Loft boarding - Celotex

[spugh99]

Hi,

I'm just about to start boarding my loft as I need some extra storage space. But need some advise before I start.

At present I have 100mm rockwool between the joists.

I have been given some Celotex 100mm PIR boards that my dad had left over.

I am planning on putting these boards above the joists so I can get the recomended 0.16W/mK U Value.

When I'm laying the celotex do I need to put chipboard down first for the celotex to rest on. Or can I lay celotex directly onto the joists with chipboard on top.

I.e

---------Joist
Layer 1. Chipboard
Layer 2. Celotex
Layer 3. Chipboard

or

---------Joist
Layer 1. Celotex
Layer 2. Chipboard

I would rather do it the second way as that would save me buying alot more chipboard. Will the celotex be be strong enough to store things on without chipboard underneath ?


Also what are the best screws to use, I guess I will need 150mm ?

Thanks

 

[PrenticeBoyofDerry]

If I was doing it, i would get some 125mm timbers run them at right angles to existing joist, every 400mm and drop the celotex between them.
This will leave a 25mm space for electric cables to be routed and prevent any de-rating of the cables current capacity.
If you have electrical cable totally surrounded be thermal insulation, it effects the safe current that can be run through them and they can overheat. This will be an issue if any socket/shower circuits are run through the loft space but could also effect the lighting circuits, depending on the fuse ratings and cable size used on these circuits.

 

[spugh99]

Thanks,

There is only the lighting circuit (6amp) in the loft, and all the cable is 1.5mm t&e.

The cables are currently clipped to the joists & have the rockwool around them. Im fairly sure there won't be enough slack to raise them any higher, even if I unclip them.

Is it ok to put more insulation over the 1.5mm cable ? If so I think i'll just put the Celotex on the joists and board over it. To save costs

 

[PrenticeBoyofDerry]

1.5mm Twin & Earth can carry a maximum of 20 amps when clipped direct.
Worst case is a 0.50 de-rating factor, with insulation covering over 500mm of cable route.
This sounds likely in your installation, the calculations say your cable will safely carry 10 amps, so as the protective device on the circuit is rated at 6 amps, which is less than the 10 amp maximum after de-rating.
It should be safe.

 

[spugh99]

Cheers,

I'll lay the celotex across the joists and lay the chipboard above it. Then use some 150mm screws to attach them to the joists.

Will this be safe/strong enough to store items & occasionally walk on ?

 

[PrenticeBoyofDerry]

Though Celotex at 100mm thickness should be quite rigid, it's not something I would do or over trust, as the celotex will be under pressure over the joist support and could fail at that point and as result you may find your loft boards becoming uneven and sinking, worst case could be that the downward pressure would put force on to ceiling and also any cables that may be routed at right angles to the joists.
Saying that if the loft is not being used for foot traffic and light storage only, the effect should not be as great