Underfloor Insulation: Sizing and Joist Width

What is standard joist width, and what size insulation is best?

Our underfloor insulation is slightly wider than the space between standard joists, but don’t be alarmed, we designed it that way. Here’s why we choose to use 450mm wide insulation when standard joists have 430mm space in between them.

Remember, the performance of underfloor insulation comes down to two things: coverage and loft (thickness and density). The easiest way to explain why it’s so much better to have the insulation a little wider than the space is to take you through our installation technique:

  • First we press the insulation up in between the joists. The additional width makes the material fit snuggly, making excellent contact with the floor and holding in place while it is stapled in place.
  • Next we pinch and staple the material to the inside edge of the joist. This causes the insulation to “pillow” slightly giving you extra benefit of 120mm thick insulation, even though a standard joist is only 90mm deep.

Under Your Floor

Standard floor joists are installed with 430mm space in between them and 90mm depth. However, if your joists are not 450mm wide (called 450 centres in the trade), you can still install insulation under your floor. See below.

 R2.0 and R2.5 Insulation

Our R2.0 and R2.5 polyester underfloor insulation is 450mm wide and 120mm thick, this means that it fits snuggly into standard joist spacing. The R2.0 and R2.5 are both 120mm thick, the difference being the density and weight of the material.


This is a cross-section of our sample floor fitted with polyester underfloor insulation the ecoMaster way.