Loft insulation question - possibly silly
Loft insulation question - possibly silly
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Discussion

rlw

Original Poster:

3,493 posts

255 months

Sunday 31st January 2010
quotequote all
Loft has insulation but it is old and filthy following a new roof some years ago. We would like to board the loft to make it more useful for storage and line the inside of the roof to keep the dirt from coming in. I note that 270mm is the new 100mm for thickness of insulation and have no problem with that BUT.....

is that thickness of insulation as effective when compressed under boards or does it need to be fluffed up to its full depth.


58warren

589 posts

197 months

Sunday 31st January 2010
quotequote all
No, don't compress the insulation as it will be nowhere near as effective. You need to raise the flooring by adding appropriately sized timber to the joists so that the floor is just above the top of the 270mm insulation.

Edited by 58warren on Sunday 31st January 11:24

andy43

11,908 posts

272 months

Sunday 31st January 2010
quotequote all
You can leave the exisitng 75-100mm insulation between the joists and then add a rigid insulation board over the top, which I believe should be strong enough for storage. Big thing - make sure you don't block eaves ventilation around the edges of the loft or you'll get probs with condensation.

Spudler

3,985 posts

214 months

Sunday 31st January 2010
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Same as the other replys. Do NOT compress. Good idea re the insulation boards, but bloody expensive and you'll only get the 450x1200 through the hatch tho.

JR

13,321 posts

276 months

Sunday 31st January 2010
quotequote all
Throw away the existing stuff and replace with Kingspan to the top of the joists. It's about 3x more effective IIRC and then just t&g on top.

f13ldy

1,432 posts

219 months

Sunday 31st January 2010
quotequote all
I left a small space boarded and insulated with 270mm elsewhere.

This ensured that 1) I got the maximum benefit from the insulation and 2) I didn't clog the loft with junk given the small storage area available.

Sam_68

9,939 posts

263 months

Monday 1st February 2010
quotequote all
andy43 said:
You can leave the exisitng 75-100mm insulation between the joists and then add a rigid insulation board over the top, which I believe should be strong enough for storage. Big thing - make sure you don't block eaves ventilation around the edges of the loft or you'll get probs with condensation.
You may well cause condensation problems if you do this, anyway.

Without boring you with too many of the technicalities, the most basic rule to avoid interstitial condensation is that you should never put anything with a high vapour resistance on the cold side of an insulated element of the consruction.

Both rigid insulation and timber 'boarding' (I assume that you will in fact use chipboard sheets?)have a high enough vapour resistance that they will trap a significant amount of water vapour within the fibreglass insulation, where it may condense in cold weather and cause damp.

rlw

Original Poster:

3,493 posts

255 months

Monday 1st February 2010
quotequote all
Thanks for your comments guys.