Bandage for a Thousand Cuts: Sealing Leaky Floors, Spray Foam Insulation

Noting the advertised R-value performance of an insulation product is one thing, but paying attention to the quality of installation is of absolute importance. For insulation to work effectively, it needs to be closely aligned with an airtight layer of the building envelope, whether that be ceiling plaster, floor boards, wall plaster, or a vapor-permeable membrane system. Air movement between the airtight envelope and incorrectly installed insulation can seriously impact the performance of the insulation.

Underfloor insulation is one of the best things a homeowner can implement in a home with wooden floors over a crawlspace, especially when the temperature drops. Our feet spend a lot of time in contact with floors, making them a potentially major comfort problem. A big reason for this is air leakage through the numerous gaps between the floor boards. It’s a numbers game – a 0.01 millimetre gap between floor boards may not sound like much, but a typical home may have 1500 metres or more of tongue-and-grove joints. Altogether, this could equate to 150-cm2 hole in your building envelope. It’s like leaving a decent-sized window cracked open all winter.

Insulation batts can work really well in stopping heat transfer, but they have 3 major weaknesses which can affect the performance and longevity of a floor installation, mainly due to the everlasting force of gravity:

• Sagging of insulation between anchoring points
• Anchor points coming free at the ends of bats
• Compressed insulation resulting from fixing too tightly to reduce the chances of sagging

In addition, fiber batt insulation is not effective as an air barrier, acting as little more than a filter for air passing through the floor boards. Local penetrations like plumbing lines and electrical cables puncture the floor air barrier, allowing cold air to travel from the crawlspace beneath the house up through walls and other cavities. The image below shows that the floor of a bathtub in a home is cold, perhaps from cold air leaking up through the floor into the space under and around the tub.

Thermal imaging a bath tub floor before insulation.

An alternative to batt insulation is spray foam, installed by a specialist contractor. Spray foam insulation can avoid the above problems. It seals a great number of gaps in floor boards and provides effective, air-proof insulation. Among the benefits are:

• Substantial comfort improvement
• Substantially better installed energy performance
• Improved floor air-tightness
• Hot water pipes under the house may encapsulated with insulation
• Bathtub underside can be insulated completely
• Insects and bugs cannot enter through large cracks under floor boards.
• Bottom of external and internal walls are sealed, reducing air flow from crawlspace to roof, which overall improves the performance of the walls

Efficiency Matrix undertook air-tightness testing on a home before and after installing an open-cell spray foam under a 1970’s home which had Tasmanian oak floor boards. The air-tightness of the home went from 7.0 ACH@50Pa to 5.5 ACH@50Pa. This was already quite an airtight home compared to most homes in Australia, but the EqLA@4Pa (equivalent leakage area) went from 682 cm² to 508 cm², which is roughly equivalent to a 90 cm x 2 cm opening.

With thermal imaging it was apparent that the insulation was immediately effective at raising the floor surface temperatures. The infrared image below shows this effect. It was also noted that internal wall stratification temperatures also seemed to improve. This could be attributed to less air movement through internal wall cavities from subfloor to the well-ventilated loft area of the roof.

Thermal imaging of the floor as spray foam is being installed, the Exothermic reaction was raising the temperature of the floor boards as it was being installed.

The downsides of spray foam, are that it is best installed by an experienced spray foam contractor with specialised equipment, and it has a cost premium over batt insulation products. It is also not recyclable, and materials covered with spray foam maybe difficult to recycle at the end of building life.  The other thing that becomes important in Summer, is protecting windows from direct sunlight.  Uninsulated floor boards usually work to help cooling the house down in Summer, but once they are insulated you no longer have that thermal bridging effect occurring and any heat that comes from a window via sunlight is insulated from radiating downwards.  Still, comfort and air tightness benefits of spray foam as a floor insulation may more than make up for these shortcomings.

In conclusion, if you are thinking about insulating your floor for comfort and energy efficiency, think not only about the advertised performance of the insulation, but also about the installed performance and its impacts on whole-house air tightness.