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Slab Temperature in Canberra for Summer?

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Started 8 years ago by nhand42
Latest reply from Catopsilia

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nhand42
Member

Hi, long-term lurker, recent poster. I've just finished building a solar passive house and I've been collecting temperature data through this Canberra summer. I've got some questions about slab temperatures.

It's a raft slab on ground with edge insulation. Approx 150sqm with hydronic tubing throughout. Wide eaves means no direct sunlight on the slab except 3sqm near the laundry between 5-8pm. R2 walls, R5 roof, low-e double glazing. Without active cooling the slab temperature peaks at 27 degrees around 9pm, and cools down to 24 degrees by 9am. The inside air temperature varies from 22 degrees to 30 degrees over the same cycle.

This isn't what I expected from the books I'd read. I thought the thermal coupling with the ground meant the slab would average much lower, less than 20 degrees. The house gets uncomfortably warm in the evenings. I think the higher starting temperature of the slab is contributing to that.

I'm using cross-breezes and night-purges but the slab is stubbornly staying warm. Does anybody in Canberra have a similar slab and data they could share? I'd like to know if 24 - 27 is the expected temperature range for January. It could just be I have the wrong expectation.

Posted Friday 17 Jan 2014 @ 10:16:02 pm from IP #
Morbo
Member

I guess I stick my head out id no one does.
The temperature should be the average shade temperature. 20 deg sounds a bit low for January.

Posted Friday 17 Jan 2014 @ 11:09:29 pm from IP #
8688john
Member

A raft slab has limited thermal coupling to the ground below, pretty much limited to direct thermal contact at the edge and internal ribs. Your slab edge insulation also contributes to isolating your slab (thermal mass) from the environment. The movement in slab temperature you are observing is consistent with the slab being warmed and cooled by the air within the house, day and night.

In winter, the lack of thermal coupling may work more to your advantage, making it easier to maintain a slab temperature above that of the ground. Swings & roundabouts, maybe?

Posted Friday 17 Jan 2014 @ 11:19:17 pm from IP #
nhand42
Member

Thanks Morbo and John. That helps explain the observed temperatures. I guess my expectation of 20 degrees for the slab was just unrealistic. That also explains a discussion with the architect. He modelled the EER software using under-slab insulation and I said we're only doing edge-slab insulation. His reply was for modelling purposes in Canberra they're identical. I didn't quite understand that at the time, but it makes more sense now.

Because the slab isn't influencing the air temperature so much as being influenced *by* the air temperature, should I be focussing on shade and breeze paths? Apart from closing curtains during the day and opening windows at night, which I'm already doing, what's a good strategy?

Posted Friday 17 Jan 2014 @ 11:48:28 pm from IP #
Catopsilia
Member

nhand42
I don't have answers. However, I used to live in Canberra, and I notice that the climate there is very similar to that here in Manilla, except that all temperatures are three degrees higher here. So, yes, the January indoor temperatures that you observe are higher than I would expect.
My house design and management seems to be similar (but no hydronics), so my 15-year observations should be relevant. I have posted many in the thread "Low Energy Houses"
http://www.ata.org.au/forums/topic/3
You say your slab has "edge insulation", but true edge insulation generally fails termite protection rules. My slab is not actually insulated along its 150 mm edge, and a lot of heat is gained or lost there each day. However, I have insulation extending vertically about 500 mm down from the slab just inside the brick perimeter footings.
This insulation effectively isolates the under-slab mass from the surrounding subsoil. Thermometers at 750 mm depth show:
Summer (DJF): under garden 25.5, under slab 22.2;
Winter (JJA): under garden 14.3, under slab 19.0,
Winter-Summer range: under garden 9.2, under slab 3.2.

Posted Saturday 18 Jan 2014 @ 12:11:57 am from IP #
Adamc73
Member

Why can't you run cool water threw the hydronic tubing?

Posted Saturday 18 Jan 2014 @ 12:30:34 am from IP #
nhand42
Member

Catopsilia said:
My house design and management seems to be similar (but no hydronics), so my 15-year observations should be relevant. I have posted many in the thread "Low Energy Houses"
http://www.ata.org.au/forums/topic/3

Thanks Catopsilia. Your Manilla House was one of the inspirations for my build. The data you've collected and your posts here and on weatherzone have been invaluable.

So, yes, the January indoor temperatures that you observe are higher than I would expect.

That's what I thought. One problem is I don't have enough data; I'm manually reading the numbers from a cheap digital thermometer in the living room, then typing those numbers into a spreadsheet. And the living room is by far the hottest room due to an unshaded western wall copping full afternoon sun. The bedrooms are more comfortable.

I've bought a bunch of 1-wire thermometers and my goal is to automate the data collection for the main living areas and bedrooms. With your thermometers under the slab, did you retrofit those? I didn't think about thermometers before the pour and I can't see an easy way to retrofit them now.

Posted Saturday 18 Jan 2014 @ 1:06:23 am from IP #
nhand42
Member

Adamc73 said:
Why can't you run cool water threw the hydronic tubing?

The hydronic is a sealed-loop and pressurised. I know enough about the hydronic to be dangerous, but not enough to be confident making changes.

Posted Saturday 18 Jan 2014 @ 1:31:16 am from IP #
Benny
Member

So when you report slab temperature - are you measuring by just putting a thermometer on the surface of the slab ? That will be pretty close to the room temperature I would think, unless you're putting some insulation over the thermometer. Can you just drill a hole in it somewhere and put the thermometer deeper ?
I agree with others that it looks like the load from the heat ingress (through walls etc) is too high and the slab can't absorb it all to keep temps low. An infra-red camera is invaluable for measuring internal surface temps and identifying where the heat is getting in - or a remote IR thermometer to measure that W wall inside temp.
Any gaps in the ceiling insulation for downlights ?

Posted Saturday 18 Jan 2014 @ 1:33:17 am from IP #
nhand42
Member

Benny said:
So when you report slab temperature - are you measuring by just putting a thermometer on the surface of the slab ? That will be pretty close to the room temperature I would think, unless you're putting some insulation over the thermometer. Can you just drill a hole in it somewhere and put the thermometer deeper ?

Yes, it's just the wired sensor from the digital thermometer. I've got it sitting on the slab and held down with a sand bag, relying on conduction for the reading. I might be able to drill a hole but need to be mindful of the hydronic pipes.

I agree with others that it looks like the load from the heat ingress (through walls etc) is too high and the slab can't absorb it all to keep temps low. An infra-red camera is invaluable for measuring internal surface temps and identifying where the heat is getting in - or a remote IR thermometer to measure that W wall inside temp.
Any gaps in the ceiling insulation for downlights ?

There are downlights in the wet areas but no gaps in the insulation, afaik. Insulation and draught blockage seem to have been done fairly well. The glazing and aluminium frames are probably the greatest source of heat ingress. The living room has too much glass; nearly 20sqm; and even the best double-glazing is inferior to the worst wall-insulation.

Posted Saturday 18 Jan 2014 @ 3:05:16 am from IP #
Catopsilia
Member

About thermometers:

Subsoil
The two subsoil thermometer installations were done in advance. In fact, the under-slab one has the longest record in my data - from 15/12/98. Plastic conduit was inserted vertically in dug holes. The sensors were Platinum Resistance Probes, and the under-slab one is still working. The one in the garden had a lead about 20 metres long, and the readings became less believable with time. I replaced the sensor with a cheap one (thermistor?) with a radio link to a receiver in the upstairs bedroom. I have the conduits filled with sand, which I can remove with a vacuum-cleaner if necessary.

Slab-surface
I don't think much care needs to be taken with the thermometer on the surface of the slab. My sensor is actually stuck to a wall tile immediately above the tiled slab (so it won't get mopped). You can see that the slab-surface readings are quite distinct from the head-height wall readings in the graph here:
http://www.ata.org.au/forums/topic/3/page/8#post-50485
The indoor atmosphere is stably stratified in summer, unless sun shines on the floor. Warmer air above will not mix with cooler air below. When cool ambient air is let in at night it spreads across the floor as a gravity current or "doorway flow":
http://en.wikipedia.org/wiki/Gravity_current

Posted Saturday 18 Jan 2014 @ 8:23:33 am from IP #
arttt
Member

im in Canberra and have a new extension so have been watching how it performed over the last few days. I can tell you what ive observed so you can compare and contrast.

Extension is a bit over 50m2 slab on ground (not raft) footings have 50mm foam 600mm vertically externally. Extra care and detail here to almost fully remove thermal bridging from slab to outside wall.
walls are about R4, roof about R7.
All windows north facing and fully shaded in summer about 30m2 of low e, double glazed, upvc frames, no extra insulation (ie blinds), large openings for night purging. This is huge for the area but its got good air flow to the old house (2 storey 50m2 each floor) - i was still worried the heat gain might be an issue.

No AC, one ceiling fan.

over the last few days using a standard digital thermometer (one sensor on the slab surface under a bit of foam, the other at shoulder height)
Slab temp has been 21C - 23C
Room temp 20C - 25C - most of the day it is around 24C

We are very very happy with how its performing.

The upstairs in the old bit of the house doesnt do so well mostly due to western windows that i still need to shade. it would also pick up a bit of heat from downstairs.

compared to yours i guess the ground connection is the biggest difference.

another factor is the lead up. We have been mindful of not getting the slab temp up for the last month or so. I suspect it would only take one warm evening having big doors open for easy indoor/outdoor living would heat the slab up a few degrees and be hard to get back down.

Hope that helps

and thanks for sharing - its interesting to hear how other houses perform.

Posted Saturday 18 Jan 2014 @ 2:02:17 pm from IP #
nhand42
Member

arttt said:

Extension is a bit over 50m2 slab on ground (not raft) footings have 50mm foam 600mm vertically externally. Extra care and detail here to almost fully remove thermal bridging from slab to outside wall.
walls are about R4, roof about R7.

All windows north facing and fully shaded in summer about 30m2 of low e, double glazed, upvc frames, no extra insulation (ie blinds), large openings for night purging. This is huge for the area but its got good air flow to the old house (2 storey 50m2 each floor) - i was still worried the heat gain might be an issue.

That sounds similar enough to mine for a good comparison. Thanks for taking the time to respond.

Slab temp has been 21C - 23C
Room temp 20C - 25C - most of the day it is around 24C

Those are the slab temperature I'm expecting. Instead my slab ranges between 24 and 27C.

another factor is the lead up. We have been mindful of not getting the slab temp up for the last month or so. I suspect it would only take one warm evening having big doors open for easy indoor/outdoor living would heat the slab up a few degrees and be hard to get back down.

That's an interesting comment. The house was handed over in early December. I'm pretty sure the builder ran the hydronic heating for a few days before handover. I only guess this because the hydronic manifold was hot to the touch when I moved in, but the heater/pump was turned off so I didn't think too much of it. The manifold is now cool to the touch which suggests he was running the heater.

I hadn't thought about lead up until you mentioned it. Could there be a heat "surplus" in the slab that's causing the problems? And how would I go about correcting this? Adam said to run cool water through the slab. I think that's worth trying. I'll work on that today.

Posted Saturday 18 Jan 2014 @ 9:58:40 pm from IP #
Catopsilia
Member

"...run cool water through the slab..."
I am convinced (and the model your architect is using neglects this) that the surface temperature of a slab on ground reflects the temperature of a mass of material several times the thickness of the slab. It is hard to change the temperature of so large a mass, and will take weeks rather than hours.
I wonder if you have instructions on how to manage the temperature of the slab when you are using hydronic heating? I am sure that few people realise how slow the process will be, demanding control well in advance of any required temperature change. Attempting change on an hourly basis is futile.
The suggestion to use water to cool the slab assumes that the available water is cool enough to have an effect. How cool is the water now? Is it below 24 degrees?

Posted Saturday 18 Jan 2014 @ 11:25:07 pm from IP #
axess
Member

Hi nhand42,

Your situation has highlighted some of the problems about using hydronic slab heating and keeping the thermal mass cooler at other times.

I've always been concerned about the long thermal lag and slow response time for this form of heating, particularly when used in the domestic sphere. It's a good solution for instituional heating requirements for people with limited mobility or those who live in countries with long clearly defined heating seasons. Wall mounted radiators would seem to be a better solution and you generally don't have to jack-hammer the slab apart if there is a pipe failure.

Running cold water through the system might introduce other new variables and you are still faced with the same control and response problems.

We live in a house that is moderately well sealed and insulated with a fair bit of design attention paid to potential heat infiltration in summer and heat retention in winter. Heating is by solar (through the windows) and zoned gas space heating in the main area, with spot heating by electric heaters as required (rare).

Cooling is by ceiling fan only in summer. Last week the in house temperature peaked at 33 degrees when it was 44 degrees outside. Over the last 14 years, several places have been identified that contibute to the internal heat load including the uninsulated slab edge in the southeastern bedroom corner, the northerly air flow from the dog door and inadequate nightime cooling airflows via the windows. These problem areas naturally become much worse during heatwaves, As a result we are considering an appropriately sized split sustem for the sleeping zone of the house.

As you have clearly identified, keeping the thermal mass out of contact with warm air infiltration and getting rid of any accumulation is the key to remaining comfortable for longer during summer heat. Naturall the opposite applies in winter.

Posted Saturday 18 Jan 2014 @ 11:51:46 pm from IP #
nhand42
Member

Catopsilia said:
"...run cool water through the slab..."
I wonder if you have instructions on how to manage the temperature of the slab when you are using hydronic heating? I am sure that few people realise how slow the process will be, demanding control well in advance of any required temperature change. Attempting change on an hourly basis is futile.
The suggestion to use water to cool the slab assumes that the available water is cool enough to have an effect. How cool is the water now? Is it below 24 degrees?

Unfortunately there were no instructions. I was told there would be training after handover, but I've never seen the plumber and the builder disappeared once he got his final payment. My opinion of builders is unrepeatable on this forum.

I've measured the mains water and it's 21.6 degrees. The slab is currently 25.3 degrees. That is a significant potential cooling, I just need to work out how to introduce mains water to the hydronic loop.

Posted Sunday 19 Jan 2014 @ 12:13:14 am from IP #
nhand42
Member

axess said:
Hi nhand42,
Your situation has highlighted some of the problems about using hydronic slab heating and keeping the thermal mass cooler at other times.

You are right. When I specified hydronic heating I didn't understand the implications for summer. Next year I'll use the tail-end of winter to get the slab below 20C and start the process of closing during the day and purging at night in autumn. The goal being to start next summer with a cooler slab.

As you have clearly identified, keeping the thermal mass out of contact with warm air infiltration and getting rid of any accumulation is the key to remaining comfortable for longer during summer heat. Naturall the opposite applies in winter.

I think a contributing factor is the house was closed until early December. That meant the slab was capturing heat during the day and not releasing it at night. The slab captured an enormous amount of heat which I now can't get rid of in summer. Thermal mass moves slowly and night-purging alone is not enough after the slab has warmed up.

The invoice for the concrete says 25 cubic metres was delivered. That's 60 tonnes and a heat capacity of 45MJ per degree. If the slab's currently 26C and needs to be 21C, that's 225MJ of excess heat I need to lose.

That's a lot.

Posted Sunday 19 Jan 2014 @ 12:37:26 am from IP #
karmacoma
Member

I just got back to Canberra very late last night and opened the house, which had been shut up vacant for 10 days during the severe heat wave. It was quite hot inside, maybe 24 deg, that's at 3am in the morning. This I should mention is a 3-year old fully-detached energy-efficient house, 120mm thick polished slab on ground, edge-insulated vertically and for 600mm horizontally around the perimeter. i.e, is semi-ground coupled. (House has 75mm foam walls, is double-glazed, etc.) Slab has hydronic heating which I turned off early October I think.

Well anyway, I opened the doors and windows for night flush, it cooled rapidly and all is ok today. The slab is cool to touch (and was last night).

Maybe the problem with raft slabs is that they are so decoupled from ground, they just heat to the average ambient indoor temp. I'd be suprised though that the slab could just hold its heat from a test of the hydronic heating system over 4 weeks ago.

But one thing: I found that morning sun through the eastern doorset had a significant effect. As in, last year the living room would get too hot too fast. This summer, I hung an old sheet over the outside of the doorset, so shading it (it's supposed to be shaded by a pergola I still haven't put up, so the sheet's the lo-tech solution). I'm surprised at how much difference that made, of not having 4 hours of sun on the slab from that glazed area. I don't have west glazing, but I'd be shading it if I did.

Maybe you're going to have to work on shading walls and windows on the east and west with plants and blinds, and ensure the roof space vents well.

Posted Sunday 19 Jan 2014 @ 8:13:16 am from IP #
cremorne121
Member

Hi, I would like to install a set of temperature sensors and some form of logger or monitor, in our upcoming extension/renovation. I would like to install at multiple locations e.g. subsurface, slab, inside, attic, outside, hit water tank, etc. Can anyone recommend equipment and an online supplier that is reasonably priced? I have a Mac, not a PC, if that makes any difference.

Posted Sunday 19 Jan 2014 @ 9:35:57 am from IP #
Catopsilia
Member

This discussion continues in "How to Improve Night Purging?":
http://www.ata.org.au/forums/topic/12364

Posted Tuesday 28 Jan 2014 @ 9:33:59 pm from IP #

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