Concrete fill DPC cavity ?

[kirky03]

I am self building a porch, have laid trench fill foundations now ready to lay DPC - 3 courses below ground, 2 above all in class B's. My drawings show the 2 skins tied and filled with lean concrete, are these both necessary ? Above DPC the walls will be solid blocks 265mm wide so no cavity. Will I be better or worse off with cavity filled or not ????

 

[noseall]

I've not heard of using solid 265mm walls above the dpc, but i guess using such a block makes any cavity beneath irrelevant.

A conventional cavity wall would have the cavity filled to within 225mm of dpc. This is so that the walls are not crushed together whist allowing a cavity drip trough below dpc.

I guess it would not hurt then to fill two of your three courses below ground leaving a total of three clear below dpc.

 

[stuart45]

You need to fill the cavity to the top with concrete if you are building a solid wall.

 

[noseall]

You need to fill the cavity to the top with concrete if you are building a solid wall.

Why?

 

[kirky03]

What mix should I use ? And do i need any wall ties in there ?

 

[stuart45]

When building a solid wall, you always have a solid foundation. It's only built with 2 skins of brickwork to get the 265mm width.
If it was a 13inch solid wall above you would build it in solid brickwork, but 265mm is easier with 2 skins and filled with concrete.

 

[kirky03]

Detail drawing shows cavity filled to top, but when I queried with the architect who drew the details he said I can if i want or not bother ??! I'm thinking solid wall above why not be solid below ie cavity filled ?

 

[stuart45]

What mix should I use ? And do i need any wall ties in there ?

You only need a lean mix for the fill, about 8/1. Put ties on the 3rd course.

 

[noseall]

When building a solid wall, you always have a solid foundation. It's only built with 2 skins of brickwork to get the 265mm width.
If it was a 13inch solid wall above you would build it in solid brickwork, but 265mm is easier with 2 skins and filled with concrete.

What about the thermal aspect immediately at slab level? Would not having a clear cavity benefit the thermal bridge at this point?

Particularly if the block walls above are aerated.

 

[noseall]

What mix should I use ? And do i need any wall ties in there ?

You only need a lean mix for the fill, about 8/1. Put ties on the 3rd course.

What will the ties be doing?

 

[stuart45]

When building a solid wall, you always have a solid foundation. It's only built with 2 skins of brickwork to get the 265mm width.
If it was a 13inch solid wall above you would build it in solid brickwork, but 265mm is easier with 2 skins and filled with concrete.

What about the thermal aspect immediately at slab level? Would not having a clear cavity benefit the thermal bridge at this point?

Particularly if the block walls above are aerated.

The walls above will probably be 265mm Turbo or similar. A cavity may help a bit with the thermal, although there won't be much difference beyween 2 skins of brick and a 265mm solid.

 

[freddiemercurystwin]

What a bizarre spec for a porch. What's wrong with trad cavity wall with some insulation in the cavity?

 

[stuart45]

I've done an extension like that, but not a porch.
If it's not heated 9 inch brickwork is good enough.

 

[kirky03]

I've got an extension to do at the rear so using the porch as a bit of practice and also to keep all materials the same, that's why I took the decision to go with the 265 blocks same as rear extension . For info the blocks are celcon solar

 

[ban-all-sheds]

Interesting that they claim "superior thermal conductivity". You know what they mean, but really they've got that AAF - you don't want your walls to have thermal conductivity - the poorer the better. Inferior thermal conductivity, or superior thermal resistance is what they should be saying.


OOI, what's your target U-value for your extension walls, and how do you plan to achieve it?