the floor won’t save you much, if anything. Seal and insulate foundation walls to help keep that heat in the house. Most people want their basements to be at least moderately warm, at least to reduce the risk of pipes freezing and dry things out a bit. If you don’t mind having a very cold basement, you can combine floor insulation with insulating and sealing the ducts (or insulating heating pipes) to help keep the heat in the upper floors. Some floors that should be insulated are cantilevers; floors over garages, on stilts, or on piers; and slab-on-grade floors in cold climates. The open side of a conditioned walkout basement is also a slab-on-grade floor.

Insulating an enclosed-cavity floor with cellulose

Any floor that is exposed to outdoor conditions or unconditioned space and has solid sheathing on the underside can be insulated with blown-in cellulose. Examples include garage ceilings, exposed heated porches, cantilevered overhangs, and sheathed floors built on piers. The process is the same as that for filling cavities in cathedral ceilings or walls. You must drill holes in each bay, and then patch the holes when you are done. In termite-prone areas, exterior rigid foam is not recommended, because insects can hide behind the insulation on their way into the house. Some manufacturers make insect-resistant polystyrene-foam products. Typically, those products do not have insect-repelling qualities that will protect your home; they only prevent critters from nesting in the material. Check your local building code to see whether those products are acceptable for such applications in your area.

Insulating a slab floor

An uninsulated slab floor can lose a lot of heat if the edges are close to or above grade level. It can be cold and uncomfortable in the winter, and cool and damp in the summer. If the floor is unfinished, or you are planning to remodel anyway, a layer of extruded polyethylene, with 1×3 furring as a nail base, makes a good foundation for a plywood subfloor. Slab floors that are more than a few feet below grade don’t cause much heat loss, but they stay cool in the summer due to ground contact. These surfaces become magnets for condensation in humid weather. For this reason alone, a slab floor may well be worth insulating.

Insulating Foundation Walls


If you have a basement or a crawlspace, chances are pretty good that the walls are not insulated. If you have heating equipment in the space, it is probably not very cold in the winter, either—the thermal boundary is ambiguous. If you use it for anything besides storage, or if you are planning to finish all or part of the basement later, insulating the foundation walls rather than the floor above. I also recommend insulating the crawlspace walls instead of the floor joists. Remember that if you do insulate overhead, you will also need to insulate the furnace ducts or the heating pipes in the basement. Most of the heat loss in a foundation wall occurs near the top of the wall, where it’s exposed to outdoor temperatures, but I recommend insulating the entire wall. The cost to insulate the entire wall is not much more than doing just the upper half. Also, if you insulate and finish the entire wall, you will have a much nicer finished space in the basement.

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