cavity wall design to meet U values

[diydad888]

Hi,

My draughtsman submitted (and now approved) plans and building control for front and rear extensions with a timber frame with brick/block skin.
Happy with that in front but I wanted trad construction to the rear as the kitchen will be there and I want to be able to hang my units anywhere without hassle. I now need ammend and submit new wall design.
I need some advice please, i want trad cavity wall and wanted keep it as narrow as poss to maximise interior space I was hoping to get to 250 brick and block with 50 mm cavity i am not sure a 30 mm insulation in the cavity would pass U values?? would i need to go to a 300 wide wall with 100 cavity with 50mm or more insulation.
Can anyone advise ?
thanks in advance,

 

[RedHerring2]

As fas I understand there are several options:
1. 50mm cavity with 25mm insulation (you must maintain 25mm void remaining) If you want 30mm insulation then aim for 55mm cavity. But you'll need to add about 25mm insulation on the inside, e.g dot & dab including pb.
2. 100mm full fill with drytherm, no residual cavity required.

3. 100mm cavity with 50mm insulation.

You may be able to adjust the insulation requirement slightly by using lightweight aerated blocks but then you may be disappointed with the ability of the finished wall to hang your kitchen units.

I believe that if you contact the chosen insulation supplier and ask them they will give you the U values of your suggested construction.
You'll then have something to show the BCO.

 

[diydad888]

Thanks, for the help, seems I must have at least a 100mm cavity with insulation to meet a minimum U value standard.
As you suggest I will contact some insulation suppliers for detail info, thanks again.

 

[RedHerring2]

Not necessarily, but, as far as I understand, you do need minimum 50mm insulation. Whether that is in the cavity, wholly or partly is your choice.

 

[diydad888]

You're on the ball
Thanks again, I got it, I can go with cavity fill on 50 and acheive it I think, what is the general thinking, does cavity fill cause any probs or is it best to have an air gap for what ever reason

 

[RedHerring2]

I must admit to having little experience in either, part or complete fill.
I know one poster, far more experienced than I, will suggest full-fill (100mm) Drytherm.

My experience is limited to part fill (30mm insulation) in 50mm cavity with 25mm added on the inside. (30mm was chosen over 25mm only due to cost)
The preference, I think, comes from the ease of building, rather than the U values attained.
However, if you're planning on a DIY build, I would suggest that you consider a part-filled cavity, 'cos as an amateur, it's probably easier to remove/prevent snots falling in the cavity that it is to prevent then in a full-fill cavity.

But, as I say, my experience is very limited.

 

[jeds]

What u-value are you trying to achieve?

 

[theoldun]

Good question John. Not being negative, but no point in trying to give advice until current U value required is known.

To the best of our knowledge certain Scottish Standards differ considerably to Approved Documents south of the border.
oldun

 

[diydad888]

Thanks folks, I will get the value and post it later today

 

[diydad888]

Hi folks, just read my building control info, the U value to be achieved is 0.25w/m2. I would prefer the insulation in the cavity rather than interior.

Cheers

 

[jeds]

You won't get to 0.25 with less than a 100mm cavity. My suggestion would be a full fill 100 cavity with a higher grade internal block such as a Thermalite Turbo or similar. Even then you might need to source a slightly higher grade insulation such as a Dri-Therm 34.

 

[theoldun]

102mm brick facework, 100mm of full fill cavity Earthwool DriTherm 32 and 100mm standard aircrete block, will give you 0.24 W/m2K.
Ditto to above but with light weight aircrete block will give you 0.23.
Would suggest with both above dot and dab.
Do not think about partial fill, it is a pain in the backside.
By the way lower the u-value, better the insulation.
oldun

 

[diydad888]

Thanks to all, this forum is a good place with very helpful people. Nice to get advice from well experienced people doing things properly...great !

 

[diydad888]

Actually...I still need help !
Blocks, can anyone help me spec the best block to meet the 0.24w/m2k,
I see you said 100 mm std aircrete, when I contacted Knauf for U value spec sheets they asked me what thermal conductivty the block was, I looked around the web but so many types.
My front ext will be bric/block but the rear will be block/block as it will be render finish outside to match existing, what blocks to use? should either of the leaves be different density of block or both ok on the same.
Extension to rear is 3m X 5m long.
Sorry for the ping pong, the more you find out...the more you need to know, thanks

 

[DevilDamo]

Celcon Standard, manufactured by H+H is a good thermal block...

http://www.hhcelcon.co.uk/by-product/celcon-standard-grade-blocks