# Cavity size for new build



## Muffet (19 Jan 2011)

Hi,

This is my first post to this site. I have read lots of threads and the information has been very helpful.

We have submitted plans and are reasonably confident that our application will be successful. We have an architect fully involved who has directed us to ensure we are maximising sunlight etc.

We are keen to have a high performance house (possibly A3 ish) and are trying to obtain as much information as possible. One area that is already confusing us is the size of the wall cavity. Our architect is suggesting 120mm but I see differing cavity sizes in different threads and I have recently met somebody who is building a house with 200mm cavities. 

I appreciate that this is only one part of the build but would be grateful for any feedback.

Thanks
Muffet


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## Docarch (20 Jan 2011)

120mm would be the maximum size for a 'standard' cavity, using 'standard' wall ties, lintels, cavity closers, etc., etc.  A cavity any wider than 120mm generally needs the input of a structural engineer to look at and specifiy wall ties, etc.

This is probably why the architect has specified a 120mm cavity.


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## threebedsemi (20 Jan 2011)

hi. 
some of the issues have been trashed out in this recent thread:

http://www.askaboutmoney.com/showthread.php?t=131300


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## Muffet (22 Jan 2011)

Thanks for the replies. Yes it looks like our architect is sticking to the letter of the regulations in all aspects of the new build. I appreciate that we would need an energy assessment to find out exactly but could anybody shed a guess at what energy rating we might receive if we went with the "standard" i.e. standard insulation in walls, floors, roof etc.

We are not trying to achieve a passive house but we would like to achieve a standard which means that our heating system etc will be effective / economical.

I had A3 in mind as a standard to achieve but does this mean that I have to go with non standard cavity / insulation i.e. 150 or 200 etc??

I do appreciate that lots of other factors go into the rating but would appreciate any input.

Thanks


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## Docarch (23 Jan 2011)

Muffet said:


> I had A3 in mind as a standard to achieve but does this mean that I have to go with non standard cavity / insulation i.e. 150 or 200 etc??


 
Theoretically you could easily achieve an A3 rating building with a 120mm cavity - but as you allude to, there are so many other factors to be considered in a rating.  

I would sussgest you get a provisionl BER carried out to help assess your options.  If your job is a new build, a BER is mandatory.


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## onq (23 Jan 2011)

Docarch,

Allow me to be the innocent here and ask you to post a relevant link to the legislation.

Is the requirement to obtain a BER cert buried somewhere in Part L?

8.31 am on a Sunday morning..?

ONQ.

    [broken link removed]

    All advice on AAM is remote from the situation and cannot be relied                 upon    as a defence or support - in and of itself -   should     legal       action     be    taken.
    Competent legal and building professionals should be asked to advise         in            Real Life with rights to inspect and issue reports    on   the        matters    at     hand


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## Docarch (23 Jan 2011)

onq said:


> Is the requirement to obtain a BER cert buried somewhere in Part L?


 
Hi ONQ

Apparantly I am not allowed to post links on the site until I have 15 posts? 

_All new homes (even when not for sale) must have a BER certificate before they are occupied as detailed under S.I. 666._


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## Muffet (23 Feb 2011)

Hi 

My architect has come back to me with the specs and I would be grateful for some input on the proposed insulation specs. As mentioned in my previous post he is proposing a 120mm cavity. Our goal is to achieve a high level of insulation which will reduce ongoing heating bills.

Wall Insulation: Xtratherm Thin R cavity wall (80mm) in the cavity with 37.5mm Xtratherm Thermal liner on the inside wall.

Floor: Xtratherm Thin-R Underfloor (125mm)

Ceiling insulation: Mineral fibre 150mm between joists 150mm over layered to underside with 42.5mm Xtratherm XT/TL Liner

We are building a 1.5 storey house which is c. 3000 sq feet.

Any feedback would be greatly appreciated.

Thanks


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## Muffet (27 Feb 2011)

Hi,

Could anybody help on the below. Any help would be much appreciated.

Thanks


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## onq (27 Feb 2011)

Muffet said:


> Hi,
> 
> Could anybody help on the below. Any help would be much appreciated.
> 
> Thanks



Muffet,

First, your stuff appears "above" your post, not below - had me looking for a while there.

Second, in post #5 above Docarch advised you to get a preliminary BER report done. I agree with him. The assessor will advise you fully in relation to the theoretical rating you *should* achieve. Whether you achieve it or not is down to the quality of the design/details and the workmanship of yoru contractor(s).

Third, I think your ceiling spec will freeze out your attic and all the services therein and lead to problems down the line.
You're posting about a 1.5 storey house and you don't have a spec for the sloped roof that addresses interstitial condensation?
I'd suggest that you allow your insulation to follow the line of the (presumable pitched) roof, keeping water tanks and services warm.

I mean, you're having 300mm mineral fibre in two 150mm layers above 42.4mm extratherm - in certain conditions, the top of it will just get wet!
Your archtiect needs to get advice on this before going to site and decides on whether he is going to include the 50mm air cavity and detail to suit.

Finally and for the record you don't post like you have actually appointed an architect or been advised by one, certainly not one who's up to speed on all of this.
Insulation of the envelope and sun path orientation is only part of the issues here in achieving a good rating on a house and arguably it can all be undone by other things.


 What about in-use airtightness, as opposed to costructional airtightness and use of air-lock approaches?
 Why the predeliction with cavity construction when externally insulated masonry can work better?
 Why the lack of consideration for using a mechanical ventilation and heat reclamation system?
 I think you need to take a more global advice briefing on this and stop focussing on the insulation to the exclusion of all else.

ONQ.

    [broken link removed]

    All advice on AAM is remote from the situation and cannot be relied                 upon    as a defence or support - in and of itself -   should     legal       action     be    taken.
    Competent legal and building professionals should be asked to advise         in            Real Life with rights to inspect and issue reports    on   the        matters    at     hand


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## onq (27 Feb 2011)

Docarch said:


> Hi ONQ
> 
> Apparantly I am not allowed to post links on the site until I have 15 posts?
> 
> _All new homes (even when not for sale) must have a BER certificate before they are occupied as detailed under S.I. 666._



Apologies for not following up before Docarch and thanks for taking the time to reply.

ONQ.

    [broken link removed]

    All advice on AAM is remote from the situation and cannot be relied                 upon    as a defence or support - in and of itself -   should     legal       action     be    taken.
    Competent legal and building professionals should be asked to advise         in            Real Life with rights to inspect and issue reports    on   the        matters    at     hand


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## Muffet (28 Feb 2011)

Hi onq,

Thanks for the reply. I can confirm that I do have an architect. As per my initial post we are waiting for planning permission to come through during which time I am researching construction methods, insulation, air tightness etc. I am also in the process of getting a preliminary BER.

I asked my architect to provide the spec for insulation. My architect has recommended block cavity build and has provided the spec outlined below. Thanks again for you comments on it. We do plan to have an airtight house and to have mechanical ventilation & heat recovery.

I do have a couple more questions on you comments:


Could you explain more what you mean by “in-use airtightness, as opposed to costructional airtightness and use of air-lock approaches?”. As above I do plan to have an airtight house and have done a little research but I was not aware that there were different types?
When you mention getting global advice – how do you go about getting such advice? I do have an architect but as above he is keen on block build & provided the above insulation spec which he tells me is a very high spec. I am keen to keep the same architect but is it possible to get somebody else to consult in relation to the issues above?
Many thanks


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