Imcreasing loft insulation thickness when there is already a chipboard floor.....

[SH]

Hello all, I have fink trusses like these:

When I bought the house, there was already 100 mm fibre glass insulation between the fink trusses.

I bought some 170 mm thick loft insulation and I topped up the outer 1/3 of the loft floor to bring it up to a total of 270 mm.

I wanted to use the middle third as a loft storage area and to acess my server racks which are on the gable wall.

So the middle third has chipboard flooring on top of the exisiting 100 mm thick leoft insulation.

Reading around various websites seems to suggest that I could put 100 mm thick sheets of PIR rigid insulation by slimply lifting the existing chipboard floor. resting the rigid insulation on to of the Fink floor joists and then replace the chipboard floor back on top.....

This would give me the equivalent of 300 mm thermal equivalent insulation as PIR is twice as insulating as flexible rolls of loft insulation.

Really?????

I am struggling to accept that the rigid PIR insualtion will take the weight of the chipboard flooring and a persons weight without the rigid insulation developing long channels whenere the fink floor joists push into under compression, and then eventuallu compressing the 100 mm esiting insulation underneath it and the plasterboard underneath that.

Comment most welcome

[alan_m]

You can get stand-off legs for the flooring

[SH]

this is one source that is similar to what I propose to do.

Putting in stand off legs would mean cutting holes in the PIR sheets.....

[SH]

oops i forgot linky:

[SH]

those legs do not seem to have mich in the way of preventing lateral movement though.....

[Robert]

I did this with I think 90mm PIR - using the ready cut sheets for cavity wall insulation. Any weight is well spread out,and any compression of the PIR is going to very limited. I cant see the glass fibre insulation being compressed and impacting the plasterboard ceiling though. Building Inspector was happy with what I did.

[SH]

The main issue is that as PIR sheets are 1.2 m wide, I will have to cut these in half to 600 mm wide to get them through the loft hatch ( I managed to get the 600 mm wide chipboard sheets through no problem....)

[Robin]

FWIW I've had ~1 x 0.6 m lengths of 100mm PIR across 2" Victorian rafters with chipboard loft panels on top in the middle of our loft for

10+ years. I'm across them several times a month on average and they've not compressed noticeably so far.

Of course much depends on the area over which you can spread the load but IIRC what I used takes ~15 tonnes/m2 static load to compress by 10%. And I think you can get much higher with flooring products.

[newshound]

Without doing the sums, I'm also pretty sure you would be fine. Another strategy would be to put the PIR on top of the chipboard, then another layer of chipboard on top of that. If you are not going to have point loads bouncing up and down, 12mm MDF or stirling board would probably be OK for the top layer.

[Theo]

PIR isn't too hard to cut: I suggest a thin serrated knife, like a bread knife. I bought some of these:

I haven't used them on PIR yet (only furniture foam, where they work fine, although they aren't the sharpest ever).

100mm is relatively thick, so the technique is to score one side to break the foil covering and start a crack, and then make a knife-depth cut on the other. It should then snap over an edge/knee fairly easily. There's a bit of dust, but nowhere near as much as people who cut it with power tools. (if I had an electric carving knife I might try that...)

I'd probably cut the length into 4x 600mm strips of 1.2m high, as those are easier to handle than 2.4m x 0.6m strips. If you cut it oversize you can get a friction fit with a bit of trimming, or there's a school of thought that cuts undersize and then fills the gap with expanding foam.

Theo

[Chris J Dixon]

The centre area of my loft was already boarded with a mixture of materials that had accumulated over the years. Much of it was nailed to the joists, and could not have been lifted without risking damage to the ceilings below.

A few years ago I then covered this with two layers of Knauf space board insulation,(1200 mm x 500 mm x 50 mm) with the joints staggered.

As I needed to hold everything down, and the insulation will dent from point loads such as bony knees and storage box feet, I covered it with hardboard (full sheets cut across into 500 mm strips {plus off cuts} at B&Q) and screwed it all down to the boarding using M6 x 120 mm woodscrews which were on offer at CPC.

I was very pleased that the installation went exactly as planned. I did drill pilot holes, but the screws would probably have self-drilled.

The only slight problem, which became apparent after a few weeks, is that (despite the fact that the hardboard spent some of the autumn stored in the caravan on the drive, until I could get the job done) the sheets have expanded slightly and bowed a little.

It has now been in place about 11 years, and is still working fine.

Chris

[Andrew]

Or since this uk.diy make your own out of short lengths of i inch plywood about 6 inches wide and length equal to the depth of the ceiling joists plus upstand required. Cut a 2x2 notch in the upper end to carry 2x2 beams at right angle to the joists. Glue+bolt the lower end through the joists. Space them 600mm apart and fill between with rockwool to make up to 300mm then screw some 11 or 15 mm OSB (its ligher than ply) across the new 2x2 rails.

[Andrew]

If the OP is worried, he could get some 19x38 tiling battens and fit them crosswise and screwed into the joists but with a gap between, say

3x the widtth of one batten. This would spread any loading from the PIR over a larger surface area and they are pretty cheap to buy.

[Andrew]

Or make your own knife by sharpening the back of a hacksaw blade and making up a handle. Or just 'borrow' the bread knife and use the wavy non-serrated edge.

A stanley blade works fine up to 2 inches thick. Cut both sides.

[newshound]

I did wonder about hardboard; you probably don't walk about up there in stilleto heels but I could see heavier items causing damage if moved carelessly.

[Robert]

Cavity wall insulation comes in 1200 x 450mm pieces - easily manageable After researching appropriate tools I ended up using a panel saw to cut the sheets , worked well and didnt create much dust. Didnt think there was any need to purchase anything special.

[Tricky Dicky]

I have a similar truss in my loft but with a pitch of 22.5 deg. Although we do not store things in the loft when doing my recent rewire I needed to create crawl space owing to not being able to stand up at any point. I simply screwed 3?x2? lengths of timber to the W sections of the trusses @ 300mm above the ceiling plasterboard to get above the insulation. The crawl space was created by spanning chipboard flooring panels across the added timbers. It may not be the best flooring but it was only required for the electrical work.

Richard

[Chris J Dixon]

What happens in the loft stays in the loft. ;-)

Actually, there isn't the height for anybody to walk about in any attire! Some of the boxes are reasonably heavy, and so far there have been no issues.

Chris

[John Rumm]

Yup.

If you work out the pressure per unit area on the foam, you will realise that it is not particularly high. So it will not show any serious compression at the point of contact - and especially in very low traffic areas like a loft.

Use over joists and for loft floors etc is one of the recommended uses. e.g:

[SH]

yes but the devil is in the detail: it states clearly:

"Pitched Roof - Loft insulation - Between and Over Timber Joists"

Nowhere does it say that it will also support the mass of chipboard floor *on Top* of the GA4000 *and* that it will support the mass of a person walking on said chipboard without the Fink trusses compressing into the bottom of the GA400 sheets.

S.