Insulating loft

4 Sep 2013
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United Kingdom
I want to insulate my loft , I have increased the height of the joists from 80mm to 140mm and thrown away all of old mineral wall as it was damp and not in great condition. I have fixed all damp issues now.

So also want to board over joists for light storage using 22mm chip board. I therefore have 140mm depth for insulation. I know if you use mineral wool only the level required is 270mm. Obviously I cannot do this but want to retain the same insulation value that 270mm of mineral wool would give me.

My suggestion is to to use 100mm of mineral wool as a base and then 50mm insulation board (celotex) on top of this pushing down the mineral by 10ml. I know you shouldn't squish mineral wool too much but I hoped if it was only 10ml this would be ok.

So would 100mm mineral wool with 50mm celotex board on too be as efficient as 270mm mineral wool?
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Celotex is less breathable, put the more vapour resistant materials nearer the warm side, so wool on top of celotex. That and you will want to use expanding foam around the celotex, bit tricky to do if it;s on top of mineral wool.

50mm of celotex and 100mm of mineral wool = approx 0.24 u value

270mm mineral woold = approx 0.18 u value

That's the desktop answer, in reality (with expanding foam to fill gaps between the celotex), and chipboard on top of the joists, this will mean less air movement within the mineral wool insulation, so it may prove almost equal in practice, but you have no way of proving that.
Thank you very much for going the value calculations for me as I wasn't sure how to do these. Sounds as though my plan will be pretty good then and wil offer nearly if not the same insulation. I take your note on putting down the celotex first, I'll make sure I do that

One more note if that's ok, I plan to install LEDs into GU10 fittings. Can I just lay the celotex and then when I cut the holes out from below I cut through plasterboard and celotex as this should then leave a nice round hole in the celotex for the light fitting to slot up into.
With LEDs that will probably be fine, but it would be a fire risk if standard bulbs where used (they would overheat), something to bear in mind if you hand the property over to someone else.

You also want to cover the GU10 fittings with loft caps, or make little plasterboard boxes that sit over them.

Better yet, don't use downlighters :)
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Thank you. I like the idea of LEDs for energy savings etc. I'll use loft caps over the GU10 fittings then. Thanks again
My spacing between each joist is only 260mm and all the loft wool insulation I can find seems to come in either 400mm or 600mm wide strips.

Has anyone comes across this problem and what is the solution?
Cut the loft insulation while it is still in it's packet. Use a panel saw to cut through it.

Thanks Andy, my only thought is that I'd wasting the other 140mm. Unless I use x2 lots of the 140mm strips that are left to put in another joist spacing. So this would be 280mm in a 260mm spacing but squeezing it in that little amount should be ok right?
Can I ask what thoughts are in led down lights fittings pushed up and cut into celotex board??

I've read on celotex website that it is not advised due to the fitting acting as a bridge in which cold air meets hot, condenses and then drips out of light fitting.

Any advice on fitting LEDs into loft space where I want to put insulation board
If you put a loft cap over the light fitting, and insulate over that, then the light fitting should not get cold enough for condensation to occur, however you only have 140mm to play with, so it's hard to see how you will insulate over it (most light fittings being about 100 deep?, leaving you only 40mm for a gap, a loftcap and then insulation, it doesnt work).

Unless in the areas where you have light sockets, you do not overlay with chipboard (or have a "hole"), where you can then fit an insulated fireproof box over the light fitting, that laps with the loft insulation.

Again I want to stress if you do this with a standard GU10 bulb, it will overheat and act as a fire risk.

If you don't insulate the light, then yes, the metal will get cold and condensation could occur, unless it's also open to free air movement (which would then make it a hole in your ceiling, negating all your insulation work).

That's why people frown on downlighters.

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