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.

A “Hybrid” approach may be the best bet.

A hybrid approach where we have some insulation in the roof of the attic and some on the attic floor maybe the ideal approach in the majority of cases in California. Rather than sealing off the attic completely we may keep some vents open to the outside to allow any trapped moisture to escape and to prevent any stale attic air from passing into the living space. The ideal insulation for the roof is spray foam because it sticks to the roof sheathing and also creates a vapor barrier and air seal.  For the floor of the attic we can either use spray foam or blown in cellulose with air sealing around gaps as described below.

Whole house fans

These are an excellent addition to any home due the dramatic reduction in cooling costs that can be achieved. Leaving some attic vents open as described above allows the possibility of using a whole house fan which vents directly into the attic. If the attic is completely sealed a fan that vents directly to the outside through a dedicated roof vent could be a possibility. Please refer to the whole house fan section of the website for more details.

Insulation for attic floors

Although spray foam is the best insulating material as we discuss elsewhere on the website, if your budget is limited then blown in cellulose is another excellent approach for attic floors.
In addition to not having insulation in the walls, many older homes have inadequate insulation in the attic as well. The R value describes how much resistance there is to hot or cold, and the higher the R value, the easier it is to keep heat in or to keep cold out. The current R value for attic insulation for new homes in California is R38, however many older homes have just R12 or even less. In addition, older insulation can deteriorate, is often subject to rodent infestation and may have been moved due to work in the attic such as the installation of can lights.

What is “Blown in cellulose”?

Blown in cellulose is a type of loose fill insulation that is spread around the attic using a blowing machine and large hose.  It spreads out on the floor of the attic and fills the gaps between floor joists, wires, piping and other obstructions. It forms a thick insulating blanket and can be used to achieve a very high R value of R38 or above.

Disadvantages of fiber glass (bat or blown in)

Fiber glass is another material that is used for attic insulation. It commonly comes either in rolls or “bats” with a paper cover, or can also be blown in as a loose material. Fiber glass however has a number of disadvantages. With fiber glass bat, it is very difficult to achieve good insulation around obstructions such as pipes, wires, framing, can lights etc.  The many gaps and voids created inevitably mean that the true R value is dramatically less than the theoretical R value. So although the installation cost of fiber glass bat is low, the energy losses over time can significantly increase heating and cooling cost.  Here are some examples of air gaps using fiber glass bat:
 
 
Blown in fiber glass initially achieves a good R value, but its very low density means it can easily be disturbed during construction work etc. and can even be blown around by attic fans etc. Fiber glass also tend to have little or no sound absorbing qualities. Leading to “Noisy” homes.

Fiber glass allergies

As anyone who has worked with fiber glass knows, all types of fiber glass can cause allergic reactions and tend to cause extreme itching and discomfort if they are in contact with human skin. People working with fiber glass without gloves have the sensation of dozens of tiny needles sticking into their skin.  Fiber glass can also cause extreme irritation in the nose, throat and lungs. Older types of fiber glass also contain formaldehyde which is a known health hazard. Even going into an attic for a short time to retrieve stored items can result in severe allergies for some people. All in all, the mnay disadvantages of fiber glass dramatically outweigh its lower cost.

Advantages of blown in cellulose for attic floors

  • Good R value
  • Higher density so it tends to stay in place
  • Fire retardant
  • Good sound absorbing properties leading to a “Quieter” home
  • Tends to fill gaps and voids very well.
  • Based on a natural, organic material for a “Greener” home
  • Easy to move and replace if construction work is needed in the future.

Insulation removal

Very often insulation in older homes tends to be in a very poor condition. Damp, mold, rodent infestation, debris from reroofs etc  are all very typical conditions that exist as can be seen here:
 
 

Recommended process for attic floor insulation:

1.) Remove old insulation and debris using an industrial vacuum machine. 
2.) Air seal gaps and cracks using spray foam. 
3.) Add the required thickness of blown in cellulose, using baffles to ensure all vents remain open. 

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