Loft Boarding (Cross Baton & Celotex)

19 Jan 2006
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West Midlands
United Kingdom

I live in a fairly modern house (2001) with standard loft insulation laid between the joists and overlapped at 90 deg.

I'd like to board out part of the loft for light storage. I've read that one common approach would be to install new 4" x 2" joists at 90 deg to the existing and board on top. However, the existing joists are only 3" high and although the existing insulation could be compressed to fit between the existing and new joists, I would be far off the recommended 270mm depth of insulation.

One option I was thinking about was to leave the existing insulation between the 3" existing joists and continue to install new 4" joists. However, I could then install either 75mm or 100mm (tight fit) Celotex between these joists. The improved thermal performance of the Celotex, together with the chipboard, should keep the boarded space R-Value similar to that of 270mm of mineral wool insulation.

I'd be grateful of any thoughts on whether the above will work, or any better ideas for my situation.

Many thanks,

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I'd squash the existing insulation down. The boarding you are putting over has an insulation value of its' own, quite reasonable if you use chipboard.

I think it would more than compensate for the modest loss of efficiency is squashing the insulation down.

(I actually think 270mm is thicker than any benefit it gives, I don't think there is a genuine need for more than about 150mm anyways)
don't squash insulation. it will press on the ceiling and make it bow or crack.

you can lay insulating foam boards on top of the timbers, and lay chipboard flooring on top of that. No counterbattens required with this method. Your local DIY shed or builders merchant will probably have a display of the stuff.
I disagree with andy.

If you squash the insulation your insulation value is reduced to the thicknesss after squashing i.e. squash 200 to 100 and it insulates like 100. I only do this for sound-insulation in stud-walls between rooms.

Chipboard has only 25% of the insulation value of (uncompacted) fibre-wool.

Do agree however that 270 mm is just outlandish and personally don't go above 200 mm even in the Alps where I live.

From an insulation point-of-view I don't think boards are worth their money. Their extra insulating properties come from gases used in their manufacture which leak out of the structure over the years.

After 7 years or so, the insulating properties are no better than fibre-wool or expanded polystyrene but have cost 3 ( ?) times more.

Having said that, you have to use boards for their rigidity if you adopt the suggestion made by John which gives a neat solution.
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Just found this and am interested as we had a similar question after moving into our new house (which also has very thick insulation, completely covering the old joists).

JohnD I am interested in what you said, but being a relative DIY novice not totally clear on what you meant, so wondered if you could explain it a bit more please? Would the foam boards and/or the flooring need to be fixed down to the joists, and if so how can we do that when they are all covered? At the moment it is just like a sea of insulating material up there, so it would not be all that easy to fix anything to the joists unless we either squashed or removed some of the insulation, which we would prefer not to have to do! Thank you.
(Or do you mean that the foam boards replace the existing insulation? - just reread and wondered if that was what you meant!)
fibreglass rolls up to the top of the existing timbers, then rigid foam slabs on top of the timbers, then chipboard on top of the foam if you want to walk on it

look up Knauf Space Board Insulation
It is expensive if you don't need to have a walk-on deck.

your DIY shed should have a display and leaflets

I should think other brands of rigid insulating foam would also work

you have to remove any quilt that is on top of the timbers, but maybe you can re-lay it somewhere else. It must not be squashed down
I see, thank you! The link does make it a lot clearer. Will have a think then, as we don't strictly need to be able to walk on it (though it would be useful) and you said it is expensive. I might check prices next time we get to a DIY place. Thanks again.
Damien-sounds like what I am doing. Read my link.


Whats bothering me is I am reading so many different opinions on what to do. Some say add cross batons 90 degrees to your existing joists and then lay chipboards so as to create an 'air-space' above the existing insulation and boards otherwise you will create mositure if you compress the existing insulation. Others I have heard just board on top.

B & Q would not sell chipboard loft boards if they weren't meant to be in the loft. They aren't designed to hold HEAVY storage like most wood unless you added extra support ie if the loft was converted into another room. Its seems you want to use yours as I do for light storage and the odd occasion you would want to climb into the loft to move stuff around so the boards would have to take our weight.

So why am I reading so many different means of flooring the loft?

I will have to contact a local expert before I mess up :rolleyes:

Sorry for the delay in replying - boarding of the loft has been put off until now.

I think what I am going to do is:

* Cross baton the existing joists with 4" x 2" at 600mm centres

* Fix cross batons to every joist they cross and fix by diagonally drilling and screwing (70-80mm screws) through lower part of cross baton into joist below (being careful for electrics, etc)

* Board using moisture resistant (P5) 18mm chipboard - this comes in 2400mm x 600mm boards but I will have to cut these down to 1200mm length to get in the car/loft

* Infill with 100mm mineral wool (meaning approx. 200mm thickness of insulation in boarded area)

Slightly concerned about the weight of the extra timber/chipboard in the loft but should hopefully be okay with light storage. May even use some joist hangers near the dividing semi-detached wall and put the heaviest storage items in this area.

If anyone sees any problems with the above, I would appreciate any early feedback before I go and buy the stuff needed.

Many thanks,

see if you can get ply at a reasonable price, it is stronger and more rigid than chipboard and does not go soft when damp.

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