Cavity Wall & Full Fill Technique/Order

VDubDan · 23 Oct 2019

Right, apologies for the endless questions! (This is where you realise how much you didn't learn on your bricklaying course, and why people hire builders

)

I've decided to switch to full fill insulation, can I just sanity check the technique:

1. Put a board or something down between the outer leaf and inner leaf to catch mortar
2. Build up the outer leaf first, at least a few courses but as high as I like (up to the lintels, at most I guess)
3. Clean off all snots etc
4. Place insulation between wall ties
5. Lay inner course

The only bit I don't quite get is how you clear the mortar off the back of the inner course? My Mrs'll kill me if we get damp after

cdbe · 23 Oct 2019

I'm using dritherm 32. It tells you on the packaging: first skin up to wall ties 450mm vertically then one course on top to hold tie wires in place, place batts in and they'll be held in place by top tie wires then up with the second skin. I guess snots are unavoidable but if your beds and perps are reasonable there should be much. Personally I think it would be better to stagger the horizontal block/batt joint to reduce any bridging risk:

But that would involve cutting slots in the batts for the ties which would compromise the insulation. In my case I don't seem to have much oozing out of the outside joints so presumably must be a similar story inside.
On my first wall I did 450mm out/in/out/in but I was building overhand from my side so it helped to do in that way, otherwise I suppose as long as your wall is stable (a few corners) you could go higher on one skin first. I'm building the inner blockwork pretty much freehand - just check the cavity width is (75mm in my case) correct every 2 courses and you can't go wrong as long as your outer skin is plumb!

VDubDan · 23 Oct 2019

Thanks mate, gonna go and get some packs later so didn't know if they'd have any guidance

23vc · 23 Oct 2019

No hard and fast rule, most prefer to build the outer skin first, wouldnt say to all the way to lintel height tho, can’t see the point in that and could be a bit fragile, happy to be told otherwise tho.
if you’re a “mixer load every few days when you get a chance” kind of guy like me, I’d do the outer skin (which also may well be a different mortar mix to the block work) until you’re however many block courses above the inner skin to warrant a full mixer load, then do the block skin. Then repeat ad infinitum. At least that’s what I do and it works for me.

The only thing you want to avoid is going too high with both skins without insulation and trying to ram the insulation down the cavity, or laying when the opposite skin is above the level of the insulation unless you use a board to keep the mortar off, you’ve then gotta clean the mortar out of the gap. You can bend the ties a bit to pin the insulation in place if that helps.
All in all it’s not rocket science, and easier than rigid

Leofric · 24 Oct 2019

Can't resist asking but what sort of bricklaying course did you do if you think you can build only outer leaf up to lintel level

noseall · 24 Oct 2019

Leofric said:
if you think you can build only outer leaf up to lintel level

How can you build the brickwork, without backing it in with blockwork and go beyond lintel level?

noseall · 24 Oct 2019

23vc said:
wouldnt say to all the way to lintel height tho, can’t see the point in that and could be a bit fragile, happy to be told otherwise tho.

No dramas as long as you dont leave vulnerable panels without some support. Done all the time.

VDubDan · 24 Oct 2019

23vc said:
No hard and fast rule, most prefer to build the outer skin first, wouldnt say to all the way to lintel height tho, can’t see the point in that and could be a bit fragile, happy to be told otherwise tho.
if you’re a “mixer load every few days when you get a chance” kind of guy like me, I’d do the outer skin (which also may well be a different mortar mix to the block work) until you’re however many block courses above the inner skin to warrant a full mixer load, then do the block skin. Then repeat ad infinitum. At least that’s what I do and it works for me.

The only thing you want to avoid is going too high with both skins without insulation and trying to ram the insulation down the cavity, or laying when the opposite skin is above the level of the insulation unless you use a board to keep the mortar off, you’ve then gotta clean the mortar out of the gap. You can bend the ties a bit to pin the insulation in place if that helps.
All in all it’s not rocket science, and easier than rigid

Thanks mate, that's properly helpful. Feel like I've finally got my head around what I need to do now! Doubt I'll actually go many courses higher on the outer, but I just wanted to understand the limitations.

^woody^ · 24 Oct 2019

You would always tend to build the outer skin first, as this allows the openings and levels to be set out more accurately to dimensions or bricks.

If there was a Clerk of Works on site then he would only let you build 450mm of brickwork, before backing it up with 450mm of blockwork so as to load the foundation equally - and allow the ties to be set at perfect level and 90° to the wall and ensure that there are no mortar snots between the insualtion bats. But I digress.

As for preventling snots when laying the second leaf, that comes down to technique. You need to angle the back of the bed joint so that when it is compressed, the mortar does not squeeze outwards past the edge of the block.

^woody^ · 24 Oct 2019

noseall said:
How can you build the brickwork, without backing it in with blockwork and go beyond lintel level?

Use some lengths of 4x2 under the bearings.

Leofric · 24 Oct 2019

^woody^ said:
If there was a Clerk of Works on site then he would only let you build 450mm of brickwork, before backing it up with 450mm of blockwork

So you should not build the outer skin of bwk all the way

up to lintel level before building up the inn

er leaf of blockwork :-

23vc said:
most prefer to build the outer skin first, wouldnt say to all the way to lintel height tho, can’t see the point in that and could be a bit fragile,

-yes

noseall said:
How can you build the brickwork, without backing it in with blockwork and go beyond lintel level?

-

don't understand, you don't build up to lintel level in outer skin of bwk only without backing with blockwork, that is the point

charliegolf · 24 Oct 2019

How does full cavity fill stop moisture transfer between skins? It seems to fly in the face of the reason for the cavity?

^woody^ · 24 Oct 2019

Voodoo

charliegolf · 24 Oct 2019

I knew it! Iwas convinced it wasn't natural!

noseall · 24 Oct 2019

charliegolf said:
How does full cavity fill stop moisture transfer between skins?

The cavity batts are coated with water repellent.

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Cavity Wall & Full Fill Technique/Order

Leofric

Leofric

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