Raft vs Piling foundation

[deuce22]

Hi.

 

I've only ever used strip foundations in the past, so I have know experience with the costs of a raft and pile foundations. I have been advised to use a raft foundation on this development, but was wondering what the difference in cost is between the two.

What would be the preference for people on here.

 

Thanks.

[Alex C]

You only tend to use piles when you have no alternative due to ground conditions as they are expensive. A lot of self builders on here have used highly insulated rafts and they work very well and are pretty straight forward.

[ToughButterCup]

64 piles , £7,000 ish. Needed because we are on Made Ground - 100 m m raft sitting on 300 mm of EPS

 

[deuce22]

Thanks very much both.

 

I'm just doing as much research now before I actually start construction. I've never dealt with piles before and assumed it was more expensive, but somebody said it is similar to a raft. Just so I understand, the 7k for the piles was to get you to a position to then pour concrete (similarly to a strip foundation)?

 

I'll stick with the raft then and do some research into the different options.

 

Thanks.

 

[Patrick]

Just now, deuce22 said:

Thanks very much both.

 

I'm just doing as much research now before I actually start construction. I've never dealt with piles before and assumed it was more expensive, but somebody said it is similar to a raft. Just so I understand, the 7k for the piles was to get you to a position to then pour concrete (similarly to a strip foundation)?

 

I'll stick with the raft then and do some research into the different options.

 

Thanks.

 

+1 on @Alex C

We used piles. Cost around 15k. Quite normal for piles as far as I can see around the forum.

@AnonymousBosch

piles where exceptional cheap. But he used a different piling method than others. Not viable for everyone.

Generally even though I wasn't able to use it due to Ground conditions, I would have preferred insulated raft. As always @Jeremy Harris

 blog is the place to look for details.

[deuce22]

I'm going to calculate costs on the different raft options available and then I'll have something to work from.

 

Just so I understand the method properly, can somebody tell me if this is correct for a raft foundation.

 

Shutter the area to be poured, install all the steel and then pour the first slab. Wait for that to cure and then pour the second slab. Lay DPC and insulation and then screed over the top for the finished floor.

 

I have just spoken with Isoquick and they said that the insulated forms get laid and then the concrete gets poured. There is no need for shuttering and the concrete can be floated for the finished floor.

 

Am I correct?

 

Thanks. 

[Gone West]

1 hour ago, deuce22 said:

I have just spoken with Isoquick and they said that the insulated forms get laid and then the concrete gets poured. There is no need for shuttering and the concrete can be floated for the finished floor.

 

Am I correct?

We had the first Isoquick installation in England and it is a very simple system. It can only get spoilt if the concrete is badly laid. See my blog for details . I can't see any reason it can't be done DIY.

[Alex C]

Take a look at my time lapse video that is on the mbc timber frame website. Shows pretty clearly one method of installing an insulated raft

[deuce22]

Just had a look at your development Alex, it is great. I didn't see a video, but can see how it was done from the images. So the finished floor was the poured concrete? How thick was the concrete?

 

Is your blog on this site Peter?

 

Did either of you have a comparison of the costs between what you used and a standard raft to finish floor level?

 

Thanks.

[Alex C]

This is the link to the video 

 

Finished floor was tiles over concrete. You are never going to get a very good standard of concrete floor from polishing a structural slab. Concrete thickness is dependant on the design of the slab and ground conditions. Mine is technically different to the majority as it is structural with more steel in and also thicker. I wanted decent insulation with no thermal bridge so didn't look in to cost for a 'normal' raft. 

Edited February 5, 2020 by Alex C

[ToughButterCup]

64 piles , £7,000 ish. ....

 

Yes @deuce22. We used piles that are sometimes called ground improvement columns. 

Your piling contractor will take responsibility for the design.

 

[Gone West]

1 hour ago, deuce22 said:

Is your blog on this site Peter?

Yup.

 

 

[deuce22]

I didn't mean that I wanted the concrete to actually be the finished floor, just that it is finished ready for carpet/tiles. The guy from Isoquick said the floor stays warm, without underfloor heating as it is within the thermal envelope of the building. Have you found that to be true?

 

2 hours ago, AnonymousBosch said:

64 piles , £7,000 ish. ....

 

Yes @deuce22. We used piles that are sometimes called ground improvement columns. 

Your piling contractor will take responsibility for the design.

 

Thanks.

 

2 hours ago, PeterStarck said:

Yup.

 

 

Thanks. I'll look through it.

[Alex C]

36 minutes ago, deuce22 said:

I didn't mean that I wanted the concrete to actually be the finished floor, just that it is finished ready for carpet/tiles. The guy from Isoquick said the floor stays warm, without underfloor heating as it is within the thermal envelope of the building. Have you found that to be true?

 

Thanks.

 

Thanks. I'll look through it.

That depends on what your idea of warm is. It would be crazy to build a new house with a concrete slab and then not put heating under it. Running some pipe in the slab to a manifold is cheap and easy.

[Oz07]

I'm not so sure about that. Fair enough if a raft with mesh in already but in a ground bearing slab with strip founds ufh is dearer than radiators, means introducing mesh and increased complexity. 

Having said that I do think it is better for low energy homes 

[Alex C]

1 hour ago, Oz07 said:

I'm not so sure about that. Fair enough if a raft with mesh in already but in a ground bearing slab with strip founds ufh is dearer than radiators, means introducing mesh and increased complexity. 

Having said that I do think it is better for low energy homes 

The OP was talking about an Isoquick insulated raft and that is what I was responding to.

[Gone West]

13 hours ago, deuce22 said:

The guy from Isoquick said the floor stays warm, without underfloor heating as it is within the thermal envelope of the building. Have you found that to be true?

We have 300mm insulation under the concrete slab and warm air heating which keeps the house at 23C. The slab is always going to be very close to 23C unless of course the sun is shining on it when it can be warmer. With bare feet it will feel cooler because we have porcelain tiles all over the ground floor but if you had for instance cork tiles it would feel warm.

 

13 hours ago, Alex C said:

That depends on what your idea of warm is. It would be crazy to build a new house with a concrete slab and then not put heating under it. Running some pipe in the slab to a manifold is cheap and easy.

I disagree. UFH is an inefficient way to heat a house. However much insulation there is under a slab a certain proportion of the heat is lost to the ground. I agree it has advantages over radiators in freeing up wall space, but there are alternatives.

[Jeremy Harris]

Although UFH is always less efficient than a heating system like radiators or warm air, because it increases the temperature differential across the floor insulation, and hence increases the heat loss, it is worth putting some real numbers in to a typical case to see the impact of that.

 

Increasing the floor surface temperature to 23°C with a room temperature of 21°C, and a ground temperature of 8°C, delivers about 18.8 W/m² of heat to the house, more than enough to keep a reasonably well-insulated and airtight house warm in cold weather. 

 

Assuming a floor U value of 0.11 W/m².K, then the additional heat lost to the ground by having the floor surface at 23°C rather than close to room temperature, 21°C, is ~ 0.22 W/m². 

 

The total heat loss through the floor will be about 1.65 W/m² with UFH, or about 1.43 W/m² without UFH (for the case above).

 

If heating the house with a heat pump, with a COP of 3 and an electricity cost of £0.16/kWh, then the additional heat loss from losing an extra 0.22 W/m² in cold weather (the loss will be a lot less in mild weather) would be about £0.0026/m²/day.  So for a 100m² heated ground floor area the additional cost would be about £0.26/day (in cold weather).  Averaged out over the heating season the additional cost is likely to be around 1/3rd of this, maybe a fair bit less, (based on our experience of the ratio between the average and peak heating requirement in winter).

 

The question is then whether that is a price worth paying or not.  For our house, with ~75m² of ground floor UFH and an annual heating requirement of about 1,550 kWh, the additional cost of using UFH over the warm air system we have (but don't use) is about £0.85, in terms of paying for the additional heat loss the UFH creates.  One advantage the UFH gives us is that it allows us to heat the floor slab using E7 electricity overnight, so it acts as a storage heater through the day.  That does mean that, allowing for our heat pump COP of about 3 (it's usually better than that) we pay around £0.027/kWh for the heat delivered to the house.  If using another form of heating it might not be possible to take advantage of cheap rate electricity, which would increase the running cost by ~80% or so.

[Alex C]

My experience with a slab with ufh and 300mm underneath is that the house temp is kept very constant. The slab both heats up and cools down pretty slowly and only needs to be heated to a few degrees above room temp giving a very comfortable even temperature environment. We have porcelain tiles on the ground floor and if the slab is allowed to get down to around 21 it starts to feel pretty cool to walk on without shoes, where as at 23 it is fine. This may only be a small differnence but definately noticeable. I agree if you had carpet or cork you would not notice this.

 

UFH may be claimed to be an inefficient way to heat a house, but seeing as my heating bills are about £100-£200 a year it isnt really a major worry. I will take the couple £ extra bill for the benefits of comfort.

Edited February 6, 2020 by Alex C

[Ed Davies]

There's also the consideration that warm-air heating will increase the ventilation heat losses.

 

The perceived temperature is, roughly, the average of the actual air temperature and the radiant temperature of the surroundings. If you're heating the air directly then the air will be a little bit warmer than the walls, floor and ceiling because of the interface resistance so the air will need to be about half as much warmer than the required perceived temperature. Using radiant heating (UFH particularly where about half the heat emitted is via radiation or radiators where it's maybe a sixth [¹]) means that air can be a tad cooler than the wanted perceived temperature so ventilation heat losses will be smaller. With heat-recovery ventilation this will not be a lot, but as @Jeremy Harris shows the difference with UFH is small, too.

 

[¹] Longer discussion: https://edavies.me.uk/2014/08/radiators/

[Gone West]

23 hours ago, Jeremy Harris said:

Assuming a floor U value of 0.11 W/m².K, then the additional heat lost to the ground by having the floor surface at 23°C rather than close to room temperature, 21°C, is ~ 0.22 W/m². 

Of course when the floor is well insulated then the losses will be less. As the level of insulation decreases the losses increase and then the floor temperature will have to be higher to counter the losses. As long as people are aware that UFH should only be acceptable with a well insulated floor and isn't always the most suitable form of heating. Then of course there are the costs to be considered. Not everyone is willing or able to install their own ASHP and UFH components and it can be an expensive exercise to provide the small amount of heat required in a well insulated house.

[Alex C]

7 hours ago, PeterStarck said:

Then of course there are the costs to be considered. Not everyone is willing or able to install their own ASHP and UFH components and it can be an expensive exercise to provide the small amount of heat required in a well insulated house.

I paid less than £400 for someone to lay all the pipework over 145m2 and attach to the manifold. This was then connected to a small gas boiler by my plumber. No great expense or expertise involved and after 2 years of living with it am very happy. Has the benefit that on cool, sunny days like today I can run the pump with no boiler input and it spreads the solar gain from near the windows over the ground floor.

[Gone West]

16 hours ago, Alex C said:

I paid less than £400 for someone to lay all the pipework over 145m2 and attach to the manifold. This was then connected to a small gas boiler by my plumber. No great expense or expertise involved and after 2 years of living with it am very happy. Has the benefit that on cool, sunny days like today I can run the pump with no boiler input and it spreads the solar gain from near the windows over the ground floor.

That sounds like a good setup for a well insulated floor. I like hearing about more simple systems. AIUI normally with a gas boiler there is a buffer tank to prevent short cycling and blending valves etc. How did your plumber get around those problems?

[Alex C]

6 hours ago, PeterStarck said:

That sounds like a good setup for a well insulated floor. I like hearing about more simple systems. AIUI normally with a gas boiler there is a buffer tank to prevent short cycling and blending valves etc. How did your plumber get around those problems?

I bought a blending valve that is accurate down to about 20 degrees although I have found that running it at around 26/27 works best. The boiler is a worcester bosch greenstar 12i that modulates from about 3.5 up to 12kw. I have never had issues with short cycling and with just a wall mounted temp sensor it controls the temp well. I also use it to run 4 x 600w towel rads which is all the heating we have upstairs. I have these on for an hour in the morning in the winter and overlap them with water heating as their output is even slightly less than the lowest boiler modulation. The UFH comes on a max of twice a week mid winter and often only once for about 4 hours. It was a bit of punt as I sort of designed it myself but it is dead simple and has worked really well. My plumber didnt  believe that the boiler would be powerful enough to heat the house. Getting a plumber that understands low energy housing seems to be quite tricky.

Edited February 8, 2020 by Alex C

[deuce22]

Alex C.

 

I just watched the video. Did they lay the UH directly on top of the forms before pouring the concrete? My footprint is around the same as yours, so I'd consider doing the same. As the ground floor is spare bedrooms, bathrooms and entrance hall, it won't be used very much, so I wasn't going to bother. The property is 3 stories and I was planning on using UH on the other 2 floors, has anyone done this and how well does it perform. How many metres of concrete did you pour?

 

Thanks.