Strip footings or insulated raft?

[Selfbuildnewbie]

Our ground investigation have been completed. Report says ground is clay with a bearing pressure of 140kN/m² and recommends strip footings with a suspended beam and block floor.  Foundation strips wouldn't need to be too wide given the good bearing pressure. 

However, looking through this forum, I get the impression that insulated raft foundations are most commonly used for ICF properties (is that correct?).  Please could someone enlighten me on the reason for that and whether insulated raft or strip foundations are generally preferable?
 

[Chanmenie]

Insulated rafts are used a lost for timber frame and SIP builds as well as ICF 

[Gus Potter]

2 hours ago, Selfbuildnewbie said:

Our ground investigation have been completed. Report says ground is clay with a bearing pressure of 140kN/m² and recommends strip footings with a suspended beam and block floor.  Foundation strips wouldn't need to be too wide given the good bearing pressure. 

However, looking through this forum, I get the impression that insulated raft foundations are most commonly used for ICF properties (is that correct?).  Please could someone enlighten me on the reason for that and whether insulated raft or strip foundations are generally preferable?

Hiya.

 

I'm assuming they are saying an "allowable bearing pressure" if so, that is a good value to have. 100 kN/m² is usually used for initial design in say boulder clays. 

 

Often rafts are used where say the soil is not so good.. say 30 - 50 kN/m²

 

It boils down to cost. Strip founds tend to be cheepest, assuming you don't have a basement. SE's say always start with looking at strip founds, then if that does not work you progessivly work your way up through the increasingly expensive alternatives.

 

If you have good ground then I would explore all the other options available before going for ICF even if just to rule them out for you.

 

 

 

[SteamyTea]

There are other considerations other than load bearing.

How easy is it to insulate and mitigate thermal bridges spring to mind.

And fitting services.

[joe90]

I had a quote fir a raft just fir my garage and it was huge so went with strip foundations. Fir the house I also went this way as no SE involved, BC okayed the trench and used this method to avoid thermal bridges. (My builder altered it slightly to reduce the “slip plane” above the skirting board).

[nod]

I wouldn’t go for raft unless ground conditions dictated it 

I’m not sure what the percentages are 

Probably at least double 

Most SE seam to spec B&B now 

[scottishjohn]

this post got me thinking

 I have always thought to just have a slab foundation /floor inside my old house  for the UF as it would be cheaper 

 

 

BUT if  would it be cheaper  to block and beam it  then maybe a good idea  labour to fit the blocks and beams is a nearly a free bie to me 

and cost is paramount

 

 at this time I am working on a 300mm dig out below original floor level  with some strip foundations for internal load bearing walls to support the upper floor joist 

 

Idon,t see any problem with ground loads as its very very solid 

 all thoughts welcome 

 i am just starting to clear out the inside and grading the stones into piles for small builders/arge builders   and small rubble and the tons of old lime mortar 

 

 

 

[joe90]

33 minutes ago, scottishjohn said:

with some strip foundations for internal load bearing walls to support the upper floor joist 

Can you not use the original granite walls for load bearing?, you would then need far less support for a timber internal structure?

[saveasteading]

In simplistic terms, with cost  and  simplicity being the main drivers.

 

Ground  good and also level : load bearing slab and strip footings

 

Ground good but sloping or liable to flood: beam and block

 

Poor ground:  raft.

 

So why do major developers use B and B so much? Certainty, and repetition of designs. Also less liable to problems if using unknown ground-workers.

Why are rafts specified so commonly? Not the designers' money, and perhaps don't realise the cost effect.

[SteamyTea]

3 minutes ago, saveasteading said:

In simplistic terms, with cost  and  simplicity being the main drivers.

 

Ground  good and also level : load bearing slab and strip footings

 

Ground good but sloping or liable to flood: beam and block

 

Poor ground:  raft.

 

So why do major developers use B and B so much? Certainty, and repetition of designs. Also less liable to problems if using unknown ground-workers.

Why are rafts specified so commonly? Not the designers' money, and perhaps don't realise the cost effect.

Could you expand on the above a little, especially the difference between load bearing slab and a raft.

To me they seem very similar.

[saveasteading]

1 minute ago, SteamyTea said:

expand on the above a little

Let me know if any of this needs clarification.

 

For a load-bearing slab we consider the slab, acting alone and supporting only the floor loading.  The concrete sits on stone , on ground and the floor loads therefore act directly on the ground, (spread somewhat by the thickness, if the loads are uneven)

The much heavier walls sit on footings in the ground, usually 1m or deeper, and not  on the slabs.

Reinforcement of the slab is generally only to control shrinkage cracking, and is a steel mesh, easily laid.

Footings are usually mass concrete, a lo-tech process

 

Rafts are structural slabs. The concrete is thicker and they have 2 layers of steel, top and bottom, to prevent bending and make the slab work like a big beam.  Cost are higher because the concrete is thicker and the steel is heavier gauge as well as being in 2 layers.   It may also require additional skill. The walls are supported on  edge and cross beams made as deepenings of the slab, heavily reinforced and fully linked to the slab. These support the walls but also spread this heavy load over a wider area of ground, using the slab area.

 

So a simple load-bearing slab sits on good ground. If a book-case wouldn't sink into the ground, then neither will it when there is concrete on top.

Walls are very much heavier and so need strong ground.

 

If the ground is not strong enough to support the walls then a raft may resolve the issue by spreading the load over a bigger area, and allow construction. Cost of slab and footings approximately double, perhaps more.

[IanR]

On 20/10/2022 at 18:55, Selfbuildnewbie said:

However, looking through this forum, I get the impression that insulated raft foundations are most commonly used for ICF properties (is that correct?).  

 

Since you are specifically asking about an insulated raft, rather than a traditional raft, it's not entirely fair to compare costs to strip foundations, unless your going to spend extra on the strip foundations to get their performance up to the same level as an insulated raft.

 

An insulated raft allows you to easily remove all cold bridges at the floor wall joint and with some careful design, achieve the same at the door thresholds.

 

If you are paying for removal of spoil, I'd also say they are actually cost competitive against other highly insulated floor options. If you are on flat ground then the an insulated raft is a shallow dig. A 0.11 U Value is achieved with around 300mm of EPS insulation. UFH can be set directly in the raft and a screed is not necessary, so another saving there.

 

All in all materials costs are actually lower than strip foundations + block and beam + insulation + screed for the same U value, but, an insulated raft needs a structural engineer's costs adding to them to get them through building control. For all their benefits they remain a niche product.

 

I have an insulated raft from Advanced Foundation Technologies Ltd. and wouldn't consider doing foundations any other way on my next self-build.

Edited October 22, 2022 by IanR

[SteamyTea]

7 hours ago, saveasteading said:

Let me know if any of this needs clarification.

No, that was a good explanation, now know the difference.

I think as @IanR explains, once insulation parity it taken into account, there may not be as much price difference as first assumed.

Thanks.

[saveasteading]

6 hours ago, SteamyTea said:

may not be as much price difference as first assumedi

 Agreed ' not as much'. But i think there will still be a big difference on most sites. There are much cheaper, yet efficient, ways to get the insulation at the perimeter than to insulate the footings.

 

I'm not about to calculate the difference, but would be interested if someone has. I can see the attraction of the kit foundation idea though, for certainty and speed, for which there would be a premium. Hunch price?  £15k extra on 200m2?

 

As to muck away costs. Yes this is often ignored and can be significant on a small site in the SE.  Often there is a design solution to this too, but it needs holistic design.

Edited October 23, 2022 by saveasteading

[Selfbuildnewbie]

This is what has been designed using strip foundations. Looks pretty straightforward to me but I know nothing about this sort of thing.

Won't there be thermal bridging where the ICF walls meet the foundations?  Looking at what @joe90 posted (cavity wall rather than ICF but not sure why that should make a difference), should there be some insulation to remove the thermal bridges?

Also is some blockwork below ground on the outside of the ICF walls? Above ground will be brick slips, not shown on drawings.

[scottishjohn]

On 20/10/2022 at 18:55, Selfbuildnewbie said:

Our ground investigation have been completed. Report says ground is clay with a bearing pressure of 140kN/m² and recommends strip footings with a suspended beam and block floor.  Foundation strips wouldn't need to be too wide given the good bearing pressure. 

However, looking through this forum, I get the impression that insulated raft foundations are most commonly used for ICF properties (is that correct?).  Please could someone enlighten me on the reason for that and whether insulated raft or strip foundations are generally preferable?
 

block and beam can be every bit as effective with UFH 

as you will put your insulation on top of the block+beam ,then add UFH,then screed it .

It is something I am looking at seriously as i beleive block and beam will be cheaper to do  and if self build a job you can do yourself 

you would sit your beams on the strip footings making them a bit wider to accomodate the beamsand add insulation at perimeter to isolate from walls 

[IanR]

1 hour ago, scottishjohn said:

block and beam can be every bit as effective with UFH 

 

Agreed, if only considering the performance of the floor. The air temp under the B&B being a lower temp than the ground under a raft won't have that much effect on the overall performance.

 

The external walls and internal load bearing walls however, will perform far less well in a strip foundation with B&B setup due to the cold bridging.

 

1 hour ago, scottishjohn said:

i beleive block and beam will be cheaper to do  and if self build a job you can do yourself 

 

Probably worth pricing both out to check on that. You'd need to include the cost of insulation and screed on the B&B floor to compare to an insulated raft.

An insulated raft is not difficult to do, well, at least up to the pour. On your first one it would be worth getting a ground works team in to help with the pour and power-float to de-risk that part. That would be a days work for a team of 3 or 4.

AFT will come and do it with 1 person, if you supply a couple of labourers.

[scottishjohn]

50 minutes ago, IanR said:

The external walls and internal load bearing walls however, will perform far less well in a strip foundation with B&B setup due to the cold bridging

I would like to see you  quantify  the real difference

remeber that fully insulated foundation  rafts to pour your floor and wall footing into  come from countries with much harsher climates or even permo frost 

 

my average winter temp in s/w scpt;and is +3c

 

 not -15c or worse for months at a time  likein northern european countries where these came from 

 I doubt the extra 10K for the insualted raft will ever pay itself back 

 just my view 

 

and full concrete raft floor will be more expensive than  B+B

 

 hence why  B+B they are used so much in uk they are cheaper to 

and if DIY it don,t get any easier  and you have to option to correct any mistakes on drainge penetrations  until you screed it

 I have no issues with the insulated raft system other than it seems OTT for this country

[scottishjohn]

you could go halfway and insulate the strip founds same way and use B+B+screed

 so cold bridge gone anyway

 and anayway the sub soil temp in uk is about 12c all year round at about 1m deep 

or ground source heat pumps would not work --would they 

Edited November 11, 2022 by scottishjohn

[saveasteading]

Please excuse if I have missed something but my thoughts are..

The cross sections are a bit vague. Where is the floor insulation/dpm?

Why an extra block to support the beams, is the outer wall existing?

Yes there is a cold bridge..this can be sorted but needs more detail.

2m deep footings. Presumably there are  trees about 10m away , at all sides.

Ask the local concrete company what their cheapest mix is. Leanmix is fine, but I would just ask in case they have a mix with bigger stones, or single sized, etc that might save a lot.

I don't follow what the line under the beams denotes. It looks like a slab but it perhaps is ground....it should say on the drawing.  

700 and 900 seem very wide trenches.

Others tell please...Scottish regs used to require a solum ( stone or something) below a suspended floor. Does that apply?

[IanR]

29 minutes ago, scottishjohn said:

I would like to see you  quantify  the real difference

 

Only by doing the heat loss calcs, which I did for my current property, but  I never compared insulated raft v. B&B with all else equal, so could not pick out the delta for that comparison.

 

Mine was a walk from a heat loss of around 55kWh.m²/year for a "traditional" conversion (from a cow shed) to a little under 15kWh.m²/year for what I eventually built. The big factors in that improvement was good floor/wall/roof insulation levels, high performing windows and doors, no cold bridges and very low infiltration rate. What percentage of the improvement is down to the insulated raft + elimination of cold bridges I don't know.

 

The decision was made easy for me as when I actually costed both options, and included insulation and screed costs for the B&B option, the insulated raft was cheaper.
 

[saveasteading]

1 hour ago, scottishjohn said:

ground source heat pumps would not work --would they 

No. They don't work efficiently in most uk situations. No hot rock.

[scottishjohn]

1 hour ago, saveasteading said:

No. They don't work efficiently in most uk situations. No hot rock.

they do work very well if installation is correct 

Its just the cost to do it  compated to air source  makes them not as good an option in most situations

but in cold climates they will out perfrom air source eveyr time 

 

 

[jamieled]

It's generally accepted (by the energy saving trust and others) that gshp's are more efficient across the year than ashp. This is primarily because the source-target temp difference in the heating season is much lower with a gshp (soil temps generally higher than air). But as mentioned above, the additional efficiency comes with a high installation cost which may never be recoverd compared to an ashp installation.

 

Gshp does not need hot rock, that is more akin to geothermal heat recovery.

[saveasteading]

3 hours ago, scottishjohn said:

In cold climates they will out perfrom air source eveyr time 

I don't begin to agree on this broad statement. In clay and rock, gsh is not efficient because thd heat is not replaced. In fact, boreholes freeze.

In porous ground it can work, but with great capital expense and too many disappoinments.

 

I have been going to gshp industry  presentations for decades and noted how the wild promises were gradually altered to suit reality.

Without grants the true value became apparent.

 

It is now acknowledged that most boreholes need to be reheated in summer (pump running) So it is really air source or solar, with the ground evening out the seasons. 

Slinky coils take solar energy from higher ground and can be OK in large sunny spots, but they also chill out in cold weather.

 

I know this from observing failed projects in SE clay  and the industry recently acknowledging it.

 

You have inspired me to look at the EST website. They show better savings from gs than as, but at much higher capital cost. But they are very quiet on ground conditions.

 

Air source works well, and reliably, except in very, very cold conditions.

 

Convince me otherwise  please.