Insulating: A Guide to the Basics
I haven't met many people who like to insulate houses. Maybe that's why I haven't met many people who are good at it, either.
Fiberglass insulation has the look and feel of cotton candy, but unlike the thin wisps of sugar that so easily melt in your mouth, fiberglass's thin glass fibers lodge firmly in your pores. The fibers also tend to break off and become airborne. Inhaled, they can aggravate respiratory problems.
If the thought of insulating makes you itch and sneeze, you know that it's tempting to cram the stuff into place and then cover up the job before anyone notices.The problem with a hasty insulation job, though, is that poor detailing around wires, pipes and other utilities can cut the insulation's advertised R-value by half. Even if you have a contractor do the job for you, taking on the expense of a contractor'ssurety bonds will prove less costly then the amount of money you'll waste doing an inadequate job.
You're more likely to do the work properly if you keep the fiberglass away from your skin, eyes, nose and mouth. Proper dress includes a long-sleeve shirt, a hat, goggles, a dust mask and a pair of gloves.
A TIGHT FIT
The biggest enemy of an insulation job is gaps. To maintain its R-value, insulation must fill the entire framing cavity, hugging the framing, sheathing and drywall.
It's easier to detect gaps as they occur if you use unfaced batts, and then cover the walls with polyethylene. Not only does the lack of a paper or foil facing make it easier to spot problems, but the poly is a better vapor barrier than the other materials.
Be sure to use the right size batts, which are made specifically to fit beween either 2 x 4 or 2 x 6 studs placed 16 inches on center. When insulating a narrower stud bay, the worst thing you can do is to crush up a full-width batt and force it into place; this inevitably leaves gaps in the back corners.
On the other hand, the batts should be wide enough -- say, 1/2-inch wider than the stud bay -- so that friction can hold them in place.
ELECTRICAL WIRES & BOXES
Cutting batts to width is relatively simple. Trimming them to fit around all the elements running through a wall or floor takes more time. For example, rather than simply stuffing insulation behind a wire, slice the back of the batt at wire height.
To trim around an electrical box, fit the batt into place behind the box and the wire, and then cut around the box. Remove the batt, slice it to fit around the wire, and reinstall it. The insulation behind the box stays in place.
PLUMBING & DUCTWORK
When plumbing or ductwork runs through an exterior wall, there may be no way to get the batts into place without leaving gaps. Another problem is that in cold climates, fiberglass alone may not provide enough R-value to shield pipes from freezing temperatures.
You can solve both problems by sliding rigid foam board between the pipe and the sheathing. (Rigid foam packs a lot more R-value per inch than fiberglass.)
If you're working in a new exterior wall, install the foam before the mechanicals so that you can fill the cavity's width with a single board. For existing walls, cut a few large pieces to fit the space, leaving a quarter-inch gap between them. Fill the gaps with an expanding foam.
CEILINGS
The Achilles heel of a ceiling is the recessed downlight. Insulating over a recessed light can cause it to overheat and start a fire. (Some recessed lights run as hot as 200 degrees Fahrenheit.)
Most building codes require 3 inches of clear space around any recessed fixture. Of course, a 3-inch gap can slash the R-value of your ceiling insulation.
Consider replacing the existing fixtures with ones that carry an "IC" rating from Underwriter's Laboratory. This rating certifies that the fixture can be safely covered with insulation.
FLOORS
Whether or not to insulate a floor depends on where you live. In climates where houses are heated for most of the year, insulate floors over unheated basements or crawl spaces.
In warmer climates where air-conditioned houses are the norm, take the opposite approach. The cool crawl space will draw heat through the floor, easing the load on the air conditioner.
The same rules apply with floors as with walls. Batts should contact the subfloor and both joists without being crushed or compressed, and should be carefully trimmed to fit around wires and electrical boxes.
To prevent bunching, use batts that are as thick as the floor joists are deep. They will cost more, but the extra insulation will pay for itself over the long term.
A common oversight in the process of insulating a floor is the band joist that runs around the house perimeter.
Before insulating the floor itself, lay pieces of foam or batt insulation against the band. If there's a vapor retarder facing on the insulation, make sure it faces the basement rather than the band.
Another typical practice in installation is to use wires to hold the batts in place. These wires compress the insulation and open gaps along the sides of the joist bay.
Alternative materials for securing insulation include construction mesh, housewrap, twine and plastic. They can be stapled to the bottoms of the floor joists.
WINDOWS & DOORS
Many remodelers use fiberglass to insulate around windows and doors. Yet a lot of heat escapes from these areas, and fiberglass isn't very effective at stopping drafts.You can get high R-values and an excellent seal by filling the framing spaces with a spray foam instead.
Choose minimal-expanding or nonexpanding foam. While expanding foams are useful for plugging electrical, plumbing and HVAC holes, they can cause windows and doors to bind.
To save on costs, use the material judiciously. Spread an inch of foam against the outside of the cavity and fill in with fiberglass.
Be sure to wear gloves and safety glasses, though. Foam is hard to get off your skin once it starts to dry. You'll have to scrub it off with a mechanic's hand cleaner.
Finally, to wrap up your expert insulation job, take a long, cold shower. The cold water will close your pores and prevent the glass fibers from becoming embedded in your skin.
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