Different base structures, bad idea?

[syne]

I've begun the process of designing an extension and an fairly certain I'm going to have an insulated raft with icf walls. This will be adjoining a (likely strip foundation) cavity wall house. 

 

I've had a quote from isoquick for the raft  approx 6m² but I'm not sure if it's worth extending this to cover the garage floor? 

It would essentially double the price, but the alternative is going to create a strip & Insulated raft & strip or raft monstrosity combo. 

 

Is it going to lead to problems?

should I just swollow the extra cost even though its going to be a waste of resources and is an insulated raft even suitable for a garage floor?

 

The upside I suppose, is that if the raft is continuing then it simplifies things and if someone decides they need another huge room downstairs in the future the prep will have been done.... 

 

 

 

[Russell griffiths]

For such a small area why are you bothering ?

also have you looked into getting a structural engineer to sign this raft off, icf walls are heavy and I would be concerned about uneven loading on a raft as I presume being an extension that it will not have a load on all 4 sides, I would also be concerned with differential movement between strip and a raft. 

 

For a small area you would be better just strip footing and build your icf up from there. 

 

A lot of reasons people use insulated raft is to remove the cold bridge when using a timber frame, however you don’t get the same cold bridging with icf. 

[PeterW]

That’s a very small extension - 2m x 3m..?? I wouldn’t bother with an insulated raft and you’ll probably find building control want a lot of paperwork for something where they would normally say “same depth as the house” and let you get on with it. 
 

 

 

[syne]

sorry,

The extension is 6x6m so 36m² and the adjoining garage is approx 7x7, but with a taper so it works out about 34m² 

 

Deferential movement, well that's a issue my SE can hopefully remove from the equation. 
 

I much prefer the icf to blocks and when combined with the slab takes (pretty much) all the cold bridging out of the equation, not to mention the speed factor.

Price wise the raft is looking to be similar in cost, the icf similar also. 

 

I'm just not sold on the idea of an insulated raft for a garage floor, seems like overkill. 

 

 

 

[PeterW]

I’d be doing that as strip and ICF between garage and room too, ground bearing slab over insulation will give you exactly the same result with less complexity. Insulated garage floors are a bit marmite - you’ll need to go to 150mm thickness and 7x7m will need mesh in it regardless. 

[syne]

The more I read the less likely it seems to be a good idea to do it this way, especially as there's no ground issue to do so. 

Looks like a strip it is..