Insulation under concrete slab

[Griff Boy]

Hi guys,

I'm having a couple of builders build a base for a conservatory at the moment. Foundations done, and backfilled in type 1 and sand above. Question is I got home from work to see that the slabs of celotex insulation were under the dpm directly on the sand, which the slab part laid ontop of the dpm.

Ie order from ground up:

1 type 1
2 sand
3 insulation sheets
4 dpm
5 4" concrete slab

Is this correct or should the insulation be under the dpm? I asked the builder and they said it doesn't matter, but logic to me says it should be on top of the dpm?

Any help appreciated!

[dxg]

It shouldn't matter as it's a closed cell insulation, but personally I'd prefer the insulation to be on the dry side of the dpm.

[Griff Boy]

dxg said:

It shouldn't matter as it's a closed cell insulation, but personally I'd prefer the insulation to be on the dry side of the dpm.

would you prefer it enough to ask them to dig up 1/2 the slab to change it?

They are good lads, I just want to make sure it's done properly!

[roofer]

Surface condensation is unlikely to occur on the floor surface if insulation is situated over the concrete slab. If insulation is situated below the slab, surface condensation can occur due to the slabs slow thermal response. This would normally be considered nuisance condensation and regular heating is recommended. Interstitial condensation can occur on the upper slab surface where the insulation is over the slab. If considered harmful, an adequate vapour control layer should be provided on the warm side of the insulation. Interstitial condensation is unlikely to occur if the insulation is installed beneath the concrete slab with an adequate vapour control layer which also acts as a damp-proof membrane.

Copy/Paste but correct.

[RWA441]

When we did our self build the spec from the architect was -
1. Type 1
2. Sand
3. DPM
4. 100mm insulation
5. DPM
6. Concrete

[albundy89]

Nice explanation roofer but did anyone actually understand it.

[Griff Boy]

The impression I'm getting is that it probably should have had a second Dpm under the insulation. This is a porch conservatory that they have built up the base foundations so that floor level is over almost 800mm of block laid on the flat. Being a conservatory / porch, it's situated in an internal corner section of the house, i.e. they are building an "L" shape wall to create the rectangular base. Dims are 4.8x2.5m

As they have offered to come back today to finish it (rain slowed them down yesterday) I need to know if I'm opening myself up for potential future issues by letting them carry on, or is it ok to have it done this way?

I don't know if it's relevant, but the floor will have electric underfloor heating and will be tiled. It's a separate room to the house, and sits lower that internal floor levels.

The builder assures me that it can be done either way, as it's a separate and closed floor?
Cheers
Ian

[Busa mav]

I struggle to see a reason why you would expose a product to the element when there is a simple option not too. There is no extra work involved to protect the boards.

When I spec floor insulation it is just about always above the slab .

Have a look at manufacturers web site and see where they recomend it be located.

Celotex show it positioned above the slab .

[dickymint]

^^^^ What Busa said.

Especially as you're having underfloor heating. By having the insulation under the slab you have to get the concrete up to temp before you feel any warmth. Fit insulation above the slab let alone above the DPC. I am assuming you will be screeding the floor?

I guess if the heating is on 24/7 it would be ok (ish) your way.

Edited to say - Kingspan also say insulation over slab.

Edited by dickymint on Sunday 11th September 08:55

[Griff Boy]

Hi guys,
Thanks for the replies.

The idea was to use the mat based system laid directly onto the slab bedded into the tile adhesive and covered with ceramic tiles directly. It is an external porch, not an internal part of the house. And to be honest heating it is just a luxury that I may not even use day to day?

My main concern is whether I am opening myself up to any future issues / problems by not having a Dpm under the insulation boards. I know that there may be more efficient ways of setting it out, by my main concern this second is whether or not the order of the installation will cause me practical issues in the future?

Thanks again for the good advice though.

[mrmaggit]

If you're concerned about efficiency, don't go with mat insulation, use wet instead.

ie plumbed hot water not electric.

[Busa mav]

dickymint said:

I guess if the heating is on 24/7

He will only do that until he gets the first quarterly bill

Edited by Busa mav on Sunday 11th September 15:55

[Griff Boy]

Ha ha, heating def won't be on permantly! As I said before it's a big porch, I'm not planning to sit in it, I've got a separate conservatory for that!

Hearings only a back up to stop it getting too cold in winter when full of wet coats etc. Besides it's a warmup 3ie controlled floor, so pretty energy efficient.