Cutting Kingspan type insulation board.

Agile · 11 Mar 2015

I want to add insulation board to my loft so that I can store things on top.

But would need to cut the 1200 mm width down to 600 mm to go through the loft hatch.

Whats the best way to cut it to provide as close to a straight edge as possible to enable them to butt up closely against each other as the supplied long sides do?

Tony

geraldthehamster · 11 Mar 2015

With a hand saw. Ideally outside because this will create dust.

Cheers
Richard

Jackrae · 11 Mar 2015

Before you cut each sheet, draw a cut line and put a code mark each side of this line to identify "Sheet-1-right" and "sheet-1-left" etc. That way, when you end up with a collection of half sheets in the attic you will be able to match which sheet belongs to which so that the cut lines match exactly

Agile · 11 Mar 2015

Yes, but my loft has a central walkway and so I cannot put sheets all across the whole area.

I was hoping for a suggestion which would enable me to butt up cut against a factory edge.

How bad is a circular saw cut when guided by a straight edge?

Tony

JohnD · 11 Mar 2015

I think I've seen sheets 600mm wide. Also smaller panels. It does work out a bit dearer.

I don't know if the small boards intended for CWI are in some way different.
http://www.wickes.co.uk/Celotex-50mm-High-Performance-Cavity-Wall-Board-450x1200mm/p/190546

theprinceofdarkness · 12 Mar 2015

Why not measure the gaps between the joists and make a note of them. take this piece of paper down and outside and do all your cutting there and label each piece so you know where it goes. Lot easier to cut outside then in the loft. I am sure that the gaps between the joists will be less then 600mm. Also you can gather up all the long off cuts and fit them as best as possible downstairs, then cut the last one down its length to get you another strip for between the joists.
I would make a jig for measuring the inter joist spacing. Sometimes the joist have "bent over", so measuring the gap at the top will results in the gap decreasing towards the bottom, so a carefully measured and cut thick piece of foam will not drop to the bottom of the gap. Two piece of wood, each short of the smallest gap, with square ends can be dropped in the gaps and clamped when you like it's fit. Whip it out and measure it.
Frank

valcra · 12 Mar 2015

does it really matter about any gaps.

couldn't they be filled after they have all been installed using an expanding foam filler which would also bond edges together and insulate.

water helps with adhesion, so spray mist joints before application.

or

instead of square/perpendicular butt joints, use 45 deg from the perpendicular mitre joints.

hope this is of use to you.

Norcon · 12 Mar 2015

I used a rail saw which cut the sheet accurately to place between the joists and no gaps. Cut outside and carried in.

But I think Agile just wants to place the sheets on top of the joists.

theprinceofdarkness · 13 Mar 2015

yes , could be, the trouble with that approach is that you can't walk on any overboarding. The added weight of a person will crush the insulation where it passes over the joists, leaving a very springy surface. If he is just talking about the four foot around the loft hatch where the stored goods can be reached from the loft trap, it would seem a lot of trouble for not much gain.
As a terrible compromise I had a partially boarded loft, which I lifted and increased the glass fibre underneath it. I then used the B &Q glassfibre which comes in a silver and orange wrapper. I lay my important good on this knowing that it will be crushed. This then gives me about 10' by 15' of storage. to get to the good at the far end, I roll up the top glass fibre and walk on the underneath boarding.
It is a shame that the B & Q product is badly made as the two parts of the wrapper have external "flanges", but they are on the same side of the glass fibre. This means that when the glassfibre is laid, the flanges always interfere. If the flanges were on diagonal corners then the pieces would tesselate properly like roof tiles, i.e. all the top flanges would overlap the adjacent piece of glassfibre in the same direction.
Frank

JohnD · 13 Mar 2015

start at 1 minute 8 seconds
https://www.youtube.com/watch?v=I3Qg7TwTuCc

This is the Knauf Space Boards method (looks like they have been withdrawn from the UK market and are on clearance at Wickes)

Rigid foam slabs on the joists, chipboard or similar on top of the slabs. The foam is rigid enough not to be crushed, and transmits the load from the deck. Long screws go into the deck, through the foam, and fix to the joists.

You do not walk on the foam itself as it would probably break

freddiemercurystwin · 13 Mar 2015

As above, both Kingspan and Celotex can be sat directly on the joists, provided there is a board on top (of the insulation) and the loading is not much, which it will not be anyway for a loft and provided people don't jump around up there.

geraldthehamster · 15 Mar 2015

Agile said:
How bad is a circular saw cut when guided by a straight edge?

I think you're over-estimating how hard it is to cut through Kingspan/Celotex. Power tools are complete overkill. You'll get all the way through with a hand saw in a few seconds. Follow the lines printed on the foil, or rule a line with a sharpie.

Cheers
Richard

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Cutting Kingspan type insulation board.

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