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 Wednesday at 13:52 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.

[Gus Potter]

11 hours ago, LnP said:

'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.

Do you do this as a day job / business or are you a genuine self builder?

 

The build light system shown is not compatible with a timber frame. 

Edited yesterday at 00:53 by Gus Potter

[Nickfromwales]

7 hours ago, LnP said:

@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.

 

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.

Rubbish, raft foundations are designed with the EPS basket extended to allow the pour of a second mini ring foundation to accept the masonry.

 

Don’t take this the wrong way, but you’re entering an area of major misadventure here, and what you’ve arrived at independently is already falling on its ass.

 

Stick with ‘tried & tested’ and an experienced contractor / SE to solve this correctly. It’s “kinda important”   

[LnP]

6 hours ago, Gus Potter said:

Do you do this as a day job / business or are you a genuine self builder? 

There are two ways for me to understand your question. If it's a straight one, the straight answer is, no I don't do it for a day job/business and yes I'm a genuine self builder, whatever that is. But since I opened my post by explaining why I'm asking my question, the straight answer should already have been clear. So I'm wondering if your question was rhetorical and intended to demean and denigrate, which would be a pity.

6 hours ago, Gus Potter said:

The build light system shown is not compatible with a timber frame. 

Thanks for your comment. Useful to get your opinion..

[LnP]

1 hour ago, Nickfromwales said:

Rubbish, raft foundations are designed with the EPS basket extended to allow the pour of a second mini ring foundation to accept the masonry.

 

Don’t take this the wrong way, but you’re entering an area of major misadventure here, and what you’ve arrived at independently is already falling on its ass.

 

Stick with ‘tried & tested’ and an experienced contractor / SE to solve this correctly. It’s “kinda important”   

Yes, I get it. Thanks for your alert. Choosing the foundations is one of the most important decisions in the project. The best one can hope for is to become an "intelligent customer". That's why I'm talking to people and asking questions on here.

[saveasteading]

7 hours ago, Gus Potter said:

Do you do this as a day job / business or are you a genuine self builder?

I read that as..." how much detail do we need to go into?

For examples:

A builder may have been asked to do this for the first time.

A self builder may have been "sold" the idea at an exhibition, without having the pros and cons explained.*

 

*there is a reason I see such systems at  self build exhibitions but not at trade exhibitions.

Not that it's bad, but that there are options and there is more to know.

 

[LnP]

46 minutes ago, saveasteading said:

I read that as..." how much detail do we need to go into?

For examples:

A builder may have been asked to do this for the first time.

A self builder may have been "sold" the idea at an exhibition, without having the pros and cons explained.*

 

*there is a reason I see such systems at  self build exhibitions but not at trade exhibitions.

Not that it's bad, but that there are options and there is more to know.

 

Fair point and thanks for your helpful interpretation. 

[MortarThePoint]

Not being overly familiar with this type of design I find this interesting. That thermal design asks a lot of the Marmox thermal blocks as they are the only line of defence between the cold bricks of the outer leaf and the warm concrete of the raft.

 

The Marmox blocks have vertical columns of epoxy/stone concrete surrounded by XPS, so the vertical direction is the worst from a thermal transfer perspective, but only 6.3% [1] of that sectional area looks to be solid concrete. I am not a thermal expert by any stretch - just trying to ballpark the numbers. Assuming an XPS lambda of 0.033 W/m.K and a concrete lambda of 2.3 W/m.K [0] results in an average vertical lambda of 0.176 W/m.K [2] which is noticeably higher than the value they advertise of 0.05 W/m.K. That may be because the horizontal performance is much better and they quote that value. If using the 65mm Marmox thermal blocks that's going to create a vertical thermal bridge of 0.379 W/m.K . There would also be a lesser thermal path out sideways trough the Marmox block to the soil, but using a total value of 0.4 W/m.K would probably be indicative. This is similar in magnitude to a standard steel lintel, as used above a window. At an average 10C difference for 6 months that would loose 17.3 kWh/m or £1.21/yr per metre based on a heating system COP of 3 (low) and electricity price of 21p/kWh (high).

 

The overall thermal bridge would have to consider the thermal transfer through the slab as well, which I have ignored, but that feels reasonable given the high thermal conductivity of concrete and rebar.

[0] IMPORTANT TO NOTE: the Marmox epoxy concrete may be thermally better than standard concrete and mean this value, and subsequent values, should be lower.

[1] (pi*(30/2)^2) / ((50+30)*140) = 6.3%

[2] (2.3*6.3%) + (0.033*(100% - 6.3%) = 0.176 W/m.K

[3] 0.176*(0.140*1) / 0.065 = 0.379 W/m.K

[4] 0.4*10*(24*180)/1000 = 17.28 kWh/yr.m

 

I have a make up on my garage raft which is essentially the same without the Marmox thermal block. That's an oversight, but it is a garage rather than a house. However, I think I can improve matters by putting a thin layer of XPS between the blockwork and the soil. The blockwork at the bottom of the outer leaf is a course of engineering bricks and a course of Stranlite 140mm 10N blocks. With the change as pictured below, ignoring horizontal heat flow through the insulation (yellow) and assuming perfect thermal conductivity in both the slab concrete (light grey) and engineering brick (brown), I can consider just the vertical heat flow through the Stranlite block (dark grey). It has a lambda of 0.43 W/m.K , so that works out as a thermal bridge of 0.43*0.140/0.215 = 0.28 W/m.K which is actually comparable to the Marmox thermal block. Interesting. Looks like I have an improvement worth making there.

 

Tweak to include horizontal heat loss through the added XPS: 50mm of XPS would have a sideways thermal bridge of 0.033*0.215/0.050 = 0.14 W/m.K which can probably be halved assuming a linear temperature gradient across the height of the Stranlite block. 0.28 + (0.14 / 2) = 0.35 W/m.K , so still similar to the Marmox thermal block.

 

Now:

 

Improved:

 

[MortarThePoint]

Just found the BBA certificate (BBA 19/5617 PS1 Issue 2) of the BuildLite product and that gives a psi value of 0.4 W/m.K for similar make up but different geometry (100mm wide Marmox) :