Underfloor insulation over crawl spaces and basements

Many builders and homeowners underestimate the importance of undeerfloor insulation, and homeowners end up with higher energy bills and uncomfortable floors.

Wood-frame Floors over basements and crawlspaces

Floors over basements and crawl-spaces are often left uninsulated, in wood-frame homes. A common reason for this is that it is difficult to insulate them properly, due to the framing, the tubes and other elements on the underfloor.

In wood frame homes, energy experts recommend the insulation of the basement/crawlspace walls and the floor - not the insulation of the ceiling of the crawlspace/basement.

Obviously, this approach requires also comprehensive air sealing and waterproofing, and good surface and ground drainage.

More details:
Basement insulation

Anyway, you may also consider underfloor insulation, that is, the insulation of the ceiling of the crawlspace or basement. It can be advantageous, as long as the framing and other obstacles in the floor do not hinder the expected results. This page deals with this approach, in wood-frame floors.

Recommended Under-floor insulation levels

Consider levels of floor insulation of about R-30 (Metric system: U-0.19) in cold climates and R-20 (Metric system: U-0.28) in mixed climates.

Super-insulated houses (in cold climates) may need higher values: R-40 (Metric system: U-0.14).

Underfloor Sealing

It’s important to air seal the underfloor carefully, before installing the insulation. Air leaks between the conditioned area and the crawlspace or the basement should be sealed. Pay attention to holes associated with plumbing drains, wiring, ducts or supply piping.

Many insulation materials are poor air-sealing products (that’s not their function), and you will not get an effective underfloor insulation without proper sealing.

Air barrier

You need more that a proper insulation layer on your floors. A good underfloor insulation job also requires a continuous air barrier, involving the underside of the whole floor.

In wood floor framing construction you can use OSB, plywood or taped rigid foam, as an air barrier. The use of a rigid foam has the advantage of minimizing thermal bridging through the floor framing, but if you think there is a risk of bugs or other pests getting into the floor, consider plywood and OSB.

Note: Ideally, in wood floor framing, you should use a continuous layer of rigid foam insulation and also a layer of OSB or plywood, as we explain further down. That’s key in new homes and important in home improvement projects.  

Open and Enclosed cavity insulation

Floor InsulationInstalling fiberglass or mineral wool batts, or rigid foam boards in floors with exposed joists (open floors) is straightforward.

The insulation should cover the whole depth of the cavity and contact with the floor. Installers should push the insulation up and fasten it to prevent any void or gap between the floor and the material.

Do not use faced batts (to avoid moisture condensation problems), and do not compress or split the material. You may use push rods, wire, twine or wood strips to support the batts.  

To insulate closed cavities (with solid sheathing on their underside), you may use cellulose or spray foam insulation. The installation process (in existing floors) is very similar to the one used in cavity wall insulation. You will have to drill holes in each bay, and to blow the cellulose (or other insulation materials) through the holes, and to patch them after installing the insulation.

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Without a thick layer of rigid insulation on the underside of the floor, even the best cavity floor insulation will not be effective.

Rigid insulation over the Underside of the whole floor

Floor cavity insulation is not enough, especially in cold climates. That does not prevent heat conduction through the floor framing.

You should also install a thick enough layer of rigid insulation involving the underside of the whole floor, covering the framing – at least 2 inches thick. You can use polystyrene, foil-faced polyisocyanurate or dense rigid mineral wool.

In other words: you may fill the cavity with cellulose, mineral wool, spray, rigid foams or other material of your choice. As long as the cavity is fully filled, without gaps or voids, the insulation material may not be very important. But to avoid thermal bridges and to meet the insulation requirements, you have to install a thick enough layer of rigid insulation (mineral wool, rigid foam) over the underside of the floor (and also an air barrier: plywood, OSB).





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