To cavity or not to cavity? New wall above solid wall

[dd2005]

I am planning an extension above my garage, gone through planning but now sorting BR approval. The garage wall is two skin (brick/block) but with no cavity. When I build above this what is the best option? I have been advised that I can achieve the required u value by installing a two skin wall with no cavity and 65mm of insulated board to the inner leaf. I would prefer to have a cavity wall but is it worth installing steel beams with piers to support the inner leaf. This would probably cost more to build but the improved thermal performance may be better in the long run.

Any advice would be appreciated.

 

[freddiemercurystwin]

Have you established whether the existing foundations/walls can cope with an additional storey yet?

 

[Joe Malone]

It makes no sense to try and create a cavity wall above a solid wall and there is no need whatsoever. Personally I would build solid walling with external wall insulation.

 

[freddiemercurystwin]

It makes no sense to try and create a cavity wall above a solid wall and there is no need whatsoever. Personally I would build solid walling with external wall insulation.

Assuming the existing founds and wall are suitable of course .....

 

[noseall]

It makes no sense to try and create a cavity wall above a solid wall and there is no need whatsoever. Personally I would build solid walling with external wall insulation.

Personally I think this is a mad idea.

It sounds inviting just building up off the existing masonry and achieving the thermal values by either adding insulation inside or outside. However, it can be a right pain later on and it is for ever.

I personally would bite the bullet now and have a cavity wall system from the outset. Trust me, you have got to live with it.

 

[dd2005]

The foundations and wall will be checked as part of the work (but highly likely not to be an issue). Cannot do external insulation as it has to match existing brick. My question is how to build up the wall to receive the cavity as that is what I would like to do. Any helpful suggestions from experience on other projects?

 

[wessex101]

As others have said, usually the most economical way to do it is demolish the garage and start again.

However I have done a couple of first floor extensions above 9 inch solid walls. First one we built up the 9 inch wall corbelling out the inner skin by 25mm for 3 courses in the floor void until we had the thickness to take up cavity wall above, it was only a short section of wall with light loading so probably not suitable in your case. Second one we had to insert a beam along the line of the inner skin of the cavity wall, that might work for you and don't forget the cavity tray.

 

[freddiemercurystwin]

Are you doing this on a Notice or with a Full plans submission? Self-builder or employing a builder? There are various methods, all will likely need SE backing. Seems a bit odd to have got this far and have still not confirmed for sure if the existing will be able to take it, its a bit pointless speculating until you know for sure.

 

[tony1851]

As Nose, building a solid wall and insulating on the inside is asking for trouble; a 225 solid wall will not be resistant to rain penetration and would not comply with the relevant Approved Document.

A while ago, I did a very similar job as Wessex describes, on my own garage, which has solid 9" brick walls. I corbelled two courses out with class B bricks to give a 50mm cavity. With full-fill, this would not meet current Part L regs, so I insulated internally to make up for it. And yes, don't forget the cavity tray.

 

[Joe Malone]

It makes no sense to try and create a cavity wall above a solid wall and there is no need whatsoever. Personally I would build solid walling with external wall insulation.

Personally I think this is a mad idea.

It sounds inviting just building up off the existing masonry and achieving the thermal values by either adding insulation inside or outside. However, it can be a right pain later on and it is for ever.

I personally would bite the bullet now and have a cavity wall system from the outset. Trust me, you have got to live with it.

Mad idea? There are millions of pounds of EWI retrofit work being carried out in the UK and has been for quite some time. I know all about these systems having authored an article on them for the construction, Research and Innovation Journal. Pragmatically this is the best and most cost effective solution.
With regard to the point on matching brickwork, they can be covered with brick slips to match existing.

 

[theoldun]

If the foundations are adequate all this nonsense about pulling down, throwing in steels, external insulation etc, is great if you have a shed full of money to burn.
Regards corbelling over100mm in the joist space with nothing to weight and tie corbel down is laughable.
To comply with L1B 28W/m2K, you can go 215mm brick and block wall with 77.5 Celotex PL4000.
Regards oldun

 

[noseall]

With regard to the point on matching brickwork, they can be covered with brick slips to match existing.

....
Just when you thought it could not get any worse

FFS man, they are looking for economical solutions not a 5 yearly ugly expensive maintenance headache.

 

[jeds]

I done a couple of these in timber frame. Fix floor joists on hangers. Build external wall in half brick. Build inner timber frame wall off of joists.

PS. otherwise I'd build solid wall and insulate internally.

 

[Joe Malone]

With regard to the point on matching brickwork, they can be covered with brick slips to match existing.

....
Just when you thought it could not get any worse

FFS man, they are looking for economical solutions not a 5 yearly ugly expensive maintenance headache.

Oh, didn't realise you'd costed the difference between building a cavity wall and EWI. You clearly have a jaded view on EWI that bears no relationship to fact.

 

[ban-all-sheds]

It's not the EWI that's the problem.

It's the madness of brick slips.