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

[SH]

Hello all, I have fink trusses like these:

https://www.mbctimberframe.co.uk/wp-content/uploads/2019/03/Example-Fink-Truss-1.jpg

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

https://www.wickes.co.uk/Loftleg-Raised-Floor-Loft-Legs---175mm-Pack-of-12/p/100412



--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

[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:

https://insulateyourloft.co.uk/rigid-loft-insulation/

[SH]

On 21/11/2021 14:32, alan_m wrote:

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

[Robert]

On 21/11/2021 14:12, SH wrote:
>

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]

On 21/11/2021 14:46, Robert wrote:
> On 21/11/2021 14:12, SH wrote:
>>
>> Hello all, I have fink trusses like these:
>>
>> https://www.mbctimberframe.co.uk/wp-content/uploads/2019/03/Example-Fink-Truss-1.jpg
>>
>>
>> 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.

>>a
https://insulation-uk.com/roof-insulation/pitched-roofs/loft-insulation

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]

On 21/11/2021 14:12, SH wrote:
>

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.


--
Robin
reply-to address is (intended to be) valid

[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]

SH <i.lov...@spam.com> wrote:
> 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....)

PIR isn't too hard to cut: I suggest a thin serrated knife, like a bread
knife. I bought some of these:
https://www.amazon.co.uk/gp/product/B00537PLPY/
although 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]

newshound wrote:


>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.

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
--
Chris J Dixon Nottingham UK
ch...@cdixon.me.uk @ChrisJDixon1

Plant amazing Acers.

[Andrew]

On 21/11/2021 14:32, alan_m wrote:

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]

On 21/11/2021 14:46, Robert wrote:

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]

On 21/11/2021 15:22, Theo wrote:
> SH <i.lov...@spam.com> wrote:
>> 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....)
>
> PIR isn't too hard to cut: I suggest a thin serrated knife, like a bread
> knife. I bought some of these:
> https://www.amazon.co.uk/gp/product/B00537PLPY/
> although I haven't used them on PIR yet (only furniture foam, where they
> work fine, although they aren't the sharpest ever).
>

https://www.tooled-up.com/bosch-t113-a-rubber-cutting-jigsaw-blades/prod/60497/

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.

[John Rumm]

On 21/11/2021 14:12, SH wrote:
>

> Hello all, I have fink trusses like these:
>
> https://www.mbctimberframe.co.uk/wp-content/uploads/2019/03/Example-Fink-Truss-1.jpg
>
>
> 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?????

Yup.

> 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.

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:

https://insulation-uk.com/products/celotex-ga4000



--
Cheers,

John.

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[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.

[John Rumm]

Its common in a warm deck flat roof for example to place PIR foam boards
directly onto roof joists/firings and then place a thin ply cover over
with a flat roofing material bonded / nailed on top. That will withstand
foot traffic easily (not to mention half a tonne of felt and ply).

In fact:

https://www.ikogroup.co.uk/2020/02/12/compressive-strength-of-thermal-insulation/

"IKO enertherm ALU is a foil-faced insulation board and achieves a
declared compressive strength of 175 kPa. IKO enertherm MG has a
mineral-coated glass fibre facing and achieves a declared compressive
strength of 150 kPa. This means they are capable of bearing static,
evenly distributed loads of approximately 17.5 tonnes/m2 and 15
tonnes/m2 respectively without failure."

So say your fink trusses have 35mm wide top surfaces, and you have foam
under 18mm loft grade chipboard panels. Standing directly over a truss,
you load will be spread over say 400mm of truss. Or 0.4 x 0.035 =
0.014m^2. At 15t/m^2 load capacity that would allow up to 15 * 0.014 =
210kg of applied load.

[nightjar]

On 21/11/2021 14:12, SH wrote:
>

If you put the rigid PIR insulation on top of the existing floor and a
new floor over that, wouldn't that solve the problem, as well as being
much easier to do?

--
Colin Bignell