cavity wall insulation pushing blocks out during constructio

garyo · Posted: Thu May 13, 2010 11:23 am

As I build the inner leaf of my extension, the 85mm rockwool insulation bats always seem to exert a slight outward pressure on the blocks. This means that regardless of how well they're placed, they always end up leaning out at the top.

I'm using drytherm bats. My outer leaf is made from reclaimed bricks of various sizes, which means that the cavity is only 75mm thick here and there, so this could be part of the problem.

I've been dealing with it so far by placing a spare block up against the block I've just laid to keep it in check, but as the height of the internal leaf rises, this approach is becoming impractical. It seems equally silly for me to have to hold the block in position for 5 minutes in order for it to go off.

Thanks!

Gary

Static · Posted: Thu May 13, 2010 11:46 am

So your putting 85mm insulation in a 75mm cavity? Is that what the manufacturers recommend?

garyo · Posted: Thu May 13, 2010 1:54 pm

The cavity is 85mm for 90% of the time. The odd brick is 5 to 10mm thicker.

I get the impression that the bats are nominally to suit an 85mm cavity, but in reality expand to 90 to 95mm and need to be compressed into the cavity. Hence the problem.

stuart45 · Posted: Thu May 13, 2010 2:30 pm

You should have made the cavity a bit wider. You are allowed a small bit of extra tolerance. As you can see it's not practical to have too small a cavity.

RonnyRaygun · Posted: Thu May 13, 2010 2:40 pm

I reckon you're either going to have to get some 75mm insulation (which BC might not allow) or build up a couple of courses of block at a time, let the muck set, then push in the insulation batts.

garyo · Posted: Thu May 13, 2010 2:46 pm

I think the use of thermalites doesn't help - they're so light they don't have the presence to deal with even a small amount of pressure.

garyo · Posted: Thu May 13, 2010 2:48 pm

Thanks for the replies.

Quote:

I reckon you're either going to have to get some 75mm insulation (which BC might not allow) or build up a couple of courses of block at a time, let the muck set, then push in the insulation batts.

I'm already four courses of blocks off floor level using the existing technique. I think trying to push the insulation in afterwards would be even more painful than temporarily leaning something on the blocks.

stuart45 · Posted: Thu May 13, 2010 2:57 pm

[img]
This is the accepted tolerance.[/img]

theoldun · Posted: Fri May 14, 2010 6:58 pm

First you say you have got 85mm Rockwall Slabs. Rockwall do not do 85mm slab. You then say Drytherm, What I reckon you have are Earthwool Dritherm Slabs (rock), Crown, Knuaf, which are 85mm. thick.
If this is the case, you have two options.
Either rattle it over and clean blocks up and re-lay with a 100mm laid cavity, or corbel over by 25mm and plumb up from there. Dub your bottom out with two scratch coats 4&1 sand cement. Suggest first option, unless you can lay decent block.
You are allowed to corbel over 25% of laid width. Guidance only. old un

jeds · Posted: Sat May 15, 2010 9:16 am

Build the inner leaf first.

noseall · Posted: Sat May 15, 2010 9:43 am

It is normal to get a little push.

For instance, when building 100mm cav's we actually build them 110mm so as to allow for the expansion and does not affect the lintels.

Building the blockwork first is a nightmare.

cyclonebri1 · Posted: Wed May 26, 2010 8:22 am

I wouldn't have thought you have a problem as an out and out amatuer, (assumption).

I cannot see you laying more than 2 courses, 1 batt high all round in a session. Use enough brick ties and you will have no issues inserting the stuff the next day before you mix again,

noseall · Posted: Wed May 26, 2010 5:15 pm

cyclonebri1 wrote:

I cannot see you laying more than 2 courses, 1 batt high all round in a session. Use enough brick ties and you will have no issues inserting the stuff the next day before you mix again,

Two courses of blocks, you avin a larf!?!?

There are few extensions that we can't get up to lintel height and beyond within one day. That is two brickies going at it with one dropping off to do the labouring.

stuart45 · Posted: Wed May 26, 2010 8:20 pm

[quote="noseall";p="1625750"]

cyclonebri1 wrote:

I cannot see you laying more than 2 courses, 1 batt high all round in a session. Use enough brick ties and you will have no issues inserting the stuff the next day before you mix again,

Two courses of blocks, you avin a larf!?!?quote]

He's not joking, I can do 2 courses on a good day.