Insulation spec

[bmj1]

Hi all,

 

Our project is underway, we'll shortly be finishing foundations.

 

My understanding is builder has selected block and beam for the oversite.

 

I wanted to ask two questions:

A) it is worth me suggesting concrete slab (does it perform better), and paying the difference?

B) builder does not currently have any requirements around insulation other than achieving building control sign off. Should I be specifying some specific requirements in addition to this ? E.g. types of insulation, etc ?

 

Many thanks in advance 

[Thorfun]

there are many discussions on this subject and they all, generally, come to the same conclusion that you want lots more than BR levels of insulation, especially on the ground. the thickness does depend on the insulation you choose but I would be aiming for 0.11W/m²K U-value. 

 

you can specify an insulated slab (EPS insulation under the slab (we have 300mm EPS under ours with 200mm under the basement)) which is a great solution for many reasons but it's something that your structural engineer needs to sign off as it depends on lots of factors.

[bmj1]

16 minutes ago, Thorfun said:

there are many discussions on this subject and they all, generally, come to the same conclusion that you want lots more than BR levels of insulation, especially on the ground. the thickness does depend on the insulation you choose but I would be aiming for 0.11W/m²K U-value. 

 

you can specify an insulated slab (EPS insulation under the slab (we have 300mm EPS under ours with 200mm under the basement)) which is a great solution for many reasons but it's something that your structural engineer needs to sign off as it depends on lots of factors.

If I don't want to get into the weeds on individual items, is there a sensible standard I can point to to ensure we get a decent level of insulation ?

[nod]

I’m surprised your builder has any say as to weather you use BB or slab 

It’s normally dictated by ground conditions and a Structual engineer 

Slab is by far the cheaper option 

[Buzz]

1 hour ago, bmj1 said:

If I don't want to get into the weeds on individual items, is there a sensible standard I can point to to ensure we get a decent level of insulation ?

Do you have a copy of your building regulations? This will tell you what has been specified for your build regarding the type and amount of insulation for the floor walls and roof . Post it up and I'm sure you will get some good advice on  anything that could be improved.  

 

[Adam2]

An interesting comment from my SE was that with a concrete slab, which is what we have, adding extra floor installation (we have 150 PIR) may not really make much difference as the earth around this will over time increase temp to close to slab temp, appreciate thoughts on that 

[JohnMo]

I think the comment is true that the earth will increase in temperature, but the rate of rise and the ultimate temperature is determined by the insulation. More over the heat loss will decrease with a better u value.  So he really needs to go back to school.

 

U value is W/(m2⋅K), so to calculate heat from s given U value, m2⋅delta T⋅U value.

 

So for a U value of 1, you will loose 1W for each m2 for every degree temperature difference between the inside of the house and the ground.

 

So a couple of worked examples

 

100m2 floor area, ground 7 degrees, house 20 degrees.

100 X (20-7) X 1 =1300.

 

Heat loss 1300W.

 

U value 0.1 heat loss

100 X (20-7) X 0.1 = 230

 

Heat loss 130W.

 

Even if you said the ground was 15 (very unlikely)

 

U value 1 heat loss, 500W heat loss

0.1 is  50W heat loss.

 

[joe90]

12 hours ago, Adam2 said:

An interesting comment from my SE was that with a concrete slab, which is what we have, adding extra floor installation (we have 150 PIR) may not really make much difference as the earth around this will over time increase temp to close to slab temp, appreciate thoughts on that 

This was discussed on a other forum years ago, yes the Earth below a slab will stabilise in temp and IMO be above outside air temp, but this will also depend on insulation u value (thickness) as the heat flows down from the house. It was also discussed about ground water being able to wash the heat away in extreme cases. Mine is
a slab on 200mm PIR with UFH and never gets cold even with the heating off. 

[SteamyTea]

When it is cold outside, I always put the same thin jumper on, the one I wear at work when in is 40°C.

 

[Adam2]

Interesting, we're on a step hill with no real water in the ground as it is sandy/sandstone and free draining. Will be quite interesting to see if I can get a temp sensor somehow in the ground beneath our slab for some readings through the year.

[Gus Potter]

Here is a bit of background info / my thoughts.

 

In clay, for example soils, you can design a large heated square warehouse with only insulation under the slab around the edges that preforms quite well. As mentioned by others provided you don't have significant ground water flow then you can often treat the clay as being of infinite depth. Ground water does not tend to flow that well in clay, particularly London clay (was used to make dams) say but usually if you go deep enough you will find ground water and water flow.

 

Now BRE 443 indicates that the thermal conductivity of a clay soil with some hard core over the top is 1.5 W/m.K. In other words if you have a clay layer 1000mm thick then the U value of this layer is 1.5 W/m.K .. Compare this to a window which may have a u value of 1.4 W/m.K. The window is much thinner than a metre of clay but hopefully you get the jist of it. Just because something it is more conductive than say glass wool if you make it thick enough it will perform the same function. Say wattle and daub houses.. made of clay and straw.

 

In a domestic application the foot print of the building is often quite small so the heat leaks out the sides. That is why when you look up floor insulation tables they ask you for the perimeter / floor area ratio. The smaller the ratio = less heat leaking out the sides. A lot of the software we use relies on these principles.

 

For the mathematically minded the analysis / maths are founded on the principles developed by Laplace.. a very clever mathematician.

 

 

 

 

 

 

 

 

Edited June 12, 2022 by Gus Potter
ratio described incorrectly

[joe90]

1 hour ago, Gus Potter said:

a large heated square warehouse with only insulation under the slab around the edges that preforms quite well.

Interesting, the other forum I mentioned above also had members who advocated creating an insulation “skirt” horizontally around the outside of a new build to create a larger footprint to retain the heat within the ground. Not heard of it since 🤷‍♂️

[SteamyTea]

6 hours ago, Gus Potter said:

For the mathematically minded the analysis / maths are founded on the principles developed by Laplace.. a very clever mathematician.

An even cleverer one was Fourier.

[saveasteading]

On 11/06/2022 at 21:12, Adam2 said:

the earth around this will over time increase temp to close to slab temp,

Yes, but will still leak heat to the side. 

Gus is correct and this theory requires a very large area.

The ground at the perimeter will lose heat more quickly , simply as it is a shorter distance. So if you have  a big footprint then you might insulate the perimeter more than the centre.

Also you can add vertical insulation to the perimeter down to about 500mm as the ground is not very cold below that.

 

Slab economics in simplistic terms.

On flat and strong ground use ground-bearing.

On a slope or on poor ground use beam and block.

 

(Or if your selected builder can't do slabs well (and most can't)  then B and B is a pragmatic choice.

 

 

[Andehh]

For the OP - we have beam & block flooring, original specification to be BC compliant (u of 0.25) was around 100mm? 125mm?  PIR (which gave us around 0.18 I think) for our 240sqm build we paid around £2500 extra (I think) to bring it up to 150mm PIR dropping it to around 0.12. I doubt we'll ever save enough money to actually make that worth while in the next 10 years but couldn't resist.

 

Numbers are from memory, so might be slightly off - but the reality is when you start getting to mid teens the pay back/RoI doesn't make sense (though recognising this forum exists for  people looking at more then RoI) but it depends on your priorities for your home.

 

Also remember insulation is a law of diminishing returns, you cant double it up & expect 2x performance....

 

 

 

[SteamyTea]

12 minutes ago, Andehh said:

I doubt we'll ever save enough money to actually make that worth while in the next 10 years but couldn't resist.

Lets make an assumption that for 200 days a year there is a 10°C temperature difference, that will be, over 10 years:

 

48,000 [h] x 10 [K] x 240 [m²] x 0.25 [W.m^-2.K^-1] = 28,800 kWh

48,000 [h] x 10 [K] x 240 [m²] x 0.12 [W.m^-2.K^-1] = 13,824 kWh

 

28,800 - 13,824 = 14,976 kWh

 

Assume an energy price of 8p.kWh^-1

 

14,976 [kWh] x 0.08 [p.kWh^-1] = £1,198

 

So you are probably going to be quid's in.