Closing cavity at the top of the wall

[kirstent]

Hi All,
Wondered if anyone could shed any light on closing the cavity at the top of a new 2 story extension?

We've got all the brickwork up (block and brick with 100mm cavity and 50mm Celotex) and have just had the roof trusses put on and have finally figured out what to do about insulating the roof (100mm Celotex between rafters on sloped part of ceiling and 270mm rockwool to flat ceiling part). It seems that the roof insulation should meet the wall insulation, effectively 'wrapping' the new extension in insulation and hopefully overcoming all the mildew problems we have in the existing property because of cold spots.
Our BCO says we do not need to close the cavity at the top even though our architect specd cavity closers. In addition, another BCO who came when our usual one was on holiday says we DO need to close the cavity as a fire break, and that we should stop the wall insulation BELOW the rockwool fire sock. Who do we go with?

If we cut back the wall insulation to put in a fire stop, then the roof insulation won't meet the wall insulation which seems to be the way to go.

Do we stuff a cavity sock type thing in the gap between insulation and facing brick at the top of the cavity? This would solve the additional problem of the cavity having grown a bit to 130mm in places - looks straight but clearly isn't - we are stuggling to source a cavity stop/sock for a 130mm cavity but if we could take the 50mm off that it would be easy!

Do we need to close the cavity at all? We considered putting a strip of dpm in under the last brick course that needs to be replaced now the roof trusses are on, for condensation/vermin issues but would be useless as a fire stop.

Any ideas please?

Many thanks in advance, Kirsten

 

[^woody^]

Cavity wall insulation should be continuous with the roof insulation.

You would only close an air cavity - in your case the open part of the cavity. Let the insulation run up past the last course of bricks and then either lay some bricks across, or wedge some celotex strips in tightly

 

[freddiemercurystwin]

They should be closed at the top, the Regs are pretty clear in this respect. The barrier should expand the full width of the cavity. You can't use your Celotex to reduce the width of your fire sock. Rockwool fire socks are insulation anyway so there won't be a break in the insulation bubble.

Why not just use 2 thinner ones squeezed in? Or some other nosebonk will be along to suggest a manufacturer or getaround!

 

[kirstent]

Thanks for taking the time to reply guys.

I'm sure you can see our problem now tho - one of you says close it with a fire stop and one says continue wall insulation. This is same as the two opinions of two BCOs so we don't know what to do!

I'll research building regs further - if our usual BCO goes off sick and another comes we may struggle to justify what we've done if it isn't to the letter.

Found a fire sock for cavities up to 200mm but not widely available - Cavi60 MWR 200. The costs keep rising....

Many thanks again, very grateful for your time and opinions!

 

[noseall]

We ALWAYS continue the insulation from the cavity to the roof insulation and this method seems preferable to most BCO's.

 

[kirstent]

Thanks noseall, good to know that BCOs can have differing opinions on what to do - that explains the different advice.
Our BCO must be one of the "carry on the insulation" kind and since he will be signing it off in the end we will use his method. We would prefer to do this - butt the 100mm roof to the 50mm wall insulations and seal with the foil tape we've used throughout.
Also, regarding fire breaks, we have a brick-built house with cavity wall insulation and brick partition walls - no stud work at all which probably helps re fire.

Thanks again for the advice everyone - I'm never off this site these days.
Kirsten

 

[freddiemercurystwin]

Following some further research Celotex can be retained next to a fire sock. Tis on the Celotex website!

 

[kirstent]

Perfect! Everybody's happy then. They are cheap as chips up to 100mm so daft not to put them in.
Thanks again
Kirsten

 

[kirstent]

Hi all,
Just an update on what we did for any future forum searchers with the same issue.
BC calls for roof/wall insulation to be continuous but also for cavities to be closed.
Struggled to source cavity fire socks of required depth but found Isover brand at local merchants. When we went to buy them we were told they no longer conform to fire regs so instead bought Dacatie brand they are phasing in to replace Isover (never heard of them before).
They are 120mm wide and 100mm deep and squash a treat into the 80mm cavity we have between wall insulation and outer brick skin. They have also stopped us from dropping pencils and Sharpies down the cavity!

Untitled

• kirstent
• 13 Aug 2012
• 1

Happy days!