ICF Foundation Details - Thresholds

[Jenki]

First of a few posts, Having lived in the Highlands for a long time, (5 months and counting), We have had a re-think on the construction of our house. 

The decision is two fold, firstly the increased cost of stick build and insulation, plus the delivery costs, secondly the Weather.  Its windy and I've never really experienced the issues with (constant) driving rain until now.

 If it rains its driving rain. 

ICF will be dearer, but not by much, and then with some tweaks to the design and the foundation, a simpler (thus cheaper) Air tightness detailing I'm not sure there is a lot in it.  SO that's it, were going to build with ICF, and the decision is to use POLYSTEEL blocks, with EWI on the exterior.

SO  down to the Questions.

 

 My site is rock, the house plot is currently stripped of all top and sub soil down to the bedrock. The excavation results in depths of 350 - 500mm below the original ground, but this is sloping to the SW corner approx. 800mm drop to this corner. I will be breaking out some of this rock to  reduce the slope. 

Once I've got this more level, the initial idea is to lay the radon barrier on the rock (sand blinding under the slab area) then  shutter a 600mm trench strip for the concrete footing.  The idea being that the radon barrier will not have any protrusions for services.

 

1, DPC - Poly steel says I don't need a DPC - ? Thoughts and comments please.  The sketch shows the section.  600mm block sat on the strip foundation.

If I opt for adding a DPC, then I'm introducing a moment in the footing and thus more reinforcement will be required.

2, The Visqueen would then be lapped up the wall, but not returned into the blocks?

 

3 How does my detailing of the thresholds look? - With Poly steel, the cavity is closed with 6*2 treated timber.  I was thinking that as the doors need low thresholds the the door should be positioned on the external EPC of the block, and strapped back to the timber.

 

 

Be Kind...

 

 

 

040123_Foundation.pdf

 

[saveasteading]

1 hour ago, Jenki said:

I'm introducing a moment in the footing

Consider drilling and gluing rods into the rock. You might as well make use of it.  Gus will confirm if this is stupid or genius.

The holes in the membrane will be of no consequence if the concrete is well laid.

 

1 hour ago, Jenki said:

the increased cost of stick build and insulation, plus the delivery costs

I'm surprised.  The timber will be local (Moray perhaps?) or even from Muir of Ord.

Ours was from James Jones at Forres, via a merchant.

We got better prices for insulation and timber from the local BM than from any national or web supplier.

 

I am biased towards timber because things fit together, you can always get another bit, and adjustment of dimensions is easy.

 

1 hour ago, Jenki said:

sand blinding under the slab area

On the rock and then on the membrane to stop stones punching through.

 

When we researched, 150 PIR was much the same U value and cost as 300 EPS. Depends on your construction of course. 

Higher is good for the view and allowing the moorland runoff to run harmlessly past. Not so handy when making  ramps to the door.

 

You appear to have about 400mm of MOT 1. you don't need any other than to level off over the rock.

Anywhere else I would suggest recycled stone as being much better value and environmentally. Stone may be cheaper for you, but MOT1 perhaps excessive....watch out for surprisingly big stones in it.

 

[Temp]

I've not built in ICF but looking at your drawing... 

 

The 100mm slab probably has to be reinforced? Right to the edge? 

What stops you putting a DPC/DPM between the insulation and the slab? I don't understand the "moment" you refer to.

Your EWI looks too thin? About 50mm?

Presumably the outside will be rendered? Not all render is approved for use below DPC. Perhaps think about a brick plinth detail or some other solution?

Plan ahead for level entry door threshold and drainage to avoid it looking like an obvious ramp added as an after thought.

 

Building Control will have a view on all of this probably.

 

[JohnMo]

What does your warrant say about no DPC, not sure they would accept that?

 

The concrete slab is a good cold bridge, would you be better, to run a few theromolite blocks in line with the outside edge of the blocks below to form an upstand up to finished floor level.  Then inside that have a further insulation upstand in PIR or your preferred solid insulation, then cast the concrete floor?

[Jenki]

5 hours ago, saveasteading said:

Consider drilling and gluing rods into the rock. You might as well make use of it.  Gus will confirm if this is stupid or genius.

The holes in the membrane will be of no consequence if the concrete is well laid.

I have thought about this - was going to wait for the design to make that decision.

 

5 hours ago, saveasteading said:

I'm surprised.  The timber will be local (Moray perhaps?) or even from Muir of Ord.

We got better prices for insulation and timber from the local BM than from any national or web supplier.

 

Its not so much the timber cost. The design I was given added 3 Simpson Steel Strong walls (around £700 each), double layers of OSB to both gables and then insulation.  I too prefer timber, but I do like the idea of this being concrete. 

6 hours ago, saveasteading said:

On the rock and then on the membrane to stop stones punching through.

 

When we researched, 150 PIR was much the same U value and cost as 300 EPS. Depends on your construction of course. 

Higher is good for the view and allowing the moorland runoff to run harmlessly past. Not so handy when making  ramps to the door.

 

You appear to have about 400mm of MOT 1. you don't need any other than to level off over the rock.

Anywhere else I would suggest recycled stone as being much better value and environmentally. Stone may be cheaper for you, but MOT1 perhaps excessive....watch out for surprisingly big stones in it.

 

 Yes- both sides of membrane.

 The issue with the height is that the land falls to the SW, considerably over the last 3M, if I don't sink the E elevation the W elevation will be around 1 M above ground to FFL.

 

I've already experienced some huge stones in the crushed rock we have had.

300mm EPS as you say is similar to 150mm PIR but as I have the depth to fill, this is the most economical way forward.

 

 

5 hours ago, Temp said:

The 100mm slab probably has to be reinforced? Right to the edge? 

What stops you putting a DPC/DPM between the insulation and the slab? I don't understand the "moment" you refer to.

 

Hopefully the slab, as it is ground supported wont need reinforcement - but might add steel fibres

I agree - Visqueen between top and second layer of EPS  (allowing for UFH to be stapled to top.

 

EWI is 50mm plus the ICF block. this gives a U of 0.16.

 

House will be clad, plinth to DPC level will be slate, tile, stone - not sure yet.

 

The threshold detail is one of the main purposes of the post. 

 

[Jenki]

15 minutes ago, JohnMo said:

What does your warrant say about no DPC, not sure they would accept that?

 

The concrete slab is a good cold bridge, would you be better, to run a few theromolite blocks in line with the outside edge of the blocks below to form an upstand up to finished floor level.  Then inside that have a further insulation upstand in PIR or your preferred solid insulation, then cast the concrete floor?

I'll have to change the Warrant for ICF construction, so trying to ensure I've got the details correct before  I apply for the certificate of design  / Warrant changes.

 

The section of the footing on the drawing is at the threshold. The majority of the slab is inside and sat on EPS, and butted up to the EPS of the ICF block.

I was thinking that (at the thresholds only) cutting the ICF to allow a layer of SLAB EPS to sail over the concrete core and touch the out EPS (thus stopping the cold bridge from the Concrete footing) ?

 

 

[saveasteading]

5 minutes ago, Jenki said:

might add steel fibres.

Steel fibres  expensive, plastic fibres cheap. Our local SE (we were obliged to use one)  was unaware of the product. Worked nicely.

[saveasteading]

4 minutes ago, Jenki said:

allow a layer of SLAB EPS to sail over the concrete core

Needs a detailed cross section drawing. Surely the manufacturers have approved details?

Edited January 5, 2023 by saveasteading

[Jenki]

6 minutes ago, saveasteading said:

Needs a detailed cross section drawing. Surely the manufacturers have approved details?

I was hoping my drawing would be this.. with any mods suggested here.

040123_Foundation.pdf

[Temp]

53 minutes ago, Jenki said:

Hopefully the slab, as it is ground supported wont need reinforcement - but might add steel fibres

 

I'd get it specified by an SE. The weight of the ICF walls and roof is on the very edge of the slab supported by the insulation which is compressible. The reinforcement distributes the load over the wider area of the slab.

 

 

[Jenki]

23 minutes ago, Temp said:

 

I'd get it specified by an SE. The weight of the ICF walls and roof is on the very edge of the slab supported by the insulation which is compressible. The reinforcement distributes the load over the wider area of the slab.

 

 

@Temp Are you confusing the SECTION - A-A - this is at the door threshold?. The slab is not taking any weight from the walls or roof.  Section A-A  is intended to show the detail at the door way. - I have added the Typical section to the drawing to emphasize this.  Or am I missing something? Thanks

Edited January 5, 2023 by Jenki

[JohnMo]

Do you have a structural engineer involved, as they will have to issue a certificate to cover all aspects of the structural design.  This forms part of the warrant, but is also required for the building sign off.

[Russell griffiths]

I’m afraid there’s nothing I like about that design. 

Pour your concrete and leave it very low at the threshold, bring it up to level when you have 100% worked out FFL , then bring it up and add more insulation for the door to sit on, I used XPS 500. 

[Russell griffiths]

another one. 

[Nickfromwales]

5 hours ago, Jenki said:

@Temp Are you confusing the SECTION - A-A - this is at the door threshold?. The slab is not taking any weight from the walls or roof.  Section A-A  is intended to show the detail at the door way. - I have added the Typical section to the drawing to emphasize this.  Or am I missing something? Thanks

Gotcha.

I was about to follow @Temp, as we both missed that detail.

 

Still don't like it either lol  

 

Do as I do with each slab / foundation / raft I specify, and as @Russell griffiths says, and replace the upper projecting layer of EPS with Marmox / similar load-bearing product ( 400kpa ). The upper layer of EPS in a raft design is usually only 100kpa, so no good for this detail with reduced concrete cover there, eve with steel fibres.

 

Cold-bridging is also terrible in the design you show.

Edited January 5, 2023 by Nickfromwales

[Nickfromwales]

Raise the radon barrier to underneath the 2nd layer of EPS and it will form both radon and DPM if you select the correct membrane. DPM down that low, and omitted higher up is also something I've never seen, other than with Kore, but even then they turn it up the outside of the EPS basket and back over at ToC as DPC.

 

Apart from all these things wrong with it, your on to a winner  

Edited January 5, 2023 by Nickfromwales

[Jenki]

3 hours ago, JohnMo said:

Do you have a structural engineer involved, as they will have to issue a certificate to cover all aspects of the structural design.  This forms part of the warrant, but is also required for the building sign off.

Yes.  but I want to sort the detail out, and then get him to confirm he's happy . rather than just  end up with a genreric that will do...

[Jenki]

@Russell griffiths. Looks like you had a screed after? my slab is finished floor level. so the height will be 100% correct.   

 

I take it both you and @Nickfromwales don't like the thermal bridge, and I want to get rid of that  Which I thought I had.  I'm struggling  with the detail. 

If I don't put the concrete slab into the reveals then the door will need to be sat further back? otherwise I'm going to have concrete slab, then insulation, then door threshold sat on insulation?  Also if I set the door back, this will expose the timber, that is used to shutter the blocks at the openings. 

So the door needs to sit at least  flush with the outside edge of the timber, leaving the EPS of the poly steel block and the EWI exposed? 

 

 

For Clarity my thoughts were- pour the strip foundation.  place and locate 1 row of ICF blocks,  Prepare the base for the slab, Drainage, Utilities. 2 layers of EPS, Visqueen, 1 layer EPS, UFH pipe, cast the slab.  (hence the detail at the threshold. 

Then build the blocks to wall plate. 

 

 

[Russell griffiths]

KSo floor level 100% correct, not a chance. 

Your slab will be the height it is but ffl is slab plus flooring

so slab plus self levelling compound plus tiles. 

Or you change your mind and it’s now self levelling compound and LVT 

so in the blink of an eye you have lost 15mm in height. 

So your door now sits 15mm too high. 

 

As I said stop your concrete on the inside of the walls, stop your concrete core low and build it up two days before you fit the doors. 

 

Your doors can sit on the insulation, they don’t need to sit on concrete. 

 

Regarding seeing the wood wood in the icf block core, that’s correct, you see this on the inside, you then pack this out with a skinny bit of insulation and then cover with plasterboard, you need to make your opening a bit larger than the door, or you end up with the standard shit look that people have with the plasterboard touching the door hinges. 

 

Ive just spent 2 days  cutting out a floor slab and grinding down the insulation on a new build where the door was installed too high, another company came back to put the door in and said they wished they were all this accurate.  

 

[Russell griffiths]

 

plywood to cover treated timber in cavity. 

 

 

This is how ive just done this one. 

[saveasteading]

34 minutes ago, Russell griffiths said:

stop your concrete on the inside of the walls,

That's best for several reasons. Simpler to pour or form  the screed against a board which closes off the door opening. allows the screed to shrink without sticking in the door. Thresholds detail and tweaking levels later.

 

Doors are always a nuisance.

[Temp]

14 hours ago, Jenki said:

@Temp Are you confusing the SECTION - A-A - this is at the door threshold?. The slab is not taking any weight from the walls or roof.  Section A-A  is intended to show the detail at the door way. - I have added the Typical section to the drawing to emphasize this.  Or am I missing something? Thanks

 

Ah yes OK. My mistake.

[Nickfromwales]

7 hours ago, Jenki said:

I'm going to have concrete slab, then insulation, then door threshold sat on insulation? 

100% correct. Bingo!

For the insulation, you install Compacfoam / similar and that is rigid 400kpa ( load bearing over the whole footprint of the frame threshold ) and that kills all of the cold bridge from outside to the heated slab.

7 hours ago, Jenki said:

Also if I set the door back, this will expose the timber, that is used to shutter the blocks at the openings. 

Matters not one jot, as the frame fixings ( the proper ones, not cheap shitty bits of universal plate btw ) will be long enough to grab a purchase wherever that shall be.

 

Glad you posted this so you can detail this better.

 

Think about raising that membrane, don't forget!!

[JohnMo]

This is pretty much how I ended up with mine.  The pink block being replaced with 2x thermolite blocks, I also did the upstand in 70mm PIR.  So the sidewards u value across the threshold, ends up in the region of 0.2.

 

 

[Radian]

2 hours ago, JohnMo said:

I also did the upstand in 70mm PIR.

That's impressively wide. What sort of floor finish did you use on top? We used stick-down carpet on a thin layer of self-levelling compound covering the screed and a 25mm PIR upstand in our garden room extension. After they'd gone I discovered the carpet fitters had gone around the perimeter with a bolster and caved in the PIR a bit 🙄