Restoration Guide: Roof Insulation

Editor's Note: This is article 6 of 13 in the Roofs Chapter of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.

6. Insulation

Let's start with some insulation basics. You would think insulation would be easy--throw a little into your attic and reap the energy savings. Not so.

Attic insulation involves many facets, including:

• Insulation density
• Insulation compressibility
• Air leakage
• Moisture control
• Fire safety
• Eave venting

Density. R-value is a rating of how much heat will pass through the insulation. Insulation has a density at which it has the greatest R-value per inch, but it is not cost effective to make it that dense. For instance, making a 3 ½" fiberglass batt at its maximum R-value would require enough material to make eight 3 1/2" R-11 batts.

Compressibility. Loose, or blown-in insulation, loses its R-value as it settles. If you are having insulation blown into your attic as part of a home renovation, you should specify the R-value you want after it settles.

Air leakage and moisture control. The passage of air from the living space into the attic presents multiple problems. If you have an old house, you might be especially susceptible to this problem, as less care was paid to leakage in the past. Besides heat (or cooled air) escaping, air rising into the attic also carries moisture that can result in condensation. This air can pass through vents, ducts, wiring, bath fans, skylights, recessed lighting and attic stairs--they all should be sealed.

Air also will leak through tongue-and-groove wood ceilings, and they should be backed with an air barrier, such as drywall or plastic.

Recessed lighting (also called "down lights" and "light cans") should be rated "IC" (insulation contact) so that insulation can be tight against them. Better yet, if the recessed light is airtight, its top also can be covered with insulation.

Fire safety. Any material used to block air leakage at a chimney or flue must resist high temperature. While fiberglass does not burn, heat can cause it to break down.

Note: Any electrical junction boxes, where separate wires are joined in an electrical box, must be visible above insulation. While first-time wiring probably will not have junction boxes in the attic, house renovations in old homes can lead to patched wiring. If you have a new insulation project in that old house, you may need to raise your junction boxes.

Despite precautions, some moist air still will creep into your attic, so most jurisdictions require attic ventilation. This is done with gable and roof vents as well as venting at the eaves.

Eave venting. This takes a brief explanation. Your trusses/rafters are anchored to the top plate, the top 2x4 or 2x6 of the wall. Between each truss/rafter is blocking, usually a 2x4, to help firm up the roof. That blocking usually has three 2" ventilation holes drilled through it, with screening over the holes to keep birds out of your attic (hence the common name "bird blocks"). Now if insulation in the attic covers those holes, it blocks ventilation. To prevent this problem, cardboard or plastic baffling is run from the bird blocks over the insulation, creating a clear air path into the attic.

A cathedral ceiling, where the ceiling is attached to the bottom of the rafters, presents ventilation problems because the insulation will fill the space between the ceiling and roof. In this case, there still should be a baffle creating a gap between the insulation and the roof that allows for airflow.

Even if not required by code, some warranties for roofing, roofing felt and sheathing require such ventilation.

Materials and Methods

1. Batt Insulation over Ceiling

   Batts are blankets of insulation, versus loose-fill blown-in insulation. The batts may be uncovered, or they may be faced on one side with a vapor-barrier paper. They are usually 16" or 24" wide, to fit between studs, joists and trusses. When the necessary depth of insulation exceeds the depth of the ceiling joist or truss chord (and it usually does), it needs to be in two layers. For instance, if your truss chords are 2x4s (3½" deep), and you need nine inches of fiberglass batting, you would run 3½" batts between the trusses, and the remaining 5½" in batts laid perpendicular across the truss chords. Faced batts, with the face side down, provide a partial cold-weather vapor barrier. In a hot-humid climate, the vapor barrier should be on the upper, attic side of the insulation, never on the ceiling side. (The vapor barrier goes on the side of the insulation that will be warm.)

   Batts are easy to install and do not require the special equipment of blown insulation, but they are a little more expensive. Batt installation requires care to avoid air gaps. Installers should wear protection because fiberglass insulation irritates the skin and air passages.

2. Blown-in (Loose) Insulation

   Loose-fill insulation--mineral wool, fiberglass or cellulose (made from recycled newspaper)--is blown onto the ceiling in the attic. If you have recessed lighting that is not rated IC (insulation contact), a shield must be built around the lighting to keep the insulation at least 3" away from the fixture.

   Besides cost, blown-in insulation has an advantage over batts because it more easily fills irregular areas. But blown insulation can settle, losing some of its R-value. Care must be taken to see that loose insulation is distributed evenly across the attic, without any thin spots. Blown-in insulation also will not provide a vapor barrier.

3. Insulating Cathedral or Vaulted Ceilings

   A cathedral ceiling has no attic space--the ceiling simply is attached to the bottom side of the rafters. In many climates, it requires ventilation baffles to create an air space between the roof and the insulation.

   A vaulted ceiling still has attic space. It is sloped to a lesser degree than the roof--perhaps the roof has a 6" on 12" pitch, and the ceiling a 4" on 12" pitch. This type of space is vented the same as a standard attic.

   In both cathedral and vaulted ceilings, batt insulation is recommended. Gravity will compress blown-in insulation toward the bottom of the slope over time, leaving the top of the ceiling under-insulated.

   However, loose insulation can be blown into a cathedral ceiling through holes cut in the ceiling by an experienced professional. It needs to be installed at a proper density so that it will hold its position in the sloping roof, and it would need to be in a situation where ventilation batts are not required.

4. Sprayed-on insulation in open rafters

   Foam insulation can be sprayed into rafters (with appropriate ventilation baffles if required by code or by manufacturer's warranty). It is a messy process and requires clean up. The foam may bulge beyond the rafter, requiring trimming to install ceiling drywall. Unlike loose insulation, it won't settle; however, it is more expensive.

5. Rigid insulation in a cathedral ceiling

   The R-value of a cathedral ceiling can be increased with a layer of rigid foam insulation (similar to Styrofoam) under the rafters. In most climates, it is considered wise to add the rigid insulation if the rafters are 2x6 or less, and in some cases 2x8, because batts of that depth do not provide enough R-value. If the framing is steel, a layer of such insulation is necessary to prevent condensation. The rigid insulation would then need to be covered with drywall.