Homes built before the
early 1970’s were designed in an era when gasoline was less than 30 cents a gallon, heating oil was cheap and natural
gas supplies were plentiful. Few people worried about energy efficiency because there was energy to spare. Builders and homeowners
didn’t spend money on insulating houses because it was cheaper just to turn up the heat. But times have changed. Modern
insulation standards are considerably higher.
Most homes have an accessible attic with exposed ceiling framing. That makes it easy to check the depth of
insulation and apply more when needed. Before about 1970, the most common attic insulation, if there was any, was reflective
foil (in warmer climates) or a few inches of rock wool (in colder climates). Optimum insulation still varies with location.
But homes in milder climates can benefit from 6" of fiberglass above the ceiling. In colder climates, you need 12".
If the home is heated by electricity or a means more expensive than gas, propane or heating oil, extra insulation usually
makes good economic sense.
The effectiveness of insulation is measured in R-value, which is the resistance of the material
to heat transfer. The higher the rating, the more insulating value. For example, 12" of fiberglass insulation usually
carries an R-38 rating. Six inches of the same fiberglass will be rated at R-19.
Batt and roll blanket insulation is
made to fit between joists and studs installed either 16" or 24" on center. When you see coverage figures for insulation,
those numbers include framing area. For example a roll of insulation described as "covers 80 SF" will fill a wall
area measuring 8’ by 10’.
If a vapor barrier is already in place, or if you’re installing a separate
vapor barrier, use unfaced rolls or batts. Otherwise, install Kraft-faced insulation. However, there’s one exception
when adding more insulation in an attic space. Even if the existing ceiling insulation doesn’t have a vapor barrier,
install unfaced insulation.
Loose fill insulation is just as effective as rolls or batts and it’s easy to install
in an attic. Simply pour insulation between the joists and screed it off to the right thickness.
Any time you remove interior wall finish, check the insulation. Walls in mild climates need R-13 insulation.
That’s 3-1/2" of fiberglass, which fits perfectly in the cavity of a 2 x 4 stud wall. R-19 wall insulation is appropriate
in severely cold climates. But that requires a 6" wall cavity and 2 x 6 studs, which may not be cost effective. If you
need more than R-19 from an existing 2 x 4 stud wall, 1" polyurethane sheathing can add R-6.
is easy when the wall cavity is open. When the cavity is closed, you can blow cellulose insulation through a 1"-diameter
hole cut in siding and sheathing. Open a hole at the top of each stud space. Lower a plumb bob into the cavity to find where
the fire block is installed. Then drill another hole in the same cavity below the fire block. Blow cellulose into the stud
space both above and below the block. When the cavity is filled, patch entry holes with either plastic or wood plugs.
Houses with a furnace in the basement don't need insulation between the floor joists. Basements are generally
heated. But homes should have floor insulation equivalent to the wall insulation if the floors are located above an unheated
crawl space or any other unheated space. Fit insulation batts between the floor joists. Hold the batts in place with wood
or wire strips cut slightly longer than the joist space so they spring into place. You could also staple wire netting under
the floor joists to hold either batt or blanket insulation tight against the underside of the floor sheathing. A vapor barrier
has to be on the top side of insulation placed between floor joists. That presents a problem when installing insulation above
a crawl space. Insulation manufacturers now offer roll insulation with inverse tabs so insulation can be stapled between floor
joists with the poly face-up against the floor sheathing.
You can insulate brick, block and concrete walls by applying insulation board to the interior surface. The
thicker the board, the higher the R-value. Polystyrene insulated sheathing board has an R-value of 3.5 per inch of thickness.
Polyurethane board carries a 6.25 rating per inch of thickness. Many types of insulated sheathing can be installed over the
existing siding. No tear-off is necessary, though additional edge trim will be needed around windows and doors. If the owner
wants 2" of foam insulation on a basement wall, consider attaching 2" x 2" furring strips to the wall at 16"
on center. Then install 2" insulation board between the strips and hang drywall or other wall cover on the furring. Most
insulation boards, such as polystyrene, must be covered with drywall. They’re fragile, and won’t hold paint or
wallpaper. Also, most insulation boards aren’t fire-resistant. Fire codes require that interior wall surfaces be covered
with fire-resistant material, such as drywall.
Good insulation cuts heating costs and adds to comfort by making the temperature in the house more uniform.
But simply adding insulation to an old home can create more problems than it solves. In poorly-insulated older homes, water
vapor escapes to the exterior without interference. But in a tighter home with fewer air leaks and better insulation, water
vapor tends to collect in walls. There, it cools and condenses into liquid. The result can be saturated insulation and siding.
Running a humidifier, cooking, bathing, and simply breathing aggravate the problem. Liquid moisture in a wall reduces the
effectiveness of insulation, promotes decay and supports the growth of mold. To control moisture problems, be sure there is
a vapor barrier on the (winter) warm side of walls and ceilings.
Most homes built before the mid-1930's don’t
have a vapor barrier in either the walls or the ceilings. If attic insulation has been upgraded since construction, you’ll
probably find batts or blankets with a Kraft face that resist the passage of moisture. The Kraft face should face down, against
the ceiling finish. But if the ceiling insulation is loose fill, there should be a separate vapor barrier of coated paper,
aluminum foil, or plastic film below the insulation.
Vapor-Resistant CoatingVapor-Resistant Coating
homes develop thousands of tiny air leaks around doors and windows, at the sill, around electrical outlets and where interior
and exterior walls have cracked. To be truly effective, the vapor barrier should be continuous over the inside of studs and
joists. That’s easy in new construction. In home improvement work, applying continuous vapor barrier is practical only
when interior wall cover is being replaced.
If removing the ceiling or wall finish is part of the job, staple vapor
barrier directly to the studs or joists. Lay vapor barrier on a subfloor directly under the new floor finish. Be sure to lap
joints at least 2" and avoid unnecessary punctures. Nails or screws driven snug against the vapor barrier don’t
do any harm.
Vapor movement through any barrier is measured in perms. The lower the perm rating, the more effective
the barrier. Materials with good perm ratings include polyethylene at least 2 mils thick, asphalt-impregnated and surface-coated
Kraft papers, and asphalt-laminated paper.
Insulation With Vapor Barrier
Most roll (or "blanket") insulation has a vapor barrier on one side that extends
beyond the edges of the roll to form a stapling tab. Lap these tabs over the 2" thickness of studs and joists.
Tabs on adjacent blankets should overlap. Your drywall crew probably prefers to see these tabs stapled to the inside face
of studs or joists. That makes it easier to hang drywall with a smooth, even surface. But stapling tabs inside the stud or
joist cavity will also reduce the effectiveness of the vapor barrier. Moisture will seep through openings between the tabs
and framing members. It’s better to make the drywall finishers work a little harder and protect the studs and joists
from decay, mold and termites.
If walls and ceilings don’t have a vapor barrier, the next best thing is a vapor-resistant interior coating
such as aluminum primer. Two coats of aluminum primer covered with acrylic latex offer moderate vapor resistance, though not
as much as a true membrane.
Damp soil in a crawl space will transfer moisture to the floor framing and even to occupied rooms
above. Good ventilation helps keep the crawl space dry. Promote good cross-ventilation by installing a vent at each corner
of the foundation. Taken together, foundation vent "free circulation area" should be 1/150th of the crawl space
surface (10 square feet of vent for a 1,500 square foot crawl space). Free vent area excludes space occupied by the vent frame,
screen wire and louvers. Most manufactured vents are identified by the overall dimensions and the net free area. If there’s
a partial basement, the crawl space can be vented to the basement instead of the exterior.
Attic and Roof VentilationAttic
and Roof Ventilation
If soil in the
crawl space is covered with a vapor barrier, foundation vent area can be as little as 1/1500 of the ground area (1 square
foot for a 1,500 square foot crawl space). It’s usually much cheaper to cover the soil in a crawl space with membrane
than to add more foundation vents. Ground cover should lap at least 2" at all joints . Lay brick or stones on top of
the membrane to hold it down and prevent curling.
and Roof Ventilation
Insulation laid between ceiling joists helps hold heat inside the home. But above that insulation,
air should circulate freely. Excess moisture accumulation will damage insulation and other attic materials. Good air circulation
is especially important in colder climates and when the roof cover is less permeable. With built-up roofing or asphalt shingles,
the only way to get moisture out of an attic is with vents.
Inadequately-vented attics can reach temperatures above
140 degrees F on a sunny day. Some of the heat will work its way into the house, overburdening the air conditioning system.
Properly-vented attics will stay cooler and can save up to 30 percent on cooling costs. In addition, a cooler attic will keep
asphalt shingles from degrading prematurely in the heat.
Air circulation works best with soffit vents around the edge
of the roof and outlet vents located high, near the ridge. Warm air rises in the attic, exits at the peak, and draws fresh
air through soffit vents at the perimeter. Under those conditions, inlet vents should be 1/900 of attic floor area. Outlet
vents should have the same area. A home with only gable vents needs more free vent area. Air circulation through gable vents
depends on wind blowing the right direction. Provide one square foot of free vent area for each 300’ of attic area.
roofs don’t have gables and flat roofs don’t have peaks. That reduces your venting options. The estimating section
that follows includes power ventilators that can be installed on nearly any roof.
Flat roofs with no attic require some
type of ventilation above the ceiling insulation. If this space is divided by joists, each joist space should be ventilated.
A continuous soffit vent strip is the best choice. Drill through all headers that impede passage of air to the opposite eave.
post and beam roof with a roof-plank ceiling has no attic and needs no ventilation. But the roof surface requires insulation
board coated with a walkable sprayed-on insulator such as urethane.