Insulated rafts

[Gus Potter]

My objective is to save you money.

 

It's not often I start a new topic but here are some of my thoughts. Please excuse the spelling and grammer as I'm a bit dyslexic and off duty from my day job. Bit of narative first to let you know I know my stuff!

 

Over say the last 10 years I've seen a lot of enthusiasm in the self build market for insulated rafts in one form or another.

 

I got into this as an SE and developed the design calculations for the biggest supplier in the UK of cold formed steel industrial buildings, about 750 to 800 descent sized buildings a year. These are lightweight buildings with low floor loadings, like a house.. that sit on a big thick layer of squashy insulation. an insulated raft.

 

I had a loads of fun at Capital Steel Buildings (CSB my employer), Gary Watson, their CEO, his directors in Australia not least put their money (a lot of 500k plus)  and faith in me. Gary Watson the CEO who lives in Dalry, Scotland! So this is not some abstract concept for me! He lives just down the road pretty much,  stuck his own money behind me and we sold a lot of buildings in the UK. Many thousands!

 

I know a bit about this insulated raft malarky in terms of structural and geotech design!

 

That said if anyone wants to engage with me on the theory then I would be delighted. It is a bit of a dark art, coupled with plastic and yield line theory design. I really push the boat on it but am always safe. Serviceablity needs discussed but I know the maths! For all the corners of a slab tend to be the bits that are hardest to get to work.

 

For any curious SE's  lurcking in the background. I took TR34 as a basis and then developed from first principles.. there is no "funny FE " software used! It's all verify able by excel!

 

For all in some ways the design of insulated domestic rafts is very elegant and often cost efective.  But get it slightly wrong and you are in trouble!

 

But make no mistake we were doing this stuff in a hard and competetive commercial market probably before Kore rafts etc and Tanners design became more main stream in the self build market.

 

For me I'm delighted that self builders are getting on board with insulated rafts and the whole passive house thing. Aside I wish I knew about this stuff when I did my self build, with hind sight I made some mistakes.

 

Ok here is the rub.

 

There are folk now jumping on the badwagon that I and others developed 15 years ago but they don't actually know what they are doing!

 

Manufacturer's are creating insulated formers that come at a premium cost and claim they are at the cutting edge.. they are not often in terms of buildability! I know this as I as above developed lean desings. These folk are there to sell you insulation and their building system.

 

*Kore etc are there to sell you insulation!

 

*MBC et al are there to sell you timber frame!

 

*You as self builders want to get an SE or Architect at the same price as a washing machine engineer.

 

My advice is to look at simple stupid alterantives that you can get off the shelf! that can be a bit of PIR insulation cut up to avoid waste and perfom the same function.

 

Example: someone pitched up the other day wanting to use a space frame. Someone said oh Kore do a thingy that can support the outer leaf. I think why as an SE can you not just specify a couple of braces in the frame so you don't need that complex outer "thingy" detail? Ask me if interested.

 

Now folks I hope you have found my rant informative or just funny / daft. Design should be fun.

 

The design process.. well if you don't like what I've written then you can rule it out..

 

Design is often about ruling stuff out and that lets you focus on the things that matter.

 

To finish.

 

As an SE I love, and it is a challenge to explain to a lay person how stuctures work.

 

I have Man bag, in that is ruler. I pull that out to explain how buckling of a beam works. There is a soft rubber and my hard plastic measuring tape.  I use these to show how the different layers of an insulated raft work.. the rubber is less stiff than the tape when you put one on top of the other..this lets folk see that the insultation is the critical design layer. The maths are complex but the Client immediately can grasp what we need to do, why and the raw cost implication.

 

 

 

 

 

 

 

 

Edited February 12 by Gus Potter

[AartWessels]

Sorry good man, it's probably because English is not my first language, I have no clue what you're trying to say. I guess your message is: if you want an insulated raft, buy PIR and do it yourself. Ill advice coming from an SE I'd say. 

Anyway, I'm sure it's coming from a good place. 

[Gus Potter]

3 minutes ago, AartWessels said:

I guess your message is: if you want an insulated raft, buy PIR and do it yourself.

Yes.

 

A reason is that on self build you have much more control. Mistakes will get made, people won't turn up on site when the should and so on. The Kore stuff may fit on the draiwng you give them but you'll need to pay for a good site survey. Also say you start digging and you hit a soft spot in the ground.. what do you do.. phone Kore or a friend?

 

Hey, if English is not your first language your get a prize for putting me on the spot! (calling me out)

 

You starting point is to get some base prices from say Kore, and so on. Then run it by your SE and see how you value engineer not just the foundation but also the superstructure.

 

For all we know if you get prices from different folk their interest is not yours! They just want to sell you stuff!

 

A lot of people on  BH are their own worst enemies. They want the cheapest price for everything, but in the heat of battle they end up loosing money... as they don't know when to pay for some professional advice.

 

It can be done and you can drive down the build cost a if you take time to identify what you want and then negotiate with builder.

 

 

 

 

[AartWessels]

Thanks, it's still a bit hard to follow your thoughts though. Kore and others provide guidance on what to do with soft spots. Regardless, I'd call my SE, to agree on the solution. 

Truth be told, I found it quite hard to find an SE comfortable with insulated raft foundations. 

Edited February 12 by AartWessels

[Nick Laslett]

@Dreadnaught did a DIY PIR Insulated Raft, I hope he doesn’t mind me saying this. 
 

Charlie Luxton has a YouTube on his insulated raft, he went direct to the manufacture for his EPS. 
 

A good SE should be product agnostic. With Tanners, I discussed EPS, XPS and Geocell. He cared about the ground conditions and the superstructure of the house. He could make any insulation layer work for those requirements. 

[Dreadnaught]

49 minutes ago, Nick Laslett said:

@Dreadnaught did a DIY PIR Insulated Raft

 I did indeed! Seems to be working fine. Tanners designed it to my specification.

 

Strictly speaking, my concrete team installed the PIR not me. 

 

The only thing I would do differently next time: bump-up and improve the edge insulation.

Edited February 12 by Dreadnaught

[JohnMo]

Started on our build route with passive style insulated raft in mind. But soon found no suppliers any where near me. Structural engineer scratching his head, big shipping costs, specialist contractor costs and accommodation not small. So then just went the route of a thermal bridge free conventional strip foundations. Had to do some thinking myself and feed this into the design, but all pretty straightforward.

 

Advantages

Bog standard stuff for our structural engineer.

Standard stuff for local contractors, ease of construction.

Everything available off the shelf 

Pretty cheap

 

Disadvantages 

None, it has no thermal bridges, down, up or sideways.

 

[saveasteading]

My take on this is that first time builders cannot possibly know all there is to know about every option, and their own site specifics.

The attraction of a kit is certainty.

Would another method be better technically, aesthetically or financially? The kit suppliers can't advise.

 

As a professional(design snd construction)  I have never used an insulated raft, but private houses aren't my thing. I have never used eps kits either as the headline costs were good but not the final cost.

Those eps of raft projects  I have looked at have small rooms, or lots of hidden extras in steels and foundations and elsewhere.

 

 I'm sure it can be a good choice in the right circumstances of site conditions and standard layouts, and ease of project management......perhaps at a cost.

[Alan Ambrose]

@Gus Potter I'm guessing that the advantage of rafts is that (a) they provide an even ground pressure, and (b) they should make the thermal detailing easier. I also imagine that if the ground is fairly soft at to say 0.5-1m down then that makes rafts difficult unless you also want supporting piles.

 

Starting from the other direction - when should we consider a raft, and when is strip or something else better? 

[saveasteading]

5 hours ago, Alan Ambrose said:

if the ground is fairly soft at to say 0.5-1m down

The load spreads with depth. With an eps raft, the wall loads have already been spread a long way so as not to squash the eps. almost any natural ground will be as strong as that.

[Alan Ambrose]

Ah, makes sense.

[Nickfromwales]

16 hours ago, AartWessels said:

Truth be told, I found it quite hard to find an SE comfortable with insulated raft foundations.

My advice there would have been "keep looking"  

[Nickfromwales]

13 hours ago, Dreadnaught said:

my concrete team installed the PIR

Not EPS? I can't recall.

[AartWessels]

3 minutes ago, Nickfromwales said:

My advice there would have been "keep looking"  

Which is what I did 😁

[Nickfromwales]

12 hours ago, JohnMo said:

Started on our build route with passive style insulated raft in mind. But soon found no suppliers any where near me. Structural engineer scratching his head, big shipping costs, specialist contractor costs and accommodation not small. So then just went the route of a thermal bridge free conventional strip foundations. Had to do some thinking myself and feed this into the design, but all pretty straightforward.

 

Advantages

Bog standard stuff for our structural engineer.

Standard stuff for local contractors, ease of construction.

Everything available off the shelf 

Pretty cheap

 

Disadvantages 

None, it has no thermal bridges, down, up or sideways.

 

I got drafted in by a client to assist with a foundation solution, and when I say 11th hour...I mean "5 to midnight" shitfest.

 

For simplicity, and availability of answers and products for a super-fast turnaround, I did exactly what you did. Excellent results tbh, and still a very admirable solution for a foundation (with a Nudura ICF structure atop).

 

Apart from labour, a very cost-effective route tbf.

[Dreadnaught]

7 hours ago, Nickfromwales said:

Not EPS? I can't recall.

 

Correct, purest PIR.

[Gus Potter]

On 12/02/2025 at 13:14, Alan Ambrose said:

@Gus Potter I'm guessing that the advantage of rafts is that (a) they provide an even ground pressure, and (b) they should make the thermal detailing easier. I also imagine that if the ground is fairly soft at to say 0.5-1m down then that makes rafts difficult unless you also want supporting piles.

To question (a) correct, (b) first part .. yes they can make the thermal detailing easier depending on what level of insulation you want to achieve. 

 

Sometimes it's cheeper and most effective to go for a simple strip found that a local builder can do in their sleep and add a bit of extra insulation elsewhere in the easy places.. this can be called the compensatory design approach to insulation for example. Design is often about making educated compromises on some details so you end up with a better job performing and buildable job at the end of the day.

 

It's surprising what you can get a raft to sit on in terms of ground bearing capacity. I've done some whacky ones with an allowable soil bearing capacity of 30 kPa in soft SAND near the coast... a nice value is normally around 100kPa about ten tonnes a square metre. But that is not all of the story. A low bearing capacity soil can also be a CLAY soil where is swells and shrinks. This makes things more tricky as while the soil may carry the load ok it keeps wanting to move up and down over the seasons.

 

You can do rafts down to quite a depth at times. Down to 1.0m is perfectly doable at times but beyond a 600mm depth of hardcore the NHBC want the fill to be specified and designed by an SE say. Sometimes (you need to be brave and get your sums right) we dig out the heavy soil and replace with insulation and a less dense material so the soil at the base of the excavation does not "know" we have popped a house on top. Here is an opportunity to use say Foamglass Geocell as it is light weight, good bearing capacity and has a quantifiable U value.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

On 12/02/2025 at 03:32, Gus Potter said:

My advice is to look at simple stupid alterantives that you can get off the shelf! that can be a bit of PIR insulation cut up to avoid waste and perfom the same function.

It is also easy to stick together with the correct application of adhesives and a basic jig.