Cutting loft insulation roll

[Gossamer]

Hello,
I want to insulate my loft which has narrow joist widths, the gap is between 280-330mm, less than the typical 400mm pre-cut roll.
Problem is they all seem to be pre-cut.
If you ignore the perforations and just cut to different widths, does the roll hold up ok?
Cheers.

 

[SFK]

Gossamer,
done this many times with Wickes insulation into narrow and wide joist widths, and it normally has been fine for me.
What I did, was NOT take off the plastic wrapper, using a permanent marker I marked on the width that I wanted (a few cm bigger than joists), and I cut using a long bladed bread knife or old saw.
Then only removed plastic from cut-off section just before I used it (as it then expands and becomes harder to handle.
Their pre cut 40cm section is still there, but makes no difference for the insulation.

However, you might find that putting 400mm insulation into 330mm joist gap fits perfectly well. A little side compression will reduce insulation a little, but not enough to get worried about as long as it is not compressed down.
sfk

 

[Gossamer]

I'd hoped that would be the case.
Thinking of EkoRoll (Knauf special for B&Q) which comes in 1140 width, so reckon can get four lengths per roll.
You'd have thought they would do an uncut version, or least make it readily available.

 

[TheVictorian]

I've done it just with an old saw, worked fine.

 

[SFK]

Victorian, You are right, using a saw often better as it's longer. I will change my text with thanks.

Gossamer,
If one near to you, and if you need 4 or more then it might be cheaper at Wickes: http://www.wickes.co.uk/Products/Building-Materials/Insulation/Loft-Insulation/c/1000270?q=:topSellers:brand:Knauf+Insulation&text=#


Also, when you say "you'd have thought they would do an uncut version, or least make it readily available." you are correct, but current along the pre-cut areas is faster and easier and so this is why I often put 400mm sections into 330mm joists (especially as the 400mm sections are actually 1140mm/3=380mm).

sfk

 

[Gossamer]

Cheers,
Ideally I should cut it as close to the width I need though? It should unroll and lay nice and easy filling the gap.

 

[SFK]

Gossmer,
Regarding your "Ideally I should cut it as close to the width I need though? It should unroll and lay nice and easy filling the gap." the answers is of course yes as its cheaper and more air trapped in insulation so better insulation.

BUT, when I did it, my Joists never stayed exactly 40cm apart, I could not cut straight, I had limited time, the loft is too hot or too cold to work in (or somehow both), and I did not want any air gaps whatsoever between the jousts and the insulation (usually where I cut a bit narrow and the joists were a bit far apart). So in the end I cut it wide enough to garentee that even if my cutting was bad it fitted snugly without any airgaps.

Also I found having it a tight fitting (rather than unroll and lay nice and easy) meant that the friction between the sides of the joists and the insulation kept it nice and flat rather than it forming a domed surface.

However, I think you and I are over thinking this as you will quickly find your method when you start. :> Buy the insulation, some good air filters (you do not want to inhale this stuff) and perhaps a paper suit and you will find it a easy (if uncomfortable) job that does make an improvement to your house.

sfk

 

[endecotp]

Some products are perforated in more than one place to suit different joist spacings, e.g.

http://www.insulationshop.co/100mm_earthwool_loft_roll_44.html

If yours isn't, yes just find whatever method of cutting works for you.

 

[mointainwalker]

//www.diynot.com/diy/threads/forum-rules-general-info.19448/

 

[Gerrydelasel]

Thanks for starting this thread -some of the advice hadn't even occurred to me before. I insulated my living room floor today along the same lines!
(Still a bit to do)

 

[noseall]

You will wish you had left the roll at 400mm. Cutting down to fit something only a couple of cm less is bonkers. It will fit ok left at 400mm.

 

[noseall]

We use cavity batts for ceilings (when going in from below). These stay up without the need for netting etc.

 

[Gossamer]

You will wish you had left the roll at 400mm. Cutting down to fit something only a couple of cm less is bonkers. It will fit ok left at 400mm.

The gap between my joists varies between about 330 to 280 mm ish so quite a bit less. I got some Knauf EkoRoll in the end, a good deal at B&Q.

Now the colder weather is here I desperately need insulation, after my renovation work this year I have none in the roof at all. The house this morning was 15 degrees and RH was higher than I'd like (73%).

 

[Gerrydelasel]

We use cavity batts for ceilings (when going in from below). These stay up without the need for netting etc.

Now you tell me!