California Building Performance Contractors Association
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    Older Home's Subfloor Insulation

In older homes; generally built before 1960 the subfloor of the house was constructed of 1x6 Douglas fir boards over 2x4 floor joist spaced 16 or 24 inches on center or 2x6 tongue and grove boards over 4x4 beams spaced 32 or 48 inches on center.

After this type of subfloor was installed in some cases a second floor was laid down after the home was weatherized. This could be a 5/16 hardwood floor, tile or another type of material that created an air-barrier between the second floor and the subfloor.

Some subfloors just received carpet that did not create any air barrier; these floors can contribute to a large amount of air infiltration because of the wood shrinking, creating gaps in the floor assembly over time.

It is generally known in the home performance industry that 40% of the air entering the home comes from the crawlspace. This presents issues; poor air quality, moisture, and unconditioned air entering the living space.

Adding fiberglass batt insulation on this type of floor system, to help prevent heat loss, will not stop the infiltration but the fiberglass insulation will become like a filter; the insulating value of insulation will be degraded.

The best way to deal with this type of board floor assembly is to apply spray foam or install rigid foam board between the floor joists, seal the foam board to the subfloor, and use gun foam where the foam board meets the joists, on all seams.

Choosing this option will increase the job cost, but it is a more effective way to deal with air quality and moisture control as well as providing a more effective R-value to the assembly.  

Even if you choose to go with this system always mitigate the bulk water issues surrounding the home as well as installing a vapor barrier in the crawlspace.

When you install fiberglass insulation on this type of floor system you are missing the opportunity to install an effective air barrier.  

Andy is one of the best Technical Analyst in the industry with over thirty years of experience in the construction industry performing home energy efficiency retrofits. He is skilled at explaining to contractors and others how the various facets of home performance retrofit work combine to form a whole house performance project. He is an expert on BPI and other energy efficiency standards. Andy is happy and always available to discuss any of the topics found in Andy’s corner, or any other Home Performance item. You could contact him at

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