What Gap to Leave for Cavity Insulation?

[zappastix]

Hi

My specs say I have to have a 75mm cavity in regular brick & block.

The insulation is specified as Crown Dritherm Cavity Slabs (made by Knauf).

On the Knauf .pdf it says the cavity can be partially or fully filled.

We are on a coastal site - I would prefer to fill the cavity "fully" because
it gets very cold as there is no protection from other buildings.

From your experience do you think that "fully filling" the cavity is OK to do?

Thanks.

Stephen

 

[freddiemercurystwin]

You have brickwork on the outer skin? Assuming Building Control play it by the book (they do down this neck of the woods) then (depending on your location) the anticipated severity of the weather will need to be identified which in turn will enable the compliant wall system to be specified.

Diagram 12 and Table 4 of this document http://www.planningportal.gov.uk/uploads/br/BR_PDFs_ADC_2004.pdf are the fellas you need to refer to. Come back if you have any questions about interpreting the table.

You can generally do either or and each system will be no better in terms of retaining heat. However there is a general consensus in the industry that full filling the cavity is less prone to poor workmanship so is preferred. Assuming the insulation is suitable for full filling of course, cavity wall insulation is either suitable for partial filling or full filling never both.

 

[theoldun]

Zippy
No one can answer your question without knowing what U value you wish to achieve.
Post one of two things, your target U value or date Building Regulations were approved. Can put you right then.
One other thing, how far from beach are you and east or west of Brighton?
oldun

 

[zappastix]

Thanks theoldun

I have just tried to decipher the Chartered Surveyors handwriting again for all the specifications and it now reads "fully insulated with Crown . . .

Sorry I should have picked up on that before.

So he must mean "fully" I'll run with that.

He didn't specify a U value.

East of Brighton, just over a stones throw.

Thanks.

 

[freddiemercurystwin]

These drawings been approved by Building Control? U Value required for new walls for an extension to a dwelling is 0.28, I'd be very surprised if 75mm Crown full fill would achieve that.

 

[theoldun]

I have just tried to decipher the Chartered Surveyors handwriting again for all the specifications and it now reads "fully insulated with Crown . . .

So he must mean "fully" I'll run with that.

Freddy, Might just as well chat to you as Zippy, as he appears to pay no attention what so ever to any advice you try to give him.
You are correct, even with Crown Drytherm 32 with a 75mm cavity and a block with a lambda of 0.11 you will still not get down to 0.28W/m2K.
East of Brighton, stones throw from beach, places building in zone 4 for driven rain. Still none of my business if he full fills with 75mm crown (K) value unknown depending on what Crown product he uses
One thing though Fred, even if he does the opposite of what you suggest Zippy always put’s a thank you up. Fair play to him.
By the way Fred are you still accidentally seeing your neighbours in their hot tub.

 

[freddiemercurystwin]

No accident oldun, anyway moved a while back, really had to crank me head round to get a proper eyeful though!!!

 

[zappastix]

Oldun, I'm not ignoring you - and follow/welcome the advice given.

We started this extension right at the end of the 3 years allowed - so maybe the specs have changed.

The website for Crown Dritherm Cavity Slabs 32 Ultimate state these slabs are ok for this area.
And you can fully fill the cavity.

We were going to use Solar/Turbo? breeze blocks - would that help with the U value.

If not can you suggest an alternative?

Or should we use a 100mm cavity?

Advice appreciated.

Because I am very busy with this build I cannot always reply straight away.

 

[freddiemercurystwin]

Probably a 100 would be OK but subject to exposure as mentioned in my first thread. Only sure way to confirm is a call to the insulation manufacturer who will want to know what blocks you are proposing. All this should be done by your designer though really that is what you have paid him for.

 

[zappastix]

UPDATE

I have just gone back to the website - this is what it states;

DriTherm Cavity Slab 32 Ultimate has a thermal conductivity of 0.032 W/mK

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DriTherm Cavity Slabs are for the thermal insulation of masonry cavity
external walls, either partially filling or fully filling the cavity. They are
approved for use in buildings up to 12m high in any exposure zone and
for use in multi-storey applications up to 25m in height.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Another website says;


This section includes solutions for all three types of cavity
wall insulation shown below. The introduction of higher performance
products, such as Crown DriTherm Cavity Slab 32, means that U-values
can be improved by approximately 10% compared to standard
Crown DriTherm Cavity Slab, with no increase in cavity width.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
So the slab I was going to use is 10% better that 0.28W/m2K
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Am I right?

If not can you give me any pointers at what I should do.

Thanks.

 

[mointainwalker]

Am I right?

No !

These figures denotes the amount of heat lost (in W ) so you are 10% worse , going by the quoted figures.

 

[freddiemercurystwin]

Only sure way to confirm is a call to the insulation manufacturer who will want to know what blocks you are proposing.

 

[theoldun]

Zippy. Sunday morning. Suppose you have been for a paddle.
Your regs passed in 2008, explains your 75mm wide cavity. Back then U value was 0.32, and technically that is all you need to achieve.
Don’t care what you have read you will not achieve 0.28 with 75mm cav, how ever you can achieve 0.30 by using 75mm Crown Dritherm Plus Slabs, repeat PLUS slabs (not 32s) with a block of lambda value of 0.11W/mK.
Blocks with 0.11 lambda. Celcon Solar, Durox Superblck, Thermolite Turbo, Topblock Superbloc and Toplock Toplite GT. Standard dot and dab internal finish.
If you do want to get to 0.28 or lower then increase cavity and fill to 100mm. Come back if you want spec.
oldun

 

[zappastix]

Oldun - I wish I had the time for a paddle!

As always, you're the man - you have provided me with what I need.

Just had 4 sheets of A142 and 15 x 3 metres or R10 delivered.

Hope to start the raft tomorrow.

Cheers.