Fitting the cavity insulation

[trevorbayliss]

At what level do you start the cavity wall insulation ?

You can see i have placed a joist in its finished position temporarily for today. This will be the floor level. The breeze block course with the cavity air vents is the bottom of the cavity and i have formed a ridge of mortar in this channel so any water will hopefully push outwards.

So would i set my full fill insulation down into the bottom of the cavity, or do i start it a little higher up, at the top of the first line of breeze blocks, which will be floor level ?

Also does it matter if the cavity is a little narrower than the 100mm insulation. The facing bricks are wider and so the cavity is going to be 90mm wide i think.

 

[chappers]

should be down to the same level as the insulation in the floor structure

 

[trevorbayliss]

Ok thanks

Can anyone comment if i have a problem with my full fill insulation if my cavity gap is smaller than the insulation. I have 100mm, but the gap is looking like it is 90mm as using old reclaimed bricks and they are wider. Does the insulation compress up a bit and is this ok ?

I am still at ground level so could change to insulation board and gap, but i have bought my insulation. Suppose i could change though.

 

[tomfe]

Yes you'll have problems, it can be nightmair at pushing bricks out.

Why have you gone for a suspended timber floor? It would have been easiler, cheaper and better to do a concrete slab. You sill can get rid of you hangers add 250mm of eps then do a screed.

 

[stuart45]

With full fill insulation I always make the cavity a few mm over the 100mm.

 

[trevorbayliss]

I have done a suspended floor and it is all in now (not the actual timber, but the hangers). For me it was much cheaper and easier to do a suspended floor.

I had a choice of filling back in about 8T of material or carry 15 pieces of wood up the drive. I am doing all the work so a no brainer for me. Plus the materials in suspended are much cheaper overall. My labour is free.

Ok so it looks like i will need to do the wall with rigid insulation. I think this means i have to build the block work first. What size sheets do i work with. Do i use the 450mm ones or get the big sheets and cut to size ?

 

[^woody^]

Ok so it looks like i will need to do the wall with rigid insulation. I think this means i have to build the block work first

You do realise that using rigid insulation requires a much higher degree is skill and accuracy to keep the wall flat, and have more ties (and in the right places) so that the insulation does not project anywhere?

Normally this means building the block wall from the outside so that the face is in the cavity. This also means that setting out of the openings to match the brickwork needs to be spot on.

You use the 450mm boards.

 

[trevorbayliss]

Well i have set out both leaves and didn't account for the wider brick size so using rigid seems my only option.

Not sure what you mean by building from the outside and face is in the cavity. I was planning on doing (as i was with full fill), to build some courses of block work, then fit the insulation to match and then do the outer leaf. And just work up like that slowly. A bricky will prob get the block wall up first and insulation in and then do the outer leaf.

Could i use a soft full fill which is narrower. I see Knauf makes a 85mm product. I will need to speak to Knauf to see if this is acceptable for regs.

 

[Ian H]

Yes you'll have problems, it can be nightmair at pushing bricks out.

Is that while the mortar cures or after?

I'm using 75mm stuff on mine but the cavity is slightly less in places.

 

[trevorbayliss]

Is this ok with BC then Ian ? I am doing an extension on my house. I don't know what the minimum amount of full fill is to meet regs. If it is 85mm then all ok as my cavities are 90mm. I was thinking you had to have 100mm minimum for full fill.

 

[trevorbayliss]

I think i have a good solution all round. 85mm insulation in the cavity full fill. Then use an insulated plaster board internally. The two will give me a good level of insulation to meet regs.

 

[Ian H]

Is this ok with BC then Ian ? .

The guy who drew the plans seems to think we have a 125mm cavity. We are just raising an extension by a few feet so 75ish mm is all we have.

 

[^woody^]

100mm quilt will fit in a 90mm cavity no problem. You can always oversail the block leaf 10mm or so if need be.

The cavity will get wider as the wall is built up anyway. This is a natural occurance.

 

[trevorbayliss]

I will ask knauf tomorrow. Got some people who says this is a problem with pushing the facing brick out and woody saying it is ok. I can change the insulation at this stage so no issues, just don't want to be fighting the wall as i build up.

Interesting that the cavity gets bigger the higher you go. I wonder why this is ?

 

[stuart45]

Build the brickwork up first on full fill. You may as well use 85mm insulation in a 90mm cavity as crushing it makes it less efficient.
Cavities tend to get bigger as masonry tends to go outwards out of plumb as the wall goes up rather than inwards, but in theory it should stay plumb.
Oversailing at the bottom may be your best option. Often done at joist height to reset the cavity size.