question about ground floor slab

chillit

Hi

I raised this on another thread but it got lost in the conversation.

My ground floor slab spec has the following makeup. Hardcore, sand, radon barrier, 150mm insulation, plastic membrane, 150mm concrete slab, floor coverings.

From reading here and elsewhere this now seems to be a more old style approach. A lot of people now refer to the following makeup

Hardcore, sand, radon barrier, concrete slab (various thickness but generally 150mm), insulation (various thickness), plastic membrane, thinner approx 60-70mm concrete slab/screed, floor coverings.

Can I ask what are the advantages of building the floor in the second way ?

I am guessing one advantage would be if using under floor heating you will have a quicker response time with a smaller top screed. Are there any other considerations?

Would there be any advantage to switching the approach?

Johnnyhpipe

Nothing wrong with your approach if you're not using u/f heating. Its a traditional tried and tested detail and is still used every day of the week.

Some people might also use option 2 for an easier level (self levelling screeds etc) but its not essential.

If you dont have u/f heating then personally I wouldnt bother with the cost of changing buildup and paying for an additional unnecessary screed.

Calahonda52

OP:
whats the perimeter insulation detail for your current spec?
Is the insulation going down in one layer or two with joints taped and staggered?
Make sure what you specify goes down as I have had situations where the builder has only one rigid insulation thickness on site, maybe 75 or 100, so this gets used in the floor instead of 150 and they just added more concrete.


The second design has much lower thermal mass, regardless of U/F.
The second method, having more steps is more error prone, especially in relation to getting FFL correct

The first design makes it easier for external air supplies to stoves to be fitted above the insulation layer and in theory reduces the risk of condensation/ damp in the pipe.
Ditto for bringing services to Island units in kitchens...

chillit

Johnnyhpipe wrote: »

Nothing wrong with your approach if you're not using u/f heating. Its a traditional tried and tested detail and is still used every day of the week.

Some people might also use option 2 for an easier level (self levelling screeds etc) but its not essential.

If you dont have u/f heating then personally I wouldnt bother with the cost of changing buildup and paying for an additional unnecessary screed.

we haven't quite decided about ufh yet but we think we will probably go with standard rads as the heat source is oil so not ideally suited to ufh

thanks for the comment

chillit

Calahonda52 wrote: »

OP:
whats the perimeter insulation detail for your current spec?

there is 25mm perimeter insulation listed but I think I might put 50mm based on recent comments I have read. There are thermal blocks (roadstone or quinlite to be decided) in the rising wall at the section level with the insulation and slab

Is the insulation going down in one layer or two with joints taped and staggered?

This is a self build so this will depend on the price at the time. might be 150mm or 100mm plus 50mm. Taping hasn't been mentioned but if this is to stop concrete pushing between the gaps then I thought that the plastic sheet above the insulation would do this. Is there another reason for the taping ?

Make sure what you specify goes down as I have had situations where the builder has only one rigid insulation thickness on site, maybe 75 or 100, so this gets used in the floor instead of 150 and they just added more concrete.


The second design has much lower thermal mass, regardless of U/F.
The second method, having more steps is more error prone, especially in relation to getting FFL correct

The first design makes it easier for external air supplies to stoves to be fitted above the insulation layer and in theory reduces the risk of condensation/ damp in the pipe.
Ditto for bringing services to Island units in kitchens...

Thanks for the comments

Calahonda52

Fair enough, I didn't see the plastic layer on top of the rigid, normally I see the UF pipes pinned directly to the rigid.
Twin layers/ taping helps to keep the insulation from kicking up when walked on,
it is also easier to fill the inevitable gaps that will arise its all fine in theory on a drawing but on site its a different world.
There is less work with single layer and it may be be cheaper but may end up with more waste.
Swings and roundabouts.
Where the taping/ twin layers does make a big difference is when trying to get good airtightness in walls and roofs.

chillit

thanks calahonda. very useful information