Attic Insulation

The Importance Of Attic Insulation

The attic is an important interface between the living space and the building exterior. In summer, heat from the sun called “Solar heat gain” causes the roof temperature to rise dramatically and this heat passes down into the attic space by conduction and radiation. A typical attic can reach temperatures of 140F and the objective is to stop the heat reaching the living space. In the winter we want to stop the cold outside air from reaching the living space. Attics are usually ventilated to allow hot air to escape and also to keep the attic dry.

Insulation Is For Summer As Well As Winter

Most of the densely populated areas in California are in warmer climate zones which tend to have warm or hot, dry summers and cool winters with a little rain. The humidity is low throughout the year but especially in summer. People tend to use electricity to power AC systems for summer cooling and natural gas for winter heat, and because gas is a much cheaper form of energy than electricity the highest energy cost is typically in summer months. Although people tend to think of insulation as keeping them warm, in California the most important purpose of insulation from an energy cost standpoint is to keep homes cool in summer.

Condensation Control

Because warm air can hold more moisture than cool air, when warm air touches a cool surface the water can condense. If not managed properly the moisture can lead to mold or rot. Even though California is relatively dry we need to consider condensation control when deciding on an insulation approach.

Traditional Approach: Attic Floor Insulation

The traditional approach has been to insulate the attic floor using materials such as fiber glass, cellulose or foam.  This meant that the attic space was hot in summer and cold in winter, and only the living space temperature is controlled. Advantages:

• Straightforward and simple to install.
• Easy to upgrade, add extra thickness, replace, etc.

Disadvantages:

• If heating and cooling ducts are in the attic, the ducts are passing through an “Unconditioned space”, i.e. where temperatures are not controlled. This can lead to energy losses particular in summer months.
• The large number of items on the attic floor such as pipes, wiring, framing, lights, vents etc. mean that there are many places where we do not achieve the R value that we desire. The insulation can also be compromised if can lights or other systems are retrofitted.

Until recently attic floor insulation was used in the vast majority of homes.

Alternative Approach: Insulate In The Roof To Create A “Conditioned Space”

Another method that is becoming increasingly popular is to install insulation on the underside of the roof in the rafters rather than on the floor. If the roof has a gable the gable is also insulated. This method prevents heat from passing through the roof and keeps the attic much cooler in the summer. In winter, some warm air from the living space passes into the attic to keep it warmer than the exterior.  The attic space may be completely sealed with no roof vents, or may have a limited number of vents for moisture control. Advantages:

• If AC ductwork is in the attic there is a significant energy saving due to the ducts passing through a cooler space in summer.
• In winter heating ducts are in a warmer space than in a regular attic.
• It is easier to create a uniforminsulating layer on the roof sheathing because there are fewer penetrations. The foam insulations is less likely to be disturbed by remodeling work such as the installation of can lights etc.

Disadvantages:

• The initial cost is somewhat more than attic floor insulations using a material such as blown in cellulose, but this is usually offset by energy savings over time.

California Building Code

Due to California’s aggressive energy saving goals the state is searching for ways to increase energy efficiency in homes. Attic insulation is a major topic and the idea of conditioned attics is one of the approaches being proposed. It is already being used in some new construction projects under way.

Spray foam

• Spray foam is an ideal insulating material for attics either on the floor or the roof of the attic. It not only insulates but also forms an air seal to stop energy losses due to air flowing through the insulating material itself.
• It can be applied in various thicknesses depending on the R value required.
• Closed cell foam with R7 per inch is often used where space is limited and we must achieve a high R value in a shallow depth. Open cell foam with R4 per inch can be more economical and is often used where there is no space limitation.

Spray foam can also be put on the outside of a roof in the form of a foam roof as described elsewhere on this website. This is usually used on flat or curved roof surfaces.

If we had a choice would we choose attic floor or attic roof insulation? ("Unconditioned space" vs "Conditioned space")

There are pros and cons to each approach as described above. In the hot California climate insulation in the roof will do a great job of keeping summer heat out of the attic (And out of the living space below). It will also keep AC ductwork cooler so the AC doesn’t have to work as hard. There is still value however in having some insulation on the floor of the attic to keep the cooler air from the AC unit in the living space. In winter, the attic floor insulation will help to keep warm air from the furnace in the living space as well.