Basement Insulation Options

[EquiumDuo]

Hi All,

 

been a while since I've posted due to trying to buy the damn piece of dirt, but we now seem to be weeks away rather than months as we are imminently about to sign a contract. I'm working with a structural engineer to try and sort out a basement wall/structure. I'm trying to get ahead of it and I am not understanding how the insulation will fit in. Briefly this is a house on a hill, with an integral basement garage that we have decided to extend to fully open out underneat the house, effectively a 3x story house with the basement buried in the hill. The garage opens to street level. Typical rectangle house, 4x bed. 72sqm per floor.

 

The garage/basement structure walls will look like this:

 

 

How does the insulation fit into this? I am imagining that we fit EPS300 or something on top of the footing but under the concrete base. And also EPS300 on the esterior of the 100mm outer block? Then maybe use marmox blocks to prevent a cold bridge?

Example:

 

Whats the best practise these days?

[Moonshine]

We have the same and building a semi basement (3 sided) with with an integral garage and entrance hall in the basement.

 

I don't know what is best practice, but this is what i am doing (work in progress drawings), and having the insulation inside of the retaining wall in a separate cavity wall, though i know others have done it outside. Also you could have a metal frame wall internally with insulations, though you need to think about the waterproofing.

 

if you are having a structural warranty you will need two forms for habitable spaces (grade 3 - BS 8102) and one for garage (grade 2 - BS 8102)

 

 

the reason for doubling up the masonry wall inside the retaining walls is that it forms a structural walls past the ground flor level, as below

 

 

 

Edited March 4, 2021 by Moonshine

[Thorfun]

we're having a full basement but are going for an insulated slab underneath rather than how yourself and @Moonshine are doing it. mine is the same way that @Bitpipe did his. 

 

here's our SE drawings based on a Kore insulated slab system

 

 

 

1 hour ago, EquiumDuo said:

And also EPS300 on the esterior of the 100mm outer block?

 

no point using EPS300 on the vertical for insulation though as you don't need that compressive strength. EPS100 or EPS70 should do but you'll need to get the confirmed by your structural engineer.

[EquiumDuo]

I've seen some of these details @Thorfun I like this design a lot, seems to require a lot less spoil removal compared to mine. Or at least perhaps a simpler shape. I probably should just wait to see what my structural engineer says. he's already suggested that I can ahve a more economical wall than I thought. I jsut don't know the detail yet.

[Thorfun]

with the insulated slab you don't get any cold bridges at all and no need to worry about those Marmox blocks. just seems logical to me! we're also having the same insulated slab foundations for our non-basement structures. 

 

may not be as economical as other ways (even though I've not done a price comparison) but I just like the concept of it. from the moment I saw the insulated slab it just made sense and I had to have one. ?

[Conor]

We just used ICF with 200m concrete core. Seems a lot simpler than the details above.

We stuck on additional insulation on the outside face to get us down to 0.15. Just sheets of PIR with expanding foam, then geomembrane before backfilling.

If you don't want to use ICF, I'd go with @Thorfun detail.

[Thorfun]

4 minutes ago, Conor said:

We just used ICF with 200m concrete core. Seems a lot simpler than the details above.

We stuck on additional insulation on the outside face to get us down to 0.15. Just sheets of PIR with expanding foam, then geomembrane before backfilling.

If you don't want to use ICF, I'd go with @Thorfun detail.

yeah, ICF is a lot simpler and it was a close decision for us between the two but even though the price difference was slightly in the favour of ICF our chosen Groundworker was more comfortable with RC so we decided to go with what he knew best. but, like I say, not a lot in it and obviously you can put the ICF on the insulated slab. 

[Conor]

6 minutes ago, Thorfun said:

yeah, ICF is a lot simpler and it was a close decision for us between the two but even though the price difference was slightly in the favour of ICF our chosen Groundworker was more comfortable with RC so we decided to go with what he knew best. but, like I say, not a lot in it and obviously you can put the ICF on the insulated slab. 

The guying doing our ICF was originally a concrete shutterer by trade. Says there's not much in it but prefers the ICF work as it's physically much easier and less to go wrong.

[Bitpipe]

2 hours ago, Moonshine said:

 

if you are having a structural warranty you will need two forms for habitable spaces (grade 3 - BS 8102) and one for garage (grade 2 - BS 8102)

 

We received our warranty with just one method (concrete) but that itself came with its own warranty - Sika.

 

[Moonshine]

12 minutes ago, Bitpipe said:

 

We received our warranty with just one method (concrete) but that itself came with its own warranty - Sika.

 

 

Ah good point, if its waterproof concrete (type B) then i think that you can just use that.

[Bitpipe]

32 minutes ago, Conor said:

The guying doing our ICF was originally a concrete shutterer by trade. Says there's not much in it but prefers the ICF work as it's physically much easier and less to go wrong.

 

What did you do for your slab? Just cast it over EPS?

 

I agree that structurally ICF and shuttering are the same, latter much less effort on site. The only advantage of shuttering over ICF is you can see the quality of the pour when it's struck. With ICF you need to expect that the vibration etc has been sufficient to prevent aggregate settling etc.

 

You don't even need to use specialist shapes like Kore above. We just laid a plane of 300mm EPS 200 on sand blinded compacted hardcore to cover the slab footprint with some overlap at edges and covered with membrane. The crew built the slab & structure on top as if it was solid ground. 200mm EPS 70 was applied to sides, sitting on 'toe' of the EPS 250 using LE foam to stick it to concrete and fill any gaps.

 

Total cost of insulation was about £5k in 2015 - £72/sheet for 300mm EPS200 and £25/sheet for the 200mm EPS70 (slab was 120m², walls were 2.5m x 45m linear - ish). We could have gone for 200mm in the slab and saved some money (less excavation & muck away also) but at that time were undecided about UFH in basement so erred on side of caution. Didn;t bother in the end but basement is always cosy @ 20^O.

 

Installation costs were zero as I did it  Maybe £100 for LE foam

 

[Bitpipe]

Just now, Moonshine said:

 

Ah good point, if its waterproof concrete (type B) then i think that you can just use that.

 

Probably depends on your ground conditions also, we were well above water (started 6m down, we dug to 3.5).

 

Friends had a Thames feeding stream go through their plot and needed WPC plus exterior tanking. They went with a system from Glatthar which was very high quality but not cheap. Big issue they had to contend with was hydrostatic pressure pushing the basement out of ground so the slab was a beast. Also needed de-watering during the construction.

[Conor]

3 minutes ago, Bitpipe said:

 

What did you do for your slab? Just cast it over EPS?

 

 No, couldn't get it to work due to loadings exceeding EPS300 capability in several places. Went for a standard ground bearing slab with ring beam.

[Bitpipe]

3 hours ago, EquiumDuo said:

Hi All,

 

been a while since I've posted due to trying to buy the damn piece of dirt, but we now seem to be weeks away rather than months as we are imminently about to sign a contract. I'm working with a structural engineer to try and sort out a basement wall/structure. I'm trying to get ahead of it and I am not understanding how the insulation will fit in. Briefly this is a house on a hill, with an integral basement garage that we have decided to extend to fully open out underneat the house, effectively a 3x story house with the basement buried in the hill. The garage opens to street level. Typical rectangle house, 4x bed. 72sqm per floor.

 

The garage/basement structure walls will look like this:

 

 

 

How does the insulation fit into this? I am imagining that we fit EPS300 or something on top of the footing but under the concrete base. And also EPS300 on the esterior of the 100mm outer block? Then maybe use marmox blocks to prevent a cold bridge?

Example:

 

Whats the best practise these days?

 

EPS 300 is overkill for the sides - as very little load is presented from the backfill you'll probably be fine with 70 or 100 grade.

 

If you go insulated slab then it depends on the load per m2 of the structure above it. We were able to get our SE to sign off on EPS200, original spec was EPS250. 

 

It gets a lot more expensive, heavy and harder to cut the higher grade it is - I think our 1.2m x 2.4m of EPS200 were about 35kg each. Once laid edge to edge on the blinded layer they were not going anywhere.

[EquiumDuo]

Thanks for the feedback, lots to consider. I definitely need to downgrade the amterialss where it makes sense.