Insulation depth in attic

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Ceist Beag

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I've searched the archive but didn't find an exact answer to this so apologies if this has already been answered and point me in the right direction if so. I'm looking into insulating our attic which is in an old house which never had insulation before. I spoke to a company yesterday who recommended the insulation (hemp mats) should be to a depth of 240mm to give adequate insulation. I did the measurements since and the depth between our celiing and the top of the joists is only around 110mm so just wondering what the logical thing to do is. Should we either batton on extra joists to the required depth or else simply lay the insulation between the joists to the height of the joists and then over the top of them for the remaining height?
 
Recommended is 260mm, or 10". As you said, normal joist height is 100mm or 4", so the recommended method to lay insulation rolls is 100mm between the joists, and then 160mm, or 6" across the joists. This layering at 90 degrees also ensures that there is no cold bridging at the joists.

The only problem with this is if you wish to create a storage space in the attic. You then either have to extend the joists and floor it, or you can use pre-insulated plywood sheets, such as those available from Aeroboard.

There should be a lot of old threads about this - search for attic insulation or attic flooring.

Regards,
Buddyboy
 
while it may not be the easiest method....
you should cut 100mm insulation boards from either Kingspan or Xtratherm to fit between the ceiling joists, and then fill any apparent gaps with spray foam insulation.
 
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