Green Building Forum - Measuring the temperature under my solid floor

Measuring the temperature under my solid floor

tony — Wed, 12 Nov 2008 17:25:42 +0000

I decided to measure the temperature under my solid uninsulated floor

So far at 250mm down it is 18.75 C and at 450 it is constant 18.5

I am doing a run at 750mm deep over the next 48 hours

With this very slow temperature gradient the rate of heat flow must be low I think and therefore the heat loss much smaller than generally accepted.

Measuring the temperature under my solid floor

JackyR — Wed, 12 Nov 2008 17:29:18 +0000

Would be interesting to do this for a whole year.

Measuring the temperature under my solid floor

TheDoctor — Wed, 12 Nov 2008 17:42:00 +0000

its all about your perimeter over area calculation

very little loss over a warehouse, say, but huge loss from the slab of a garden privvy.

what is the footprint of the building you are measuring?

Measuring the temperature under my solid floor

Paul in Montreal — Wed, 12 Nov 2008 17:46:59 +0000

What's the air temperature in the room above the floor out of interest?

Paul in Montreal.

Measuring the temperature under my solid floor

tony — Wed, 12 Nov 2008 18:48:44 +0000

20 C floor area is 80 m^2 and my probe is almost in the middle.

Jacky -- you give me a year and I ll do that.

Measuring the temperature under my solid floor

Paul in Montreal — Wed, 12 Nov 2008 19:00:59 +0000

So you can work out the effective U (or R) value of your uninsulated floor. According to the chart below:

http://www.professionalequipment.com/content/how-to-determine-r-value.asp

It's not looking too good - about R7 in imperial units.

Paul in Montreal.

Measuring the temperature under my solid floor

CWatters — Wed, 12 Nov 2008 19:31:12 +0000

Posted By: tony
With this very slow temperature gradient the rate of heat flow must be low I think and therefore the heat loss much smaller than generally accepted.

Other way isn't it? A low gradient implies good conductivity. A perfect conductor has zero thermal gradient.

Measuring the temperature under my solid floor

CWatters — Wed, 12 Nov 2008 20:21:27 +0000

This sketch might help. Shows the applicable equations.

It assumes the heat flow is uniform (eg that it doesn't spread out).

Four equations and four unknows. Needs solving for P. Then put P into the first equation to give R1. Note that gives the thermal resistance between T1 and T2 which may include a mix of materials depending on where T2 is measured. Hopefully the equations are independant I've not checked.

Measuring the temperature under my solid floor

mike7 — Wed, 12 Nov 2008 21:32:57 +0000

Posted By: CWatters
Other way isn't it? A low gradient implies good conductivity. A perfect conductor has zero thermal gradient.

It would do, but for a given rate of heat flow. Tony is right because the heat flow is determined by the temperature gradient, not the other way about. ...plus of course the conductivity of the material under his floor, and the floor area. The gradient, as the good Doctor says, is likely much higher near the outside walls.

Measuring the temperature under my solid floor

tony — Wed, 12 Nov 2008 22:16:13 +0000

And we know that the heat losses are much greater there too.

Measuring the temperature under my solid floor

Saint — Thu, 13 Nov 2008 08:23:11 +0000

Tony,
18 and 18.5 degC seem surprisingly high at those depths. Isn't average soil temp in the UK around 10 - 12 DegC?
That means you're heating a lot of subsoil. I know you're not an advocate of excessive insulation beneath slabs but here you're surely proving the case for it?
The 750mm depth result will be interesting.
Coincidentally 750mm is the recommended depth for burying water pipes to avoid ground frost damage

Measuring the temperature under my solid floor

tony — Thu, 13 Nov 2008 08:46:16 +0000

750 = 18.15

The assumption that the soil temperature is 10 - 12 is wrong under my house

The soil temperature will be the average air temperature a little way down.

Under a floor it will be average room temperature. This will be true with insulated floors too.

I am dying to measure the temperature under an insulated floor now.

It will be 13 C a lot further down but heat loss depends on temperature gradient and thermal resistance. ( between the two steady state points )

Measuring the temperature under my solid floor

CWatters — Thu, 13 Nov 2008 12:21:42 +0000

What's the temperature 750mm down in the garden? For that matter what's the outside air temperature?

Measuring the temperature under my solid floor

CWatters — Thu, 13 Nov 2008 12:32:50 +0000

deleted

Measuring the temperature under my solid floor

Mark Siddall — Thu, 13 Nov 2008 13:43:44 +0000

It may be looking dated but I'd have a look at "The thermal performance of ground floor slabs - a full scale in-situ experiment" by Thomas and Rees 1998 (from the Geo-environmental Research Centre, Cardiff) in Building and Environment. Though the technology may have moved on a little I imagine that this will give you many of the pointers that you need.

Mark

Measuring the temperature under my solid floor

tony — Thu, 13 Nov 2008 17:11:49 +0000

Can you give us a link to see that paper please Mark?

Measuring the temperature under my solid floor

fostertom — Thu, 13 Nov 2008 18:38:12 +0000

Posted By: Saintyou're heating a lot of subsoil. I know you're not an advocate of excessive insulation beneath slabs but here you're surely proving the case for it?

tony's aim is to heat the subsoil, until it's 'filled up' - isn't that right tony? The point is, do these measurements support the hope that it's not dissipating continuously but, having spent a few years 'filling up' is now losing little?

Measuring the temperature under my solid floor

Mark Siddall — Thu, 13 Nov 2008 18:38:44 +0000

Tony,
Here you go (apologies for the length of the link): http://www.sciencedirect.com/science/journal/03601323
Cut and paste the title above into the sites search engine

Mark

Measuring the temperature under my solid floor

mike7 — Thu, 13 Nov 2008 20:12:06 +0000

Tony -
To determine the heat flow you'll be needing some idea of the thermal conductivity of the ground that you are measuring the temp difference across.The table of properties below may help. Note the big differences in conductivity for wetter ground - so heat loss near outer walls may be high not just because of the nearness of the outside temp, but also due to the greater conduction if, as I suppose is likely, the ground is damper here than at the centre. Interesting project - all power to your elbow.

Soil Properties (from GS2000) (hope the tabulation survives)

Material.....water......Conductivity....Diffusivity....... Vol Sp Heat
................ %............W/moC ..........mm/s......... Ws/m3.oC x106

Sand dry...... 2..............0.60...........0.42................1.4
.....damp.....7..............1.50...........0.72 ................2.1
...saturated..12.............2.20...........0.72.................3.1
Sandy soil.....2..............1.30...........0.48.................2.7
.................5..............2.50...........0.84.................2.9
.................8..............2.8............1.10.................2.5
Silty loam.....2..............0.60...........0.38.................1.6
................10.............1.30...........0.56..................2.3
................20.............2.3............0.68..................3.4
Silty clay/clay 2.............0.40...........0.33..................1.2
.................10............0.80...........0.48..................1.7
.................20 ............1.10...........0.57..................1.9
Basalt -lwr quartile 1.20 0.60 2.0
- mean 1.60 0.71 2.3
Granite lwr quartile 2.80 0.81 3.5
- mean 3.40 1.28 2.6
Limestone - lwr 2.20 0.98 3.5
- mean 3.40 1.28 2.6
Shale â€“ lwr qtl 1.60 0.49 3.3
- mean 2.40 0.74 3.2

Water.......100..............0.60...........0.14................4.2

Measuring the temperature under my solid floor

fostertom — Thu, 13 Nov 2008 20:23:34 +0000

tony, 80m2, but what length to width ratio? - makes big difference.

Measuring the temperature under my solid floor

CWatters — Thu, 13 Nov 2008 20:24:41 +0000

Posted By: fostertom
Posted By: Saintyou're heating a lot of subsoil. I know you're not an advocate of excessive insulation beneath slabs but here you're surely proving the case for it?
tony's aim is to heat the subsoil, until it's 'filled up' - isn't that right tony? The point is, do these measurements support the hope that it's not dissipating continuously but, having spent a few years 'filling up' is now losing little?

Humm. I've not heard of a way to change the thermal conductivity of soil by heating it. Not unless you plan to boil off any water it might contain :-)

Measuring the temperature under my solid floor

mike7 — Thu, 13 Nov 2008 20:56:44 +0000

Posted By: Paul in MontrealSo you can work out the effective U (or R) value of your uninsulated floor. According to the chart below:

http://www.professionalequipment.com/content/how-to-determine-r-value.asp" >http://www.professionalequipment.com/content/how-to-determine-r-value.asp

It's not looking too good - about R7 in imperial units.

Paul in Montreal.

This method is for a wall isn't it? The boundary layer/contact resistance if that's the right term might be quite different for a floor, I'd guess.

Measuring the temperature under my solid floor

Paul in Montreal — Thu, 13 Nov 2008 21:08:24 +0000

Posted By: mike7This method is for a wall isn't it? The boundary layer/contact resistance if that's the right term might be quite different for a floor, I'd guess

It is for a wall but there's no reason it shouldn't work for a floor. The "outside" temperature is simply the deep ground temperature (i.e. equal to the annual average air temp). It should be a good first order approximation of the effective resistance of the ground. We used the IR thermometer trick to build the models for hot2000 of my old house - it had been about -12C outside for a couple of days so we felt it would be reasonably accurate.

Paul in Montreal.

Measuring the temperature under my solid floor

tony — Thu, 13 Nov 2008 21:58:49 +0000

I would say that the soil under there ,s about 0.6 W/moC -- shortest heat path from centre of floor to outside= 5m so U value = 0.12 with no insulation

But the heat loss is a sum of all the heat paths not the shortest one and --- it all quite complex and bigger heat losses from edges- shape dependant too.

Tom, re charging up well yes but once charged up ( = steady state ) then the ammount of heat flow to keep it charged = heat loss = smaller than all calcs give.

Measuring the temperature under my solid floor

mike7 — Fri, 14 Nov 2008 00:09:18 +0000

Posted By: Paul in Montreal
Posted By: mike7This method is for a wall isn't it? The boundary layer/contact resistance if that's the right term might be quite different for a floor, I'd guess

It is for a wall but there's no reason it shouldn't work for a floor. The "outside" temperature is simply the deep ground temperature (i.e. equal to the annual average air temp). It should be a good first order approximation of the effective resistance of the ground. We used the IR thermometer trick to build the models for hot2000 of my old house - it had been about -12C outside for a couple of days so we felt it would be reasonably accurate.

Paul in Montreal.

I'm thinking that a static layer of air cooler than the general 'ambient' would likely be present over a floor, thicker than the boundary layer against a wall - which would be limited by convection currents. I recon this would skew the result and give a worse picture than actual.

Measuring the temperature under my solid floor

fostertom — Fri, 14 Nov 2008 00:30:37 +0000

Posted By: CWattersI've not heard of a way to change the thermal conductivity of soil by heating it

Didn't mean that - not sure how you got that from what I said?

Measuring the temperature under my solid floor

fostertom — Fri, 14 Nov 2008 00:42:13 +0000

Posted By: Paul in MontrealThe "outside" temperature is simply the deep ground temperature (i.e. equal to the annual average air temp)

But that's modified after pouring heat into the ground for a while. the temp gradient between inside and that primeval deep ground temp gets flatter in gradient as it gets longer in distance, so heat flow diminishes year on year, never quite reaching steady state. It's quite wrong to see the ground as maintaining steady "equal to the annual average air temp" whatever you throw at it, quite wrong to see the ground as a perpetual source of heat loss. tony's right:

Posted By: tonyonce charged up ( = steady state ) then the ammount of heat flow to keep it charged = heat loss = smaller than all calcs give.

except as I say it never quite reaches steady state. All the above ignoring things like groundwater flow.

Measuring the temperature under my solid floor

Mike George — Fri, 14 Nov 2008 00:54:21 +0000

Paul, Do you know if is possible to isolate heat loss data in Hot 3000? Say by comparing the same models only one with and one without inulation?

I've done this in Tas and my findings [though roundly criticised by some] reinforce Tony and Tom's opinions

Measuring the temperature under my solid floor

Paul in Montreal — Fri, 14 Nov 2008 02:00:22 +0000

Posted By: mike7I'm thinking that a static layer of air cooler than the general 'ambient' would likely be present over a floor, thicker than the boundary layer against a wall - which would be limited by convection currents. I recon this would skew the result and give a worse picture than actual.

Maybe - I guess Tony should measure the air temperature just above the floor - also depends how the room is heated as to how much stratification there is.

Posted By: fostertomIt's quite wrong to see the ground as maintaining steady "equal to the annual average air temp" whatever you throw at it, quite wrong to see the ground as a perpetual source of heat loss. tony's right:

I don't disagree with you - but it's probably a close enough approximation for now to calculate the heat flow into the floor. After all, he is measuring a temperature gradient. It might be interesting to measure the temperature of the ground at similar depths that aren't under the floor slab. You're right about the ground not being a perpetual source of heat loss (or gain for that matter) - that's why it's at the annual average air temperature for those places it's not covered by buildings etc. - that's the equilibrium temperature it has come to over a long period of time. I think what could be interesting here is to determine how much umbrella insulation is required to effectively isolate the ground under a house and make it into a usable store (without requiring insulation under it). A lot depends on the water table and transport through the area under the house. I believe this is one of the reasons the Drakes Landing Solar Community is successful - the ground is dry and so the area losses around the edge of the store are small compared to the volume (plus the top is insulated).

Posted By: Mike GeorgePaul, Do you know if is possible to isolate heat loss data in Hot 3000? Say by comparing the same models only one with and one without inulation?

Absolutely. There's a mode where you can calculate the efficacy of an "upgrade" (e.g. basement insulation) in a model. Actually, I've only done this in hot2000, not hot3000 but I have no reason to believe it's any different. Of course, it really depends on how realistic the modelling is. At least test houses have been built to try and validate some of the modelling performed. I'm not sure what was done in the area of slab insulation versus umbrella insulation versus no insulation at all in real test buildings - but there are models for all these cases. From experiments I've performed, underslab insulation doesn't make as much difference as umbrella insulation.

Paul in Montreal.

Measuring the temperature under my solid floor

mike7 — Fri, 14 Nov 2008 02:38:58 +0000

Posted By: tonyI would say that the soil under there ,s about 0.6 W/moC -- shortest heat path from centre of floor to outside= 5m so U value = 0.12 with no insulation

But the heat loss is a sum of all the heat paths not the shortest one and --- it all quite complex and bigger heat losses from edges- shape dependant too.

Perhaps as an aid to thinking about all this, I offer the sketch below. It is somewhat of an artist's impression, but not entirely so as there are some restraints I've tried to include, such as the need for isotherms and streamlines to intersect at right angles. I guessed the wall to measurement distance at 3m (from your figures, Tony, the house must be almost round?) and 18.0degC at 0.9 metres. As the isotherms get wider apart, the streamlines must also separate further, so that the lengths between intersections are similar, making a sort of distorted square. It assumes the conductivity is uniform.