# Rockwool rwa45 100mm in wall cavity question?



## Rob Cheetham (1 Dec 2021)

Im currently about to start insulating my workshop. I have all the insualtion to hand. Rockwool 100mm for the walls. Celotex 100mm for the roof and celotex 50mm for the floor.

I have bought 13 packs of the rockwool for the walls with 4 100mm slabs per pack. I now realise that I probably should of bought a few more to cover for off cuts.

My question is, is it ok to just fill in the remaining bays with offcuts when i have no full slabs left or should I only be filling each bay with a whole full slab?

I hope I can just use the offcuts as it will seem a lot of offcuts to waste if not lol

I have included some screenshots of my sketchup wall framing.

TIA


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## Garden Shed Projects (1 Dec 2021)

Off cuts are ok as long as there are no gaps. Start the next bay with the off cut of the previous and your waste should be minimal.


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## Fitzroy (1 Dec 2021)

Great question I plan to use the same insulation and I’d just assumed it’d be fine to use the offcut from the last bay in to the next. A straight cut will be the important thing to prevent gaps. I’m also expecting to have to use a little spray foam in a few places where I have narrow gaps <50mm.


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## Jones (1 Dec 2021)

You can use offcuts wherever you need them. You can cut thinner slabs with a stanley knife by squashing it down with the straight edge or use a saw or bread knife .The slabs should be cut just oversized and sprung in so there's no risk of them falling out, it's flexible enough that you can butt up unless you cut it blindfold. There's a place for gun foam but bodgeing in insulation is not it. Where possible the offcuts should span from stud to stud otherwise they may fall out.


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## sammy.se (1 Dec 2021)

pro tip when cutting the rockwool:
Use a bread slicing knife (like the mercer one) and a magnetic tipped tape measure - you can measure and cut in one fell swoop, with the knife attached to the tape measure. 
I had to cut loads of this stuff last year, and the bread knife was the best thing to use, hands down.


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## Cabinetman (1 Dec 2021)

Jones said:


> You can use offcuts wherever you need them. You can cut thinner slabs with a stanley knife by squashing it down with the straight edge or use a saw or bread knife .The slabs should be cut just oversized and sprung in so there's no risk of them falling out, it's flexible enough that you can butt up unless you cut it blindfold. There's a place for gun foam but bodgeing in insulation is not it. Where possible the offcuts should span from stud to stud otherwise they may fall out.


Plus 1 for this, I found a kitchen carving knife worked well, put the insulation against the stud and use the other stud to run your knife down, with very quickly gained experience you will be able to spring them in - perfect fit. Ian


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## Molynoox (1 Dec 2021)

I used 100mm rockwool for the partition wall in my build (PIR in the external walls). I used offcuts and didn't even think about it, its so compressible that you can get a really tight fit without any effort. Like others have said oversize the cuts then push it in for a tolerance fit. I used a new stanley knife and cut it against the floor and it went through it like butter - didn't see the need to look for another cutting solution. It did take 2 cuts to do it @ 50mm depth per cut, but it took maybe 2-3 seconds per cut so seemed pretty efficient - I was happy with 6 seconds anyway  

I bought 400mm x 1200mm rockwool and my studs are 400mm centres (355mm gap) - thats the perfect tolerance fit for the rockwool on the width. Also, half my apertures were 1200mm tall - so basically zero cuts on the rockwool for those ones just take it out the packet and place it in the wall .

My tip is to wear gloves before you open the first bag, and don't touch *any* part of your body or clothes until you have taken your gloves off again. I learned that the hard way, and had an itchy neck for two weeks. Also, try and make quite gentle movements when handling the rockwool and pushing it into place or the particles go into the air and you will feel it landing on your skin (and your lungs if you don't wear a mask - but you should wear a mask)

Martin


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## Molynoox (1 Dec 2021)

oh by the way, you didn't ask for it, but a small comment on your wall design, I would align the top of the window with the top of door - it looks a lot cleaner if you do that.
Or maybe that's just my OCD.
Martin


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## Rob Cheetham (2 Dec 2021)

Thanks everyone for the replys. Some great advice. Ill try and post some pics after my long weekend off from work. Should be able to get it all done  



Molynoox said:


> oh by the way, you didn't ask for it, but a small comment on your wall design, I would align the top of the window with the top of door - it looks a lot cleaner if you do that.
> Or maybe that's just my OCD.
> Martin



Haha i know where your coming from Martin. I did want to have them both level but I decided to raise the window a bit as that is where my new laguna table saw will be going and i wanted to make sure that the bottom of the window was above the level of the table by a bit. Plus I wasnt sure on where the electrics wer going so wanted to leave some leway. Too late now anyway as its all built lol. Just got to insualte, board the walls/ceiling, floor, final fix on electrics and then move everything in . Thanks again for the help


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## Molynoox (2 Dec 2021)

Ah that makes sense  be good to see your pics of the build.
Martin


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## Rob Cheetham (5 Dec 2021)

Molynoox said:


> Ah that makes sense  be good to see your pics of the build.
> Martin



Ill be doing a thread on the finished build at the end so keep an eye out. Ill try and tag you in it. Half way through insulation atm. Going good so far   

Cheers, Rob


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## Molynoox (5 Dec 2021)

I'm nearly done with my build thread, it's taken almost as long as the build has


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## Rob Cheetham (5 Dec 2021)

Molynoox said:


> I'm nearly done with my build thread, it's taken almost as long as the build has



haha yea i can imagine mine will take a while too. you will have to send us a link to your thread when its done. can you write and add as many photos as you want on a comment in a thread or do you have to do mulitple comments?


just got the rockwool complete and ive ended up with three packs left haha. and theres me thinking id need more. going to sell them for some money back i think. on to the 100 mm celtotex for the roof tomorrow.


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## Molynoox (5 Dec 2021)

I ended up with a load of rockwool left over too - I think the fact that the utilisation efficiency is almost 100% catches you out and you over order.
my thread is here - you are restricted to 10 photos per comment, I split mine into multiple comments, one per stage of the build, it works pretty well
Martin


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