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Insulation
Insulating a loft with 75mm depth joists
Anonymous user 9 September 2022 - 3.16 PM
Hello, I’m looking to re-insulate the floor of my 1920’s loft. The joists are only 75mm deep but most reasonably priced thermal roll loft insulation starts at 100mm depth. Do I need to raise the height of the joists so that I can lay the 100mm layer before adding the 170mm layer perpendicular? Or is it ok to just lay the 100mm and the 170mm on top (perpendicular) without worrying about the short joist depth? If I do need to add to the joist height, should I add wooden batten to the top of the joists? How should I fasten them to the joists please? Thank you.
3 answers from MyBuilder tradespeople
as long as you have the space it will not matter if your insulation is higher than the current regs, just make sure you don't block the eaves to allow air circulation in the loft space or else you will end up with a major condensation problem creating dampness. good luck Alex
Answered
9 September 2022
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Anonymous user
Currently the house insulation regulation (domestic and not rental) requirements are min 270mm. This is a combination of 100mm between joists (base layer) and 170mm top layer which is laid at 90 degrees to the direction of the joists. In the event you joists depth is lower than 100mm, the fact that your joist are 75mm does not impact the laying out of the insulation. This is a very common issue right now as is that older properties have joists which are 150mm depth. The only time you would look to add timber onto the joist would be if you intended to floor the loft after installing 270mm of insulation.
Answered
9 September 2022
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Your insulation depth would only be an issue if you're planning to board your loft for storage? Otherwise the rafters should have no bearing on the depth of your insulation, the recommended insulation levels are now 270mm if that helps, most rafters will only be 100mm with the old insulation running between them.
Answered
14 September 2022
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