BUILDING ENVELOPE: Insulation

Questions:

• What are the suggested R-values for the various components of a home?
  
• Which areas in a home would benefit the most from insulation?
  
• How does one achieve an R-38 in the attic near the edge of the roof?
  
• Which is better for insulating attics, fiberglass or cellulose?
  
• Should I use a radiant barrier in the attic instead of conventional insulation?
  
• How can I insulate my floored attic?
  
• How do I seal my attic access panel?
  
• How should I insulate a slab-on-grade floor?
  
• How can I seal and insulate the opening for a whole-house fan?
  

What are the suggested R-values for the various components of a home?
The best, most current information suggests the range of R-values in the table below.

The value chosen within the range you should pick depends on where the house is located in the state. Lower R-values are more appropriate in southeastern Kansas, while homeowners in northwest Kansas should consider higher R-values. Those living in the central part of the state should aim at a value somewhere in the middle of the range.

An R-value is the measure of a substance's resistance to temperature change. Select a building system that will provide R-values within or above these ranges, and see that materials are installed so as to create a well-sealed structure.

Because the attic is usually accessible, it is an area that is easy to insulate. If the attic has not been insulated, first install a vapor barrier directly above the ceiling, then place insulation up to an R-38.

Some types of insulation have a vapor barrier attached directly to them. This insulation should be installed so that the vapor barrier is toward the warm side of the house in winter.

If there is already insulation in the attic, don't install another vapor barrier over the old insulation. It is acceptable to mix types of insulation, such as adding cellulose over fiberglass batts.

Of equal priority to insulating an attic is to seal and insulate any exposed ductwork that runs through unheated areas, such as crawl spaces and attics. These ducts should be insulated with a minimum of an R-11.

If the ducts are used during the summer for central air conditioning, the insulation should have a good vapor barrier on the outside of the insulation. This will prevent condensation from forming on the cold duct due to the humid summer air.

The next priority is to insulate unheated crawl spaces either directly beneath the floor or on the foundation walls. If insulating below the floor, install the vapor barrier on the warm side in winter, or facing up.

Be sure that any plumbing in the crawl space is on the warm side of the insulation to keep the pipes from freezing.

Insulating crawl-space walls is appropriate only in unventilated crawl spaces. Insulation on these walls should run from the band joist down the foundation wall and extend at least 2 feet across the floor of the crawl space. The band joist is the area between the foundation wall and the floor of the room above the crawl space.

The dirt floor of the crawl space should be covered with a polyethylene film.
Insulating basement walls is the next priority and is just as important as crawl-space insulation. It is possible to add furring to the wall, insulate between the furring, and add a finished surface, such as wood paneling. Or, attach rigid foam directly to the basement wall and cover it with a noncombustible material, such as gypsum board.

Although not generally considered a do-it-yourself project, installing wall insulation can be very cost effective. This requires drilling through the siding or removing some of the siding and drilling through the sheathing under the siding. Knowledge of building construction is helpful to make sure that all wall cavities are filled with insulation. Wall insulation installed at the proper density and with no voids will not only significantly reduce conduction heat loss through the walls, but can reduce air leakage as much as 30 percent.

How does one achieve an R-38 in the attic near the edge of the roof?
The problem arises at the joint where the roof, wall, and ceiling come together.
Full-depth insulation may cut off continuous ventilation.

It is necessary to maintain one inch to one and one-half inch of air space over the insulation from the soffit area into the attic. Full-depth insulation to the outside face of the wall is desirable.

If this insulation is not firmly fixed or protected, it may be moved by winds and air pressure moving through the soffit vents. This may lead to moisture problems on the interior sheetrock finish.

The best solution to both problems is to use raised-heel roof trusses with sufficient depth over the wall for the necessary insulation. Regardless of the roof construction, the edge of the insulation over the wall should be protected by baffles, which are flush with the exterior face. The baffles should turn up and follow along the truss, maintaining a vent space under the sheathing.

This should prevent wind-driven movement of insulation and reduce the possibility of moisture problems at the ceiling perimeter.

Which is better for insulating attics, fiberglass or cellulose?
Both products are excellent insulating materials. Either can be used for insulating an attic, but, generally, cellulose is easier to install and is usually less expensive. Cellulose also has a slightly higher R-value per inch thickness and is more effective in reducing air leakage.

Some studies have shown that cellulose insulation retains its insulating value at lower temperatures when compared to fiberglass. Based on these points, cellulose is the preferred insulation for most attic arrangements.

However, be sure to seal all holes in the attic floor before beginning to insulate, regardless of which material is used.

Should I use a radiant barrier in the attic instead of conventional insulation?
No. In Kansas's winter climate, conventional types of insulation are necessary to cut heat loss from the interior of the house through the ceiling. Installing insulation properly and careful attention to air sealing will reduce air leakage through the ceiling.

A radiant barrier provides the greatest savings in the summer by reducing radiant heat transfer from a hot roof to the attic floor. However, they generally have not proven to be cost effective in Kansas's climate.

How can I insulate my floored attic?
One of the simplest methods is to drill holes in the flooring and then blow cellulose, mineral wool, or fiberglass into the opening. This method is like blowing insulation into walls. It is possible to use holes as small as 1 inch in diameter, but larger holes provide better coverage. For each joist cavity, drill at least three holes. Holes should be located at both ends of a joist cavity and in the middle.

Another approach involves opening the center section of the floor and then using an insulation blowing tube. This tube is inserted through the floor opening between the ceiling joists (attic floor joists). The tube should be long enough to reach the far end of the joist cavity.

Next, blow insulation through the tube to fill the far end of the cavity. When insulation stops flowing, withdraw the tube about 18 inches. The flow will resume as the tube is withdrawn. Continue the process until the entire cavity is filled. The blowing tube is typically a 2-inch diameter, clear vinyl tube that is attached to the insulation blower's regular tube.

How do I seal my attic access panel?
Many people place a single piece of sheetrock or a quarter-inch-thick plywood piece over the panel. But this is not an effective way to reduce heat loss or form a tight seal with the frame.

A better solution is to add insulation to the top (attic side) of the panel. The insulation can be either fiberglass batt or rigid foam, and it should be thick enough to equal the R-value of the attic insulation. If there is loose-fill insulation in the attic, some of it may spill into the home when the access panel is opened. The easiest way to avoid this is to build up a frame around the opening.

The frame can be made with plywood, lumber, or even heavy cardboard. Apply weather-stripping to this frame to reduce air leakage. The drop-in panel should be heavy enough to form a tight seal with an adhesive foam strip.
Finally, caulk the ceiling trim around the opening to further stop leakage.

How should I insulate a slab-on-grade floor?
A concrete slab floor should be insulated first at the edge of the slab where it is exposed to the outdoor air and then down the face to the frost line or below. A foam board type of insulation is most suitable, usually extruded polystyrene with enough thickness to achieve an R-value of at least 12.5.

Insulating beneath the floor depends on a number of factors. If the slab is to be covered with carpet or other insulating materials, insulation is not needed underneath. Definitely insulate under the slab if there are any buried or in-slab heating systems, and do so in consultation with the manufacturer's and installer's recommendations. If the slab area is small or exposed on two or more sides, insulating the sides and underneath will tend to keep the slab warmer.

If the slab is to be used for direct-gain passive solar storage, insulation will reduce the heat loss to the earth below and keep the floor more comfortable. Insulate wherever the sun will strike the floor and where desired for comfort.
In larger slabs, a 4-foot-wide band near the edge may be sufficient.

If the slab rests on damp, wet soil, it will tend to lose heat more rapidly and insulation will help retard this loss. In general, 1-inch thickness of polystyrene should be adequate for most installations.

How can I seal and insulate the opening for a whole-house fan?
The metal louvers under a whole-house fan offer little protection against heat and air loss to the attic in winter. Consider attaching an insulated panel directly below the louvers. To make this an easy, seasonal task, build a frame using 1 by 2-inch lumber to hold the insulated panel.

Cut the panel from five-eights-inch rigid insulation board, and mount it in the frame with fabric or other decorative material. Four wing nuts mounted in the frame will hold the panel in place. Taping a sheet of plastic under the louvers will help stop air leakage but will provide little insulation.