what size cavit inwall is best 100 or 50?

10 Nov 2010
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United Kingdom
i'm starting to build the walls on my extension, i'm not up to dpc yet but after looking on forum i have a few queries as to what cavity size is best?

my architect put plans in for 100mm light concrete external block, 50mm cavity and 150mm light conc interior wall with 35mm celotex in cavity. i was unsure whether i was allowed to leave 15mm gap in cavity but build regs say its ok.

Can i not use 100mm thermalites for exterior, 100mm cavity, 100mm thernalite interior and fill cavity with rockwool type insulation?

if so can i start laying 100mm blocks on top of the 150mm blocks as theres only 1 course of 150mm blocks down at present.
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100mm outer masonry (brick block etc) + 100mm full fill dritherm cavity batts + 100mm 3.5n block internal.

Piece of cake.
Just to add to what Noseall has already said, bearing in mind that I'm only a DIYer, and perhaps to aid your understanding a bit more .
I think it's Part L of the building regs that require 0.3 W/M²-K to be achieved for walls in extensions. (New build is something like 0.27w/m²-K , I think).

How you achieve that is really a matter of choice, 100mm part-filled or full-filled cavity, 50mm part-filled cavity with insulite blocks inner skin and insulated plasterboard on the inside.
Whether you use 100 or 150mm blocks is not the criteria, 0.3w/m²-K is the criteria to be achieved.

I thought 25mm was the minimum residual cavity after insulation, but I'm not sure.

If you google Kingspan K8, similar for Celotex and for Xtratherm you'll find some tables with the various constituents for walls so you can make your own choice.
You’re architect sounds like a tool, 50mm is the minimum residual cavity, tell him to read Part C! Be wary of anything else he draws too! 35mm insul sounds slightly ambitious too although I’ve not done a U Value check so might be OK.
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Just spoke to BCO and he insists that 25mm is the minimum residual cavity after insulation, bearing in mind that my BR spec's were passed about 3 years ago.
I'm also exploring options. My spec' was for 50mm cavity, part-filled with 25mm Kingspan K8 (with 25mm additional on inside). Kingspan K8 is about £3.20 each board. Xtratherm, 30mm is £2.75 per board. (I need 100 boards so cost saving worth the effort)
BCO suggests corbelling blockwork out to gain extra 5mm to maintain 25mm residual cavity after insulation.
You’re architect sounds like a tool, 50mm is the minimum residual cavity, tell him to read Part C!

50mm is the minimum recommended for exposed areas, otherwise 25mm will do.

Part C is not required reading, there are other routes to compliance.
Just spoke to BCO and he insists that 25mm is the minimum residual cavity after insulation, ...

He's right and wrong. The approved document says 50mm and that's really what the BCO should stipulate. However, as with all the ADs, they are just one way and there are always other ways to skin the cat.

A 25mm cavity can be acceptable providing the insulation material and methodology has been approved. i.e. as long as it carries a BBA certificate. Kingspan and Celotex, for example, both have BBA certificates for 25mm resudual cavities. But some other insulation which does not have a BBA certificate should really have a 50mm cavity.

Other factors include exposure and the height of the wall etc.

To the OP; I would check that your spec for 35mm insulation complies with the U-value. Doesn't sound like it would off the top of my head.
You’re Inspector wants to check out Part C too!

That's what I thought FMT, after doing some research myself. But it does appear confusing.
e.g. from the Kingspan web site (K8 boards):Dated Oct 2010.
Residual Cavity Width
For wall heights up to 12 m a minimum 25 mm cavity is
recommended, and a minimum 50 mm cavity for wall height
greater than 12 m, and up to and including 25 m after the
installation of Kingspan Kooltherm® K8 Cavity Board
irrespective of the thickness specified. By maintaining a clear
cavity, problems associated with fully filled cavities can be
avoided. For further details please refer to current BBA
Certificate 94/3047.
NB A residual cavity width of 50 mm nominal will be required by the NHBC where nominal
standards of tolerance and workmanship are accepted.

Further on in Part C:
Alternative approach
5.14 The requirement can also be met by
following the relevant recommendations of
BS 5628-3:2001108. The code describes factors
affecting rain penetration of cavity walls.
Cavity insulation
5.15 A full or partial fill insulating material may
be placed in the cavity
between the outer leaf
and an inner leaf of masonry subject to the
following conditions:
a. The suitability of a wall for installing insulation
into the cavity should be determined either by
reference to the map in Diagram 12 and the
associated Table 4 or following the calculation
or assessment procedure in current British or
CEN standards. When partial fill materials are
to be used, the residual cavity should not be
less than 50mm nominal; and
b. A rigid (board or batt) thermal insulating material
built into the wall should be the subject of
current certification from an appropriate body
or a European Technical Approval and the
work should be carried out in accordance
with the requirements of that document....................

I've struggled to find a definition of "Residual Cavity". Is it before insulation or after?

I've reached the conclusion that:
a) 50mm is the minimum cavity
b) This cavity can be filled fully or partly with appropriate materials.
c) if partly filled, then 25mm cavity remaining is the minimum up to 12metres in height. After that 50mm is the minimum.(Edit, re Jeds & Woody's posts, depending on location)

Please feel free to correct me, disagree or enlighten me further.
I have no building qualifications and I'm only now researching the subject.

Also, don't forget my reference to the fact that my spec was passed about 3 years ago and I'm aware that the regs changed last April'ish.
When alls said and done, 25mm is probably OK on an extension but when the cavity’s full of snots and the insulation’s been fitted by a gorilla on a building site 25mm is just not good enough. On a domestic extension if the bricky can be closely monitored or it’s a self build with more care and attention it’ll be fine. NHBC require 50mm too BTW. As mentioned full fill largely eliminates these issues anyway.

Its really a given in the industry that the ‘residual cavity’ is the air gap left over.
thats great chers guys, i first started to question the 50mm cavity after reading a few threadson the forum about having to leave 50mm residual cavity after insulation.

From the replies given it seems that the 100mm cavity is preferrable for me and more straight forward.

thanks alot for the help, (although i may be after more help with different aspects of the build in the future - i read the 'how to's' and search previous threadsbut i find its best to ask you all as it clears things up in my head sometimes.)

ALSO - do you put insulation in under the dpc or not?
I think it has to extend down past the dpc by at least 150mm.
NB. Use wall ties at 600 centres to support the lowest row of boards.
Use wall ties at 600 centres to support the lowest row of boards.

With boards, for the first row (and probably for the whole lot) its best to ignore the standard spacings, and space ties so that they catch the corners and centres of the boards. This stops corners kicking out slightly into the cavity, and you may get two ties close together

As for the wall, then personally, I'd use a small a cavity as possible (50 or 65mm) with one of the full-fill foil backed quilts and a standard 100 or 115 aircrete block. This will keep the wall thickness well below the horrible 300mm ones common nowadays

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