Protection of ICF blocks in foundations

[Joey]

Hi all,

 

Currently in the early stages of planning an ICF build and I’m trying to get some specific detials clear in my head!

 

I’m hoping for some advice on how you might best go about protecting a couple of courses of ICF blocks used partially below ground as part of the foundation - these are between the strip footing and DPC level. I’m specifically talking about the front face of the outer insulation. The outer face will be rendered above DPC level, but what should be done between this and ground level, and then below level? I’ve attached a basic detail to demonstrate what I mean, where the red question marks highlight the surface I’m referring too. 
 

These two and a half courses of blocks are filled with waterproof concrete as recommended in some manufacturers detials, however the specifications for the front of the blocks generally just say something like ‘render to appropriate depth below ground level’, or something like that. Maybe I’m just over thinking things, but wouldn’t a render applied continuously down the wall to below ground level physically bridge any DPC and provide a route for moisture to do the same, or is the render waterproof enough for this not to be an issue? Would it be possible to simply just treat the face of these courses of blocks with a water-based waterproofing agent, or would this be damaged by contact with the ground?

 

Would really appreciate any advice or experience!

 

Joe

[Nickfromwales]

OK, firstly....

350mm of XPS?

 

Secondly....

Which ICF system is this? Woodcrete / EPS ?

[Gus Potter]

2 hours ago, Joey said:

specific detials clear in my head!

It's not easy!

 

Starting at the end.. with the render.. you may want a bell cast bead at the base of the render. It needs a sound base.

 

A big thing is for me is that it all looks great on a generic detail along wall detail but how do you detail the door thresholds? Can you later alter the building easily.. add an extension? Do you care?

 

I don't think you need a water proof concrete, no basement and your internal floor slab is isolated with DPC / DPM.

 

If you are really wanting to go ICF then consider an engineering brick course say up the outside, it you want a clean detail at basecourse level.. they do look smart in the right place. Stop the slab short and take extra insulation down on the inside.

 

I know you are seeking the perfect solution but always ask.. at what cost and how durable will it be. Once you get a handle on this do you do a trade off.. I would look at putting more easily installed thicker insulation elsewhere and accept I may fall a bit short in other places.

 

Remember you could have the perfect insulation design, avoid cold bridging.. but if your trades persons can't make a good job of it then it is not just false economy for you but also bad for the environment.

 

Lastly ICF.. there is a time and a place for this.. I support the concept and understand how you reinforce the concrete, detail the corners rebar wise say / openings and design a raft found if need be and so on.. frankly folks this ICF stuff is nothing that new, it boils down to the fact that we are using insulation as shuttering (yes we have some cute wall tiea) and using insulated raft technology that has been proven for the last 50 years..

 

For insulated rafts the calculations are a bit longer.. it is about two lines of sums and one short written paragraph of explanation of how you have derived the values. You compare the different elasticity of the EPS insulation at a certain compression (usually 10% and convert that down to say 1 -2% and derive a bearing capacity from that) cf say clay or sand soil.. I am not kidding you! And folk think this is magic.. it's not.. it's basic first principles.

 

An experienced Contractor say @saveasteadingthat  has done tens of thousand of square metres of insulated floors will tell you what works and what does not.. we can use these same principles for our houses and that will drive the cost down.. we just need to convince say the NHBC and BC that it does work.

 

The theory is internationally well understood. The problem is getting it to the mass market in the UK and knowing when it is the right solution for the self build market at this point in time.

 

And that is the current issue.. unless you are building a very big house not in Scotland or Wales then the economics don't stack up?

 

And after that rant @Joey is ICF your thing?

 

Gus

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

9 hours ago, Gus Potter said:

An experienced Contractor say @saveasteadingthat  has done tens of thousand of square metres of insulated floors

Well, many thousands insulated and tens of thousands not....if the building is big enough, floor insulation is not needed, certainly in the middle. 

The manufacturers used to make that clear but now seem to have forgotten to tell us.

 

re the drawing.  350mm of EPS is a lot. If a thinner construction suits you then 175mm of PIR will do the same job at the same cost.

or 75 of eps under 100 of PIR would be a pragmatic spec.

[Iceverge]

I would counter @saveasteading and stay with the 350mm. Probability use EPS.  A literal bigger chunk of insulation will reduce your geometric thermal bridge 

 

Spread over the lifespan of the building ( maybe 500+ years)  insulation is tremendous value. 

 

As for the EPS below ground level I would consider installing a drip and then some slates neatly glued to the EPS below that. Mainly for rodent and impact protection. 

 

An external French drain would ensure the floor and walls stayed dry and any backfilled gravel would be a belt and braces to keep the slate in place. 

 

 

[Iceverge]

Also drop your DPM below the top layer of insulation. This will allow you to staple the UFH pipes to the EPS through a thin slip membrane. 

[saveasteading]

37 minutes ago, Iceverge said:

A literal bigger chunk of insulation will reduce your geometric thermal bridge

No it won't if it performs less well, and eps performs significantly less well than PIR.

The actual thickness is not a 'thing' in itself.

 

EPS provides  half the u value of PIR, at half the price, so either 350 EPS or 175 PIR, is what I am saying. Either or, to give the option to suit construction detail.

Buying twice the volume with EPS might have a transport implication too.

 

There is another option of  grey EPS with a 10% better u value than white. Something to do with graphite, so 350mm of that would be like 200mm PIR, roughly.

 

We costed all the options in great detail recently, and there isn't much cost difference in the material purchase, so it is down to whether you have more or less excavation/fill as a result.

[Iceverge]

14 hours ago, saveasteading said:

No it won't if it performs less well, and eps performs significantly less well than PIR.

 

I just had a look. You're right, it makes no difference for the actual heat loss. 

 

Thicker insulation will make a smaller psi number for the external thermal bridge but this is all soaked up in the calculations anyway as you'll have more external area with a thicker build up. 

 

 

14 hours ago, saveasteading said:

EPS provides  half the u value of PIR, at half the price

 

Our EPS Graphite was 0.031 vs 0.022 for PIR ( if you believe them!) 

 

It was about 2/3 of the final cost for us to use EPS vs PIR. Obviously that may differ depending on where and when you build. 

 

 

 

 

 

 

 

[DrewG]

Reading through this tread about costs EPS v PIR what hasn't been taken into account is the volume of muck away over the whole footprint will be significantly higher for the EPS assuming a flat site...just saying