Cavity 10mm wider than insulation?

[Raine]

 

I once read that external brick & block wall cavities need to be 10mm wider than the insulation.  I.e. 150mm Dritherm 32 would need a 160mm cavity. 

 

I can't find where I read that now.

 

Is that correct, or should I ignore / forget it and go for 150mm cavity?

 

[nod]

You need 50 mil wider than your cavity bat Unless you full fill

[joe90]

I full filled my 200mm cavity with Dritherm, two layers of 100mm batts.

[HughF]

You can get away with 10mm gap if you use interlocking PIR with taped joints, but that's such a faff to do properly, just got full-fill rockwool.

[Canski]

We always add 10 mm to full fill cavity widths as the dritherm tends to expand (especially when wet) and push against the block walls a bit. 

[Iceverge]

Or use EPS beads. 

[mjc55]

  On 18/03/2024 at 09:58, joe90 said:

I full filled my 200mm cavity with Dritherm, two layers of 100mm batts.

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I seem to remember you saying that you built using masonry walls (please ignore if wrong) in which case what did you use for wall ties?

[nod]

  On 18/03/2024 at 10:00, HughF said:

You can get away with 10mm gap if you use interlocking PIR with taped joints, but that's such a faff to do properly, just got full-fill See if you can find a bricklayer that can work to a 5 mil tolerance 

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[Moonshine]

  On 18/03/2024 at 11:07, mjc55 said:

I seem to remember you saying that you built using masonry walls (please ignore if wrong) in which case what did you use for wall ties?

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stainless steel wall ties  i did 150mm cavity with wall ties.

 

You can used Ancon ST1 ties for 200mm cavity but they are expensive.

 

I found vartry very helpful

 

http://www.vartryengineering.com/products/extra-long-wall-tie-large-cavities.html

[Temp]

Pricing error on Jewson site?

 

Says...

 

Ancon ST1 Type 1 Heavy Duty Wall Tie 200mm Pack of 250

 

£3.08 inc VAT

 

https://www.jewson.co.uk/p/ancon-staifix-standard-heavy-duty-wall-tie-200mm-l-CIWT0275

 

They obviously mean per tie but it does say "box of 250" in several places. The page for the longer version doesn't.

[Moonshine]

  On 18/03/2024 at 14:59, Temp said:

Pricing error on Jewson site?

 

Says...

 

Ancon ST1 Type 1 Heavy Duty Wall Tie 200mm Pack of 250

 

£3.08 inc VAT

 

https://www.jewson.co.uk/p/ancon-staifix-standard-heavy-duty-wall-tie-200mm-l-CIWT0275

 

They obviously mean per tie but it does say "box of 250" in several places. The page for the longer version doesn't.

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Let's find out, just ordered 5 boxes for £13 🤣

 

Edited March 18, 2024 by Moonshine

[joe90]

  On 18/03/2024 at 11:07, mjc55 said:

I seem to remember you saying that you built using masonry walls (please ignore if wrong) in which case what did you use for wall ties?

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My builder supplied longer stainless wall  ties.

[joe90]

for 150mm cavity….

https://www.wickes.co.uk/Stainless-Steel-Housing-Type-4-Cavity-Wall-Tie-250mm-101-125mm---Pack-of-50/p/263631?gad_source=1&gclid=CjwKCAjwzN-vBhAkEiwAYiO7oDKQ4uQf84JBkQLArRQUChf3VZT2FmNyO91GhmH9wFRYv6DutBiTVBoCe20QAvD_BwE&gclsrc=aw.ds

Edited March 18, 2024 by joe90

[Raine]

  On 18/03/2024 at 15:07, Moonshine said:

 

Let's find out, just ordered 5 boxes for £13 🤣

 

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What's the news @Moonshine?  Did your 5 boxes arrive yet?

 

[Moonshine]

  On 22/03/2024 at 15:11, Raine said:

 

What's the news @Moonshine?  Did your 5 boxes arrive yet?

 

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They have a 4-8 week lead time on them,so i have a bit of time to wait

[Raine]

  On 18/03/2024 at 10:05, Canski said:

We always add 10 mm to full fill cavity widths as the dritherm tends to expand (especially when wet) and push against the block walls a bit. 

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Yes, I think that's the same explanation that I read / heard before.

 

The problem with that though is that cavity closers (which need to be a tight fit to prevent cold bridging) seem to be sized to match the insulation batts.

 

So, for a full fill cavity, if I go with 150mm Dritherm or Rockwool Full Fill Cavity Batts, and allow 10mm for insulation expansion, that would be a 160mm cavity, but I can't find any 160mm cavity closers.  Plenty at 150mm though.

 

I guess I'll have to spec the cavity at 150mm then, and hope that it doesn't push the blocks out of plumb. 

 

 

[joe90]

  On 22/03/2024 at 15:22, Raine said:

guess I'll have to spec the cavity at 150mm then, and hope that it doesn't push the blocks out of plumb. 

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It didn’t with my 200mm cavity.

[Hughgo12]

Nearly pulled the trigger on the Ancon's at £3 but realised i'll probably never build anything with a 50-75mm cavity

Edited March 22, 2024 by Hughgo12

[Iceverge]

  On 22/03/2024 at 15:22, Raine said:

 

Yes, I think that's the same explanation that I read / heard before.

 

The problem with that though is that cavity closers (which need to be a tight fit to prevent cold bridging) seem to be sized to match the insulation batts.

 

So, for a full fill cavity, if I go with 150mm Dritherm or Rockwool Full Fill Cavity Batts, and allow 10mm for insulation expansion, that would be a 160mm cavity, but I can't find any 160mm cavity closers.  Plenty at 150mm though.

 

I guess I'll have to spec the cavity at 150mm then, and hope that it doesn't push the blocks out of plumb. 

 

 

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Don't bother with cavity closers. 

 

Fix some airtight membrane to the wall and window followed by a J bead screwed to the window and then some plasterboard. 

 

 

 

 

[joe90]

  On 22/03/2024 at 15:28, Iceverge said:

 

Don't bother with cavity closers. 

 

Fix some airtight membrane to the wall and window followed by a J bead screwed to the window and then some plasterboard. 

 

 

 

 

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That’s what I did 👍

[Iceverge]

 

 

All the more reason to use EPS beads too. 

[NandM]

  On 22/03/2024 at 15:31, Iceverge said:

 

 

All the more reason to use EPS beads too. 

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Does the window strap or membrane go across the entire cavity to stop blown in cellulose from going everywhere?

 

*Edit* or perhaps I do the fill after the windows are in place?

Edited March 24, 2024 by NandM

[Iceverge]

  On 24/03/2024 at 07:29, NandM said:

Does the window strap or membrane go across the entire cavity to stop blown in cellulose from going everywhere?

 

*Edit* or perhaps I do the fill after the windows are in place?

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The straps will be intermittent. You could also use angle brackets to bolt them to the outer leaf. 

 

The airtight membrane will be continuous around the perimeter of the window.

 

If you have batts then these will be in before the windows. 

 

If you use EPS beads then these will be done after the windows and plasterboards. 

[Moonshine]

  On 22/03/2024 at 15:11, Raine said:

 

What's the news @Moonshine?  Did your 5 boxes arrive yet?

 

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Apparently they are now at my local store awaiting collection, lets see what i find when i get there 🤣

 

I wonder what my legal options are if there are just 5 ties

Edited March 27, 2024 by Moonshine

[joe90]

  On 27/03/2024 at 13:42, Moonshine said:

wonder what my legal options are if there are just 5 ties

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Receipt clearly states “pack of 250, quantity 5” 🤷‍♂️