Joist insulation query

dougens

Hello

I'm insulating floor joists in my loft and covering with chipboard loft flooring.

My joists are 4" or 100mm.

I am going to use Knauff mineral wool, I don' want to use Celotex / Kingspan etc.

I have a question:

Should I use 100mm or 75mm thickness?

I've read differing opinions on what is best. As some state that an air gap between the wool and the flooring is beneficial and some say it should be flush.

Many thanks in advance for replies / advise

System

100mm joists? are you sure they will take the weight of the flooring plus whatever else you are putting up there?

dougens

Yes, just light storage, removal boxes etc.

I have 22mm T&G chipboard flooring up there now but no insulation hence putting some in.

Thanks

Doozergirl

100mm isn't exactly great, but it's better than nothing. At 75mm I'd be wondering why to bother. Personally, I would use celotex or similar and get it properly insulated. The pay off will come.

dougens

Thanks for your reply.

My query is, does there have to be an air gap?

Irrespective of the size of the joists I have.
For example, if it was 200mm joists can you use 200mm insulation or better to leave a slight gap as some people say??

Thanks

Gloomendoom

I'd fill it to the top. It will probably settle over time anyway.

ariba10

It is the static air that is trapped in the insulation material that gives you the insulation.

As long as it is not packed too tightly you can fill the void.

Davesnave

dougens wrote: »

Thanks for your reply.

My query is, does there have to be an air gap?

Irrespective of the size of the joists I have.
For example, if it was 200mm joists can you use 200mm insulation or better to leave a slight gap as some people say??

Thanks

Perhaps you are remembering this thread?

https://forums.moneysavingexpert.com/discussion/5132406

dougens

Many thanks for all the replies

I've read that post and looks like the best option is to leave a gap. Even if I am leaving a gap at the eaves / soffits as well just to be safe.

Thanks again