Thermal properties, insulation and R values.

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Thermal properties, insulation and R values.

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Nick

Joined: 02 Nov 2004
Posts: 34535
Location: Hereford

Posted: Wed Jul 02, 14 9:23 am Post subject: Thermal properties, insulation and R values.
I can find lots of technical information on the web about how well concrete insulates, but I don't know how to use it. I do not understand R values, and such. I have a simple question. If I have a hot fire, say 700'C on a concrete slab, 5 inches thick, will it get hot below the slab. How hot? Regular concrete, nothing special.

sean
Downsizer Moderator

Joined: 28 Oct 2004
Posts: 42207
Location: North Devon

Posted: Wed Jul 02, 14 10:28 am Post subject:
How long will your fire be there?

Nick

Joined: 02 Nov 2004
Posts: 34535
Location: Hereford

Posted: Wed Jul 02, 14 10:35 am Post subject:
Dunno. 2 hours?

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Thu Jul 03, 14 6:45 am Post subject:

Here's a pretty good explanation of heat transfer.

Here's the relevant formula:

Rate of transfer (Watts) = k•A•(T1 - T2)/d

k is different for high and low density concrete (0.2 - 1.5)
A is the area (sq metres)
T1, T2 Inside and outside temp
d thickness (metres)

This will tell you how many watts come through the concrete and then you can use that to heat up whatever is on the other side (air?).

Simples

Nick

Joined: 02 Nov 2004
Posts: 34535
Location: Hereford

Posted: Thu Jul 03, 14 6:58 am Post subject:
Thanks. So, how hot is 2,500W ?

OtleyLad

Joined: 13 Jan 2007
Posts: 2737
Location: Otley, West Yorkshire

Posted: Thu Jul 03, 14 7:35 am Post subject:

Nick wrote:

Thanks.

So, how hot is 2,500W ?

Enough to raise the temp of 2.5 litres of water by one degree C in one second. So it could be boiling in less than 2 minutes.

Wood has a much lower specific heat than water so in theory it could reach burning temp (232C) in a similar time (if it were pressed up close to the concrete).

Nick

Joined: 02 Nov 2004
Posts: 34535
Location: Hereford

Posted: Thu Jul 03, 14 8:13 am Post subject:
Thank you. A clear and helpful answer.

onemanband

Joined: 26 Dec 2010
Posts: 1473
Location: NCA90

Posted: Thu Jul 03, 14 12:21 pm Post subject:
Building regs for a fireplace hearth are min 125mm thick concrete with a 50mm air gap(to combustable material), or 240mm with no air gap. So yeh 5 inches of concrete slab probably will get hot on the underside.

Nick

Joined: 02 Nov 2004
Posts: 34535
Location: Hereford

Posted: Thu Jul 03, 14 12:30 pm Post subject:
Well, watch this space with interest. I hope all will be well. Hope, of course, is not a strategy.

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