Insulated Raft Foundations and Build Lite

[LnP]

I'm shopping around for foundations for a timber frame structure with brick outer leaf. I don't plan to do it myself but will engage a ground works company to do the install.

 

I've not had a ground investigation yet, but indications are that I have good ground conditions, about 600 mm top soil with compacted gravel and sand below. I've had to dig holes and trenches down to about 1.2 m around the plot for various reasons, and never seen water.

 

I'm most interested in speed of installation, simplicity and cost. Regarding energy efficiency my expectation is that whatever foundations I go for, they can be made be good enough.

 

So I'm not wedded to any particular type of foundations and have been getting prices and talking to installers, suppliers and structural engineers to make my decision. I had been talking to AFT and an installer they had recommended, but as I mentioned in a previous post, AFT have ceased trading in the UK. The recommended installer is looking at carrying on his business with a different insulated form work supplier and is looking at Build Lite. 

 

I'd be interested in thoughts or comments on my approach but also on the Build Lite insulated raft system. I've attached their sketch from their website.

 

Thanks

 

[-rick-]

That's a pretty stonking great thermal bridge there. Not keen. Both AFT + Kore systems have thermally breaks between the inner and outer leaves.

 

With that thermal bridge I'm not sure why you would bother with the extra cost of an insulated raft vs traditional foundations.

 

What U value are they claiming?

[LnP]

13 minutes ago, -rick- said:

That's a pretty stonking great thermal bridge there. Not keen. Both AFT + Kore systems have thermally breaks between the inner and outer leaves.

 

@-rick- Thanks for your comment. Just checking, did you see the whole image? For some reason, after I uploaded it, I only see the whole image if I click on it and open it. Here it is uploaded a different way. Does that answer your question? They have a toe on the raft which the bricks sit on, but with a course of Marmox blocks. 

[-rick-]

I missed that before, sorry. Still not sure it really changes my view. Marmox blocks are good but they are still much lossier than the other mentioned solutions. There is also nothing stopping using marmox blocks with a traditional foundation and save all the custom eps/engineering costs (use it on the inner leaf and ensure the non-load bearing floor slab is thermally broken from the foundation).

Edited 9 hours ago by -rick-

[-rick-]

Expanding on my thoughts. If you have ground that suitable for this solution and are happy with the performance then it's a valid option if it is cheaper/quicker/easier to install. But I suspect you can get the same performance from a traditional foundation with somewhat lower cost (can get any groundworker to do it vs a specialist) and the industry is just much more comfortable with the engineering of traditional slabs which may make other things (insurance, future sale, warranty, etc) easier.

[JohnMo]

You have basically taken a passive slab and killed all its benefits. You have no continuity of insulation. Take a look below, you can follow the insulation in the floor and up the walls. The slave takes the structural loads the block work sits outside the insulation. Your Marmox block works but is an expensive bodge. 

 

 

[Nickfromwales]

2 hours ago, JohnMo said:

You have basically taken a passive slab and killed all its benefits.

In a nutshell.

 

A lot of mis-information and simply "bad" ideas here, with a significant price tag to boot. 

 

400mm of EPS means a load more unnecessary excavation and muckaway......... Buildlite, erm, no thanks, I'll pass. 

[saveasteading]

Not for me. 

8 hours ago, LnP said:

speed of installation, simplicity and cost.

Reasons to support that system?

IF  the ground doesn't need to be excavated and tipped to get to that level

IF there are no experienced builders available or it's diy

 

Reasons against?

The holistic cost is likely to be high,

A local builder may be suspicious of some new trendy system

A local builder can do traditional build much more readily.

300mm of eps on the middle areas where it isn't so important but 200 at the critical perimeter is weird.

 

you have great ground conditions  so don't waste it on a floating slab.

 

I have an inherent dislike for a heavy structural beam to the perimeter ( compare it to a big floor beam holding an upper car park floor?) sitting on top of 200m of eps. 

 

trench footings and masonry to Floor level

masonry or timber above. 100-150  mm concrete slab with crack control only

150mm pir or 250mm eps then a screed.

 

that will likely save you half your cost to floor level.

 

 

[LnP]

4 hours ago, JohnMo said:

You have basically taken a passive slab and killed all its benefits. You have no continuity of insulation. Take a look below, you can follow the insulation in the floor and up the walls. The slave takes the structural loads the block work sits outside the insulation. Your Marmox block works but is an expensive bodge. 

 

 

@JohnMo One insulated form work system provider I spoke to, said that NHBC and other warranty providers are not comfortable with having the masonry outer leaf sitting on a different foundation than the timber frame. They're concerned about differential movement. They could supply their system with a separate outer ring beam for the masonry, but recommended a toe off the raft similar to the Build Lite one I shared, for an easier ride past BCOs and warranty providers.

 

20 minutes ago, saveasteading said:

Not for me. 

Reasons to support that system?

IF  the ground doesn't need to be excavated and tipped to get to that level

IF there are no experienced builders available or it's diy

 

Reasons against?

The holistic cost is likely to be high,

A local builder may be suspicious of some new trendy system

A local builder can do traditional build much more readily.

300mm of eps on the middle areas where it isn't so important but 200 at the critical perimeter is weird.

 

you have great ground conditions  so don't waste it on a floating slab.

 

I have an inherent dislike for a heavy structural beam to the perimeter ( compare it to a big floor beam holding an upper car park floor?) sitting on top of 200m of eps. 

 

trench footings and masonry to Floor level

masonry or timber above. 100-150  mm concrete slab with crack control only

150mm pir or 250mm eps then a screed.

 

that will likely save you half your cost to floor level.

 

 

One such local builder I spoke to came by with a machine with a large (1.5 m long?) augur and drilled some holes to see what kind of ground conditions I have. He was polite about my interest in insulated raft, but was of the opinion traditional foundations would be a doddle.

 

I'm aware that there are those on here who say that insulated raft is cheaper, when you account for not needing screed, can fix the UFH pipes to the rebar etc. And I've heard it from insulated raft suppliers. I'm getting quotes both ways so will soon find out how it is in my case.

 

Maybe insulated raft is best suited when you don't have the masonry outer leaf to worry about.