Floor insulation - under or over

[londonman]

...the concrete slab?

Final finish will be oak floorboards onto battens. One suggestion is to put the insulation on top of the concrete slab and in between the battens. Seems to me like a good idea especially as I could also use the space to route pipes etc. and it will save a smidgeon on the amount of excavation required.

My book on Building Regs says that 'there is no compulsion to install floor insulation' and so I thought I'd limit my choice to 65mm Kingspan.

The build is the renovation of an old cottage and is replacing a suspended timber floor.

Comments very welcome.

 

[JohnD]

what is the purpose of the battens you mention?

 

[freddiemercurystwin]

Over the slab is always better, that way you're not wasting energy trying to keep your slab warm too.

When was your building regs book written? 1975? I'd recommend you get a new book.

You'd need approx. 80mm Celotex to comply although it sounds you're not doing it the legal way?

 

[londonman]

what is the purpose of the battens you mention?

To fix the oak flooring using secret nailing

When was your building regs book written? 1975? I'd recommend you get a new book.

It's "The Building Regulations Explained and Illustrated 13th edition 2007"

Page 8.41 says 'If required, insulation may also be incorporated'....but I can now see that later on in the book it does talk about an improved thermal element...my fault for jumping in!

So, applying the principle of a little knowledge is a dangerous thing....working out the perimeter/sqm ratio (perimeter 34m and area 58.5 m2...the ratio is 0.58 ) and looking up the tables, 65mm of Kingspan gives me a u-value of 0.22 which is below the target value of 0.25. That's how I got to the figure.

 

[JohnD]

you could lay it on top of the insulation, glued, floating, no need for battens.

 

[freddiemercurystwin]

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADL1B_2006.pdf

Page 24 (of the document) 51, its not optional!

 

[londonman]

Since anything I do to the property will be an improvement in terms of insulation (ie it has none), it does seem to me that the requirements are ridiculously onerous...to the point where one is inclined to say 'stuff it - I'm leaving the floor/building as it is' !!

 

[Nige F]

hear, hear

 

[freddiemercurystwin]

True, but to be fair, I was responding to the claim that you actually did'nt need to use any insulation. The regs are also there to give peace of mind to a future purchaser and the confirmation that their house has been built to high standards and someone hasn't just plastered over the cracks, as it were.

 

[Deluks]

There's no reason to grump about insulation requirements. It's one of the few regs that's actually beneficial to the homeowner. Whether you care about your 'carbon footprint' or not, insulation will stop you from burning cash in the winter with high heating bills. Insulation is relatively cheap, seems like common sense to me.

 

[DenzB]

Insulation under the slab is preferable because the concrete will increase the thermal mass of the building, making for a more controllable heating/cooling cycle.

 

[noseall]

we always fit the inso' under the concrete slab. i would be quite nervous of only having a brittle screed between my floor and the inso'.

 

[londonman]

True, but to be fair, I was responding to the claim that you actually did'nt need to use any insulation. The regs are also there to give peace of mind to a future purchaser and the confirmation that their house has been built to high standards and someone hasn't just plastered over the cracks, as it were.

Understood and apologies if my post came across in the wrong way. I agree with your and others comments later on in the thread and agree that insulation is a good thing. However, I guess that my concern stems from the fact that much of what is written relates to a 'new build' in which case it is relatively easy to implement all the various aspects of insulation.

But if you take the case of my property the rafters (made of oak) are about 80mm thick. The rooms are small. Previously there was no insulation between the lath and plaster and the sarking felt. Following the regs to the letter, my impression is that there needs to be an airgap of 50mm between any insulation and the underside of the felt? So that would leave me with 30mm of insulation before I start to encroach in the (already) small room.

If you then start talking about thermal bridging of the rafters (and does anyone know the relative thermal resistance of oak compared to softwood?) I get the impression that the BCO would like me to fix 125mm of Kingspan across the whole of the underside of the rafters.

Is that 'reasonable'?

 

[londonman]

Insulation under the slab is preferable because the concrete will increase the thermal mass of the building, making for a more controllable heating/cooling cycle.

I can see the logic of your argument and then I read elsewhere the comment 'put it on top of the concrete slab...why heat up the slab'.

So I'm confused.

 

[londonman]

we always fit the inso' under the concrete slab. i would be quite nervous of only having a brittle screed between my floor and the inso'.

No screed as the battens will fit straight onto the concrete slab, the oak flooring then going on top of those.