Is fleece or board best for underfloor insulation?
Posted by greentomatoenergy . on 14 December 2012 at 10:37 am
Q: Underfloor insulation: good access to 126sq m underfloor but joist widths vary from 32 to 40cm. Fleece or boards best? (it arrived via Twitter, hence the brevity)
A: In terms of insulation there's little to choose between them: insulation boards are generally thinner for the same performance but this it not usually an issue unless space beneath the floor is very restricted.
The difference comes when you think about how to make the floor airtight. About half of the heat loss from a suspended floor is cold air coming up from the space below, so unless the floor is made airtight the effectiveness of the insulation is severely reduced.
Both systems can be made airtight, but the approaches are different: for insulation boards we usually tape the joints between the boards and the joists and then use expanding foam around the perimeter, for fleece you need to spread an airtight but preferably vapour permeable sheet on top of the insulation which in our opinion is more work to make airtight as it is difficult to attach it permanently to the walls around the edge.
Foam insulation boards are generally more expensive, but once you factor in labour cost there is generally not a lot of difference between the two.
Boards will need to be cut to fit between the joists. Cutting them about 1.5 cm narrower than the width of the joists allows them to fit, and the limited air convection in the narrow gap between the board and the joist mean heat loss is minimal. It's worth getting the correct type of saw for insulation board as it cuts faster and makes a lot less dust.
The exception to the advice above would be if you have good access to the floor from below (e.g. you have a basement, or there is an unusually deep void below the floor), in which case we have a technique using blown cellulose insulation that performs well and keeps cost down.
By Alexander Rice
Photo by Bryn Pinzgauer
About the author: greentomatoenergy specialises in cost-effective renewable technologies and low carbon building.
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