Rim Joist Air Sealing and Insulation

Convection is the single largest contributor to heat loss in conventionally designed and constructed residential buildings. This convective cycle is caused by the stack, or chimney, effect. Warm moist air leaves the upper portions of the building through holes in the envelope which causes a pressure change in the building which in turn draws cool dry air to enter the lower portions of the building to take its place. The rate at which this happens is measured in air changes per hour.

Too many air changes per hour mean that the building is not as efficient as it could be and this can be improves through air sealing. Too few air changes per hour leads to poor indoor air quality and this can be remedied with mechanical ventilation. It is important to get the balance between these two ideals (energy efficiency and adequate ventilation) correct. This balance point is 0.35 air changes per hour and can be determined by conducting a blower door test.

The amount of air infiltration in most residential buildings is higher than 0.35 air changes per hour. If the home has a basement and/or crawl space air sealing the rim joist is a good way to reduce the amount of air infiltration. This article describes two methods that require professionals to do the work and one that homeowners can do themselves. I’ve also included what I consider to be the pros and cons of each technique.

Professional Installation

• It has an R-value of 5-7 per inch, depending on the type of foam, so it can be used to maximize the insulation in a closed cavity.
• It acts as an air sealer and insulator.
• The installation process is relatively quick.

• It outgases as it cures. Different manufactures use different formulas to create their foam which in turn outgas at different rates as the chemical reaction takes place and introduce various chemicals into the air both inside and outside of the building.
• It can pull away from the surfaces onto which it is applied which can allow moisture vapor to get into the cavity and condense.
• It cannot be applied to cover more than six electrical wires. There are three wires in a piece of insulated Romex and if there are more than two pieces side by side the wires can heat up to the point of causing a fire if they are covered with spray foam.

• It acts as an air sealer and insulator.
• It is made from shredded paper (usually recycled or over issue newspaper and paperback books) treated with a borate solution so it is insect and fire resistant.
• When cellulose comes into contact with an open flame it smolders instead of melts.
• There is no curing process so it doesn’t outgas. Some people have chemical sensitivities to the petroleum based inks used in the printing process so they need to make sure they don’t come into contact with the product before it gets covered up.

• It has an R-value of 3.7 per inch, so it requires a deeper cavity to achieve the insulation level of two-part spray foam.
• If the product is not installed at the proper density, it will settle which leaves gaps, which means it’s not as effective.
• The preparation time necessary to create cavities that can be filled means that the job takes a little longer.

Homeowner or Professional Installation

• The products and tools to install it are readily available at most hardware stores and/or building suppliers.
• It doesn’t require special training although attention to detail is important.
• It can be done as time and finances allow.

• The one-part foam used to seal the edges of the blocks has a lower R-value per inch.
• The one-part foam needs to be installed carefully so there are no gaps to allow air to come in or moisture vapor to be trapped.
• Time consuming.

These three methods are all viable means of air sealing and insulating rim joists. It is up to the installer (whether it be a professional or a homeowner) to decide which is the most appropriate to use for a particular situation and that it is done correctly.

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