Insulated Raft - How much better?

[dsaunders]

This is our third self build, and hopefully third time lucky will make it our last!

 

This time we're opting for a timber frame with much more consideration given to fabric of the building to ensure it performs well.

 

We hadn't considered different foundations as the budget is non-existent and assumed anything other than standard strip footings will be silly expensive. I've looked into what I can and spoken to a few insulated raft suppliers and their general consensus is that we will end up at cost parity when comparing with strip footings and concrete slab. However, this doesn't take into account muckaway. Surely the insulated raft will cost us more when factoring this in?

 

Trying to get an answer on this is difficult and appreciate varying factors will influence cost, but to do traditional strip footings I have a local groundworker who is great on the machine, but not the type to sit and cost items up so I cannot get a true comparison of cost. His simple view is that an insulated raft is very expensive and overkill - however, I can't help feel there's an element here that his experience/knowledge in traditional methods means he's slightly biased.

 

A little bit of a crossroads now as I need to instruct the engineer, if opting for traditiional trench fill then it would be one engineer, if opting for insulated raft then that is included by the supplier. Until the engineer advises requirements on trench fill, difficult to know the cost difference, however ground survey has been done and groundworker has said our ground is very good, be surprised if building control want more than 500mm deep but with existing trees being in the plot it may throw a curveball.

 

I suppose the question I'm trying to ask here, or confirm my thinking is - surely an insualted raft will be more expensive than the traditional trench fill if we have good ground? And are the benefits of insulation and omitting cold bridges worth the extra; how much is the extra and what is the payback etc. We don't want to opt for a well performing timber frame and then feel we've not paid enough attention to the foundations, but of course there are budget constraints to work within!

 

Thanks in advance.

[IanR]

8 minutes ago, dsaunders said:

this doesn't take into account muckaway. Surely the insulated raft will cost us more when factoring this in?

 

Muckaway should be less for a raft.

 

10 minutes ago, dsaunders said:

surely an insualted raft will be more expensive than the traditional trench fill if we have good ground?

 

Depends. There's no reason for it to be more expensive, in fact it should be cheaper when you compare like-for-like, including performance and include the screed you need on a traditional foundation but don't require on a raft. However, insulated rafts remain niche in the UK and you need to do your own ground works (or hire your own team) otherwise you will be paying a premium for labour, concrete, steel etc. from a company that packages it all up for you.

A previous discussion:

 

[dsaunders]

24 minutes ago, IanR said:

 

Muckaway should be less for a raft.

 

 

Depends. There's no reason for it to be more expensive, in fact it should be cheaper when you compare like-for-like, including performance and include the screed you need on a traditional foundation but don't require on a raft. However, insulated rafts remain niche in the UK and you need to do your own ground works (or hire your own team) otherwise you will be paying a premium for labour, concrete, steel etc. from a company that packages it all up for you.

A previous discussion:

 

Thanks - I guess I meant it would be cheaper if not comparing like for like, and trying to then workout the cost increase vs benefit of the improved performance.

 

Thanks, I'll have a read through.

[JohnMo]

A little detailing and thinking through any cold bridging, you can exactly the same thermal performance. Both can be thermal bridge free.

 

Locals you know and trust is half the battle. But do not expect them to understand thermal bridging, airtightness etc, you may have to drip feed them.

 

1 hour ago, dsaunders said:

And are the benefits of insulation and omitting cold bridges worth the extra;

You don't need to comprise with either system and it doesn't have to cost more than a rubbish design, think, plan, think a bit more, tell your architect exactly how you want drawn. Be an active client, that reviews and has final sign off, on all aspects of the design. I was the final approved on all the architects drawings.

[nod]

If your looking for better 

Insulated raft is for you 

If your looking for cheaper You will be disappointed Raft is more expensive 

Or they would be more commonly used 

[Oz07]

Do you also have to consider what your cladding the frame with?

Lightweight works well with raft, brick or block can be a bit trickier. Didn't someone here have a bit of bother with this and cold bridging years ago?

[Stratman]

I'm in a similar position, weighing up cost Vs performance. I love the apparent simplicity of insulated raft, simple one level excavation, no need for foundation walls or screed.  It's just the potential cost that I'm trying to figure out, getting quotes from ISO quick, Greenraft and MBC.

[-rick-]

2 minutes ago, Oz07 said:

Didn't someone here have a bit of bother with this and cold bridging years ago?

 

joe90 maybe? I remember something about rebar ties between the exterior (uninsulated) brick support concrete and the main slab creating a thermal bridge. Not a fundamental flaw with the system more a product of last minute changes that weren't thought through I think.

[Oz07]

Yeh wasn't it terrye that rings a bell

[Oz07]

Joe90 had a simple wide cavity with solid eps fill. Off the denby wood design or whatever it was called

[-rick-]

TerryE sounds right. Had a quick search and couldn't find anything though.

Edited 14 hours ago by -rick-

[Nick Laslett]

2 hours ago, Oz07 said:

Do you also have to consider what your cladding the frame with?

Lightweight works well with raft, brick or block can be a bit trickier. Didn't someone here have a bit of bother with this and cold bridging years ago?

 

2 hours ago, Oz07 said:

Yeh wasn't it terrye that rings a bell

@TerryE, talks about the thermal bridge detail of the plinth for his MBC insulated raft in this thread and on this blog post. 
 

On 17/08/2017 at 19:30, TerryE said:

Hilliard decided to link the inner and outer ring beams by dozens of 20mm rebar links so that they act as a single structure.  This was a flawed design thermally, but luckily I was able to mitigate the bridging as I described this in my blog post

 

 

@Oz07, you are right to highlight the importance of the junction between the raft and the superstructure. People also need to consider the requirements for external doors. These details should really be designed into the insulated raft.

Edited 13 hours ago by Nick Laslett