Floor slab insulation - above or below?

[biryani_boy]

Our extension project is progressing nicely and building control is super happy with the drains and the foundation. We’re laying bricks up to DPC and the idea is to pour the slab ewrly next week.

I’ve been given two options by my builder:

1. Compacted aggregate, sand, DPM, insulation, reinforcement, slab

Or

2. Compacted aggregate, sand, DPM, reinforcement, slab, insulation, screed

Which one of the two ahould I go for if I’m looking for more energy efficiency?

1950s house, brick cavity wall, cavity insulation (little white balls), new central heating, double glazing overall, solid floor (concrete).

The extension is brick and breeze block construction with solid insulation panels (celotex).

Heating on all winter, on a constant temperature. What are the pros and cons of each option?

Thanks!

 

[RonnyRaygun]

Doesn't really matter as far as energy efficiency goes. If you want better thermal efficiency put in more insulation.

Either way, make sure there is 25mm insulation around the external perimeter of the slab / screed as that will prevent the thermal bridge to the masonry.

A concrete slab will either need to be power floated or levelled with self levelling compound before finishes go on.

A thicker concrete slab will have higher thermal mass than the screed which can help to stabilise temperatures, especially if using UFH.

 

[biryani_boy]

Got it. No UFH and the existing slab is not insulated, still warm enough, so we’ll be ok either way.

Are there other considerations when deciding which layer to lay first?

 

[^woody^]

Only insulate above if there is underfloor heating.

 

[biryani_boy]

Got it, so insulation first, slab on top. Thanks!

 

[mordzy]

With the slab above the insulation you get better thermal mass energy storage. Ie when the sun shines and warms the slab, it radiates that heat back into the room at night thus keeping the room at a nice even temp.

I guess it depends what sort of room it is and which way it faces. But in my mind free heating is always good.

 

[biryani_boy]

It’s a kitchen extension, roughly 24 square meters, west facing, so a lot of sun from around 11am until sunset. Mainly used as an entertainment space, we spend 90% of the time in this room.

 

[mordzy]

If it was mine, I’d go with the option that gives you the most thermal mass. You will have less temperature fluctuations and enjoy that heat collected through the day radiated back into the room during the night. Even during winter it will be noticeable.

Are you having the roofline overhanging? In Australia they do this to reduce the energy collected by the slab during summer months (like a shade) and when the sun is lower in winter the slab will collect the suns energy. Doesn’t matter so much in the uk but I thought it might help you understand a bit.

 

[Mr Chibs]

I've got insulation above the slab, and similar facing room... it holds heat well.