Effective floor insulation


The underfloor insulation shown here is not doing its job. No wonder the occupants find the house gets too hot and too cold too easily!

What’s wrong? Let’s start with some definitions: air barrier and thermal barrier. For buildings to provide good shelter they need both. Think of it like wearing a woolly jumper on a windy day, the wool provides insulation but because the wind can move through it, you still feel the cold. A spray jacket keeps the wind out but provides no insulation so you still feel cold, but wear them both and you give yourself the best chance to feel comfortable. Your home needs both layers and to be effective, the thermal barrier (the insulation material) needs to be right up against the air barrier (the floor material or plasterboard wall/ ceiling lining) so that no air can circulate there.

So in this photo you can see that the batts have been installed with some attempt to hold them in place but unfortunately the net is failing miserably. Cold or hot air can easily pass around the batts and affect the flooring sheet as if there were no batts at all.

What’s the solution? There are some options: the whole lot could be removed or left there and then  rigid polystyrene boards fixed under the joists ensuring there are no gaps at their joins. That would mean those boards become both the air barrier and the thermal barrier. Another option would be to remove the current batts and fit polystyrene boards against the flooring, between the joists. Another would be to remove it all and use sprayfoam which provides a continuous thermal barrier and also functions as an air barrier. It seals any gaps in the flooring material too. Manufacturers of batts do have more rigid batts suitable for flooring but the key is to ensure they are snug against the floor and will not sag over time.

Often in homes like this with existing ceiling and wall insulation, installing effective underfloor insulation will give better bang for your buck than topping up the ceiling to R4 or more. Of course, sealing up unwanted sources of air leakage needs to be done as a top priority- more on that in another post.

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