Hi all,

We have just started clearing our site and have run into a problem which may spell the end for our planned insulated slab foundation due to additional cost.

The site has a lot of topsoil on it, approx 1.7m at the deepest point. Removing it is not a massive problem, but the level of the house cannot be dropped due to the need to get light into the interior over neighbouring buildings. Piling or pads have been suggested with ground beams and then the insulated slab on top. This would be the best suggestion, but I think it will be prohibitively expensive - basically like paying for 2 sets of foundations.

Our architect is of the opinion that we should remove the soil, dig strip foundations and form a block basement. The site is dry and free draining sand, so we think full waterproof concrete would be overkill. We would not be able to complete the basement at this point in time, but would like to have the option at some point in the future.

Basically, if we are going to have extra expense at this point, we would rather try and get a basement out of it. I am currently thinking of insulation to the base of the strip founds and up to ground level topped with a cast slab. Would this be possible to design in such a way as to mitigate thermal bridging to the basement below?

Any suggestions for a cost-effective way around this problem, much appreciated.

Thanks,
dj

Ah, well there's a slight problem about continuing the insulation up - it's a twin wall timber frame, so the insulation is between the studs, not on the outside.

This is the complexity, trying to join together 2 different building methods. My architect suggested just building the basement walls in blockwork and making the timber frame taller, so it would effectively be one structure with continuity of insulation. It would sit on the warm side of the blockwork wall.

In fact, if we did that, would we need the external insulation to the blockwork? Would we just put the insulated slab at the bottom of the basement and have a suspended timber floor to the main living area?

Hmmmm.

If I understand your proposal correctly, your architect is suggesting building the timber frame inside a blockwork 'bucket'? You'd need to tank the blockwork to keep water out, and then would you also be at risk of condensation on the inside of the blockwork and/or outside of the timber frame?

If you build an insulated slab at the bottom of the hole and then externally insulated blockwork for the basement, I feel sure you could then plant a timber frame house on top, including a suspended timber ground floor. The connection between the blockwork and timber would need a bit of thought but I'm sure it's not rocket science. A bit of external insulation carried up the outside of the timber frame for a little way seems to answer most similar problems.

edit: PS crossed with Pile-o-Stone; he makes an important point - you do have to decide now whether you want the basement to be 20°C or 10°C.

If we manage to do it, the living room will eventually be living space for elderly parents, so we would need it inside the thermal envelope.

djh - to be honest, there's been a lot of kicking around of ideas. We had a reasonably hideous quote for an insulated slab (34K!) and the news that there will be additional cost on top of this has not been a happy time. So, we're trying to get the best possible value out of the extra money that we have to spend.

I think the architect was thinking that it may be the easiest way to maintain continuous insulation and airtightness - we would look at either spray-on tanking or an external drain/membrane solution (cost dependant). However, if you think it's possible to manage the joint efficiently with little thermal bridging, that might be the way to go.

It's a bit of a kick in the teeth - they're not joking when they say getting out of the ground is the most expensive bit!

I'm at home now. The Passivhaus Details book has several details of a timber frame on top of a basement so it's definitely possible. Admittedly the details in the book use a concrete slab as the roof of the basement rather than suspended timber; I don't know whether there's a fundamental reason for that or just Germanic preference/law. If your architect is designing to passive standard then he certainly ought to have a copy of the book.

djh, I"ve just had a look at my copy of the details book and found that example, thank you for reminding me. Should have thought to look in there, really! The only added complexity is that we have a brick facing on some of the build, so not sure how that would work with the foundation for the outer skin and the recommended drain.

Ed - someone else recommended Tetris, which is a similar system to the Jetfloor, but unfortunately you can't use it above an inhabited basement, due to the risk of the insulation melting in the event of a fire. Bit gutting really, as it looked really good.

Hi Viking,

only just got your email and this - I've been out a customer site all day. In answer to your question, no, all we've done is cleared the site - there was some demolition and rubbish removal to be done.

The 1.7m is all black earth, then subsoil, then medium sand. I was told that all the black earth has to go - is that not right?

We are actually getting a quote for a block basement now, as we think it probably won't be too far off the piling/pads solution cost-wise and would give us a significant extra area that we could potentially convert to living space in the future.

Intrigued about not having to remove the soil though ....

cheers

<blockquote><cite>Posted By: divorcingjack</cite>The 1.7m is all black earth, then subsoil, then medium sand. I was told that all the black earth has to go - is that not right?
</blockquote>

It depends. If your structural engineer says it has to go then I would say yes, it has to go!

I think the general rule of thumb we use on site cost wise is that it becomes cheaper to pile when you get beyond 2m ish in the ground. I can't remember exactly but it is something like that