Extension wall with partial earth

[iwanttodiy]

hi

I’m looking for advice on getting an extension wall built that will be partially under ground. The current wall is a retaining wall as attached, which is about 800mm off ground level. The extension wall will be up to this height with windows sitting on top.

I have had a couple of builder provide their ideas, one suggested digging further in and create another retaining wall then leave a gap and do a cavity wall.

Another suggested when filling the foundations (500mm wide, 1 meter deep), fill all the way up to the desired wall height.

I personally have an idea of doing a standard insulated cavity wall and either double layer the outer wall or lay the blocks flat to gain additional strength. The outer layer would then be ground level or slightly lower than ground level to fit a drain.

To complicate the matter I also need to fit air vents from top of this wall to circulate air back to the existing suspended timber floors. Not yet figured out the best way, but was thinking of having the vent either go up the cavity, or inline with the middle layer of blocks.

Anyone has had any thoughts as to any of these, or suggest and better ideas? Any advice would be much appreciated. Thanks in advanced

 

[freddiemercurystwin]

There are three ways that immediately spring to mind, some as suggested by your builders, the first would be to keep the existing retaining wall and just have a little gap eg



The second building a new retaining wall further back and a normal house wall eg



And the third to build the wall as a retaining wall eg



The first two maintaining the underfloor vents is easy, the third I guess you could build in some extended telescopic vents.

 

[iwanttodiy]

Thanks the designs don’t allow keeping the existing wall and leaving a gap. The extension is just about 3m up to the wall.

For #2 creating another retaining wall will result in wasting space between the house and the new retaining wall.

I believe #3 is what I am looking for. From the looks of it, the image shows a cavity wall and 2 skins of blocks being laid flat. So that’s a wall of approximately 700mm at its widest?

 

[freddiemercurystwin]

Sure but No. 3 is higher than yours so you could probably lose the additional block leaf on the outside, there are many ways to do this, a thinner RC wall with internal insulation for example, this is just one example but it's a simple solution and maintains a consistent insulation bubble around the cavity.

 

[^woody^]

There are three ways that immediately spring to mind, some as suggested by your builders, the first would be to keep the existing retaining wall and just have a little gap eg

Ho do you get the ground to stay up vertically like that?

 

[freddiemercurystwin]

It's made of Silly Sand.

 

[tomfe]

It's made of Silly Sand.

Minecraft?

 

[iwanttodiy]

Great, thanks. Yes, Building control have also stated using telescopic air vents that go under a concrete floor to vent air into the existing void.

Assuming we are putting a vent in the cavity, we won’t be able to insulate the area. Is this loss of insulation suitable?

I suppose when it gets tanked, can apply external PIR insulation to the outside to help?

Ill get this drawn up to see if BCO are satisfied

 

[freddiemercurystwin]

Assuming we are putting a vent in the cavity, we won’t be able to insulate the area. Is this loss of insulation suitable?

The insulation bubble can be maintained, because the vent ducts can go up the cavity between the insulation and the block wall. You will need to be fastidious how you detail/build how the vents are sealed around the tanking where it’s penetrated. You generally only get once chance to get external tanking right!

 

[iwanttodiy]

The cavity wall that’s spec’ed is a 100mm Block 100mm cavity 100mm Block.

We were planning to put 100mm PIR insulation in the cavity.

Based on what you mention about the vent going up the cavity, the general size of the vent is 50mm thick. Do you mean either use 50mm PIR or make the cavity bigger (150mm)

I agree about tanking, however as the vent opening will be coming out the cavity above ground level, I would have thought it would be be above the tanking thus there won’t be any breaks.

 

[freddiemercurystwin]

100mm PIR? That's usually over double what you would need. You should check your PIR is suitable to be up against the outer skin with no gap. But if the vents are 60mm then you will still have 40mm PIR to maintain the bubble.

The tanking bubble will be penetrated at the bottom where the vent duct goes through from under the floor into the cavity.

 

[iwanttodiy]

I’ll check with BCO about full fill insulation. Products like the xtratherm full fill offers a layer of moisture protection.

Do you see any issues with this setup for the retaining wall:

300mm aerated foundation Block - 100mm cavity - 100mm aerated block.

The rest of the walls that are not retaining I will use 100mm standard dense blocks.

I hear aerated blocks offer better moisture protection than standard blocks so makes sense to use them under ground?

 

[iwanttodiy]

Ok, I’m thinking now a 200mm outer wall 100mm cavity, 100mm inner wall should be sufficient and that leaves 100mm either side the foundations. The existing wall has stood for over 30 years at 200mm, so I’m hoping there shouldn’t be any issues with a new 200mm wall.

Cost wise a 200mm and 300mm retaining wall is roughly the same, due to being able to use additional 100mm blocks instead of ordering an additional lot of 300mm blocks.

 

[iwanttodiy]

Also just checked with celcon, they don’t recommend using any of their blocks, be it the foundation Block, or the hi 7 Block for the retaining part of the wall. Guess it’s the standard dense concrete block it is then @ 2 layers

 

[^woody^]

Also just checked with celcon, they don’t recommend using any of their blocks, be it the foundation Block, or the hi 7 Block for the retaining part of the wall. Guess it’s the standard dense concrete block it is then @ 2 layers

Are you designing this yourself or is there a designer involved?