Housing - New Build: Using floors as thermal mass

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- CommentAuthorMike George
- CommentTimeSep 23rd 2007
Posted By: Jeff Norton (NZ)

I am involved with designing and overseeing the construction of some lightweight buildings on ICF foundations/slab on grade homes. Timber construction suites our conditions well as we have plenty of timber plantations and sheep's wool to insulate them.

A well insulated building requires minimal heating and a lot of NZ has dry well draining ground ideal for thermal storage. The light weight building can us the ground as the thermal mass by insulating the foundation edges and with wing insulation. The trick is to balance the energy demand of the house with the sites passive and active solar harvesting abilities and sizing a thermal store to blend the two.

These homes are design to maintain a minimal internal temperature of 18 degC year round (from solar, occupied or not) and with additional spot heat when required. Most clients want a fire place in the living room any way.

By having the 3 elements to work with (load, mass and generation) you can look for the most cost effective solution, passive house standard construction might be to expensive compared to solar panels or compacted hardfill!

Because every site/project is different, I always push for a good simulated computer model of a design at concept stage to establish the future path.

Jeff's post prompted me to start this new thread.

In the UK we are ever pushing for lower and lower u-values in floors. Clearly insulation is needed for suspended floors. However, it seems curious to me that while the principle of using thermal mass in walls is generally acceptable it is not really taken advantage of in solid floors. I have long considered a similar method to that suggested by Jeff above, I.e. continuous cavity insulation down to formation level. It is after all acceptable above ground, and has been advocated as an alternative to conventional floor insulation in CIBSE Guide A. ' Insulation wings' could be added to minimise heat loss further. Should this type of construction be adopted in the UK? If not, why not?
- CommentAuthortony
- CommentTimeSep 23rd 2007
Should be adopted and the larger the g/f area the more economic it becomes and the greener.

High water tables could scupper it though these are rare in general.

The creation of a warm bubble under the house is a great idea.

Jeff, I worry about yous because you insulation does not extend any where near far enough -- it needs to more than just the edge of the slab.
- CommentAuthorfostertom
- CommentTimeSep 23rd 2007
Posted By: Mike GeorgeShould this type of construction be adopted in the UK?
I've done this for a long time - makes perfect sense. Bldg Insp never queries it.
Posted By: tonyyou insulation does not extend any where near far enough -- it needs to more than just the edge of the slab
No it doesn't - what do you mean? - you seem to be contradicting your first para.
- CommentAuthortony
- CommentTimeSep 23rd 2007
I mean that if you are going to try and use the ground under the house as thermal mass then the insulation needs to extend down say 1m below floor level. Jeff only insulates the edge of the slab ( 200mm ?) and nothing below so heat can seep out from underneath the house.

How far down do you take your insulation then tom? Do you get away with no insulation under the floor?

Although it an alternative to insulate vertically my LA's wont accept no underfloor insulation.
- CommentAuthorMike George
- CommentTimeSep 23rd 2007
Yes Tom, how do you get around the u-value requirement for floors? My LA does not accept vertical insulation as an alternative to underfloor. Also not likely to with the new lower u-values on the way soon.
- CommentAuthorJeff Norton (NZ)
- CommentTimeSep 23rd 2007 edited
I use ICF (Insulated concrete forms) foundation details so thermal break continues to about 600mm into ground, 3 blocks high. I can dig deeper and install more blocks if required.
- CommentAuthorMike George
- CommentTimeSep 23rd 2007
I like it! It seems NZ legislation allows for more flexibility in design than UK regs. Here we have maximum backstop u- values that must be complied with [I believe] ruling out this kind of design

There is the possibility that this kind of design can attain compliance via thermal modelling, though the guidance on whether this is permissible is unclear to me. The government recommended software for dwellings [SAP] is not suitable for this kind of analysis.

Anyone know if L2 accredited dynamic modelling software can be used to gain compliance for dwellings? If so, are backstop u-values then irrelevant, given that passive measures can be used to compensate?
- CommentAuthortony
- CommentTimeSep 23rd 2007
Jeff, nice picture and thanks. To my mind your insulation is insufficient in thickness and it could do go a lot deeper too.
- CommentAuthorJames Norton
- CommentTimeSep 23rd 2007
Where can I get building inspectors like Toms...

In terms of the slab specifically I have seen an article in this very magazine on this subject. My understanding is that the more heat is lost through the slab the warmer the ground gets... but the warmer the ground gets the less heat is lost... etc etc...

http://www.buildingforafuture.co.uk/winter05/How_much_insulation.pdf

RE: edge insulation I've seen ref to this in US strawbale designs with horizontal insulation extending say 900 outward, s'pose it depends on local ground temperatures at different depths...?

J
- CommentAuthorJeff Norton (NZ)
- CommentTimeSep 23rd 2007 edited
NZ legistration for energy efficientency is pretty loose at the moment (just about to go through another review) but we are able to use dynamic simulated software for compliance (not required in this case).

This house was simulated on IES and has a heating load of around 4500 kwh per year so the question is, In this climate is the added expense warrantted to increase insulation?
My college did the simulation (newish IES user) but we are still trying to work out how to simulate the solar gains from the dry ground? Passive and active (www.floorstore.org)
- CommentAuthorMike George
- CommentTimeSep 24th 2007
Posted By: Jeff Norton (NZ)This house was simulated on IES and has a heating load of around 4500 kwh per year so the question is, In this climate is the added expense warrantted to increase insulation?

Jeff, Did the software suggest that it isn't economic to increase the thickness? or does your final paragraph mean that the results are inconclusive? Sorry I am not familiar enough with IES to help with your question regarding solar gains
- CommentAuthorJeff Norton (NZ)
- CommentTimeSep 24th 2007
Mike,
The software was only used to establish heating load and I guess I was answering tony's comment with a question. My college (Jessica who posts on this forum occasionally) is still learning the software so will do more research when knowlege allows.
Like you have suggested in the past it is correct amount of insulation (not to big and not small) for the specific design that is important.
- CommentAuthorfostertom
- CommentTimeSep 24th 2007
Posted By: Jeff Norton (NZ)the solar gains from the dry ground
What's this mean - am I missing something?
Posted By: tonyDo you get away with no insulation under the floor?

Although it an alternative to insulate vertically my LA's wont accept no underfloor insulation.
Come to think of it, so far my downstand insulation has always been in addition to u/floor - but lately the underfloor is thickest around the edge, minimal in the middle - and even around the edge it's less, deeming the downstand to be part of the edge insulation - Bldg Insp's OK with that. The u/floor is because of u/floor heating - come the time when I can do without that, then it'll all be downstand, and I feel confident that the building inspectors will play ball. Probably in couple of current projects, with aid of Tas modelling maybe with Mike Georges's assistance.
Posted By: tonyJeff, you insulation does not extend any where near far enough -- it needs to more than just the edge of the slab.
Now I see what you mean - but Jeff's "insulating the foundation edges and with wing insulation" - which is the other way to do it, probably making even more thermal mass available to the floor, than with downstand insulation - though with much other complication.
- CommentAuthortony
- CommentTimeSep 24th 2007
But too much is getting out of the sides of the slab isnt it?

No insulation under the floor -- you nearly had us moving down your way!

Re: Mikes article in BFF should the U value of the soil under the house be taken into account? It sure seems like a waste of time, money, resources and thermal mass to be putting any insulation under solid floors.
- CommentAuthorMike George
- CommentTimeSep 24th 2007 edited
Hi tony, soil conductivity was looked at in the follow up article here http://www.buildingforafuture.co.uk/spring06/25-50.pdf
- CommentAuthorJames Norton
- CommentTimeSep 24th 2007
This issue seems to be a real concern and presents a limitation in most current standards including AECB and Passive Haus

However it seems to me that the articles support some underslab insulation to current ADL standards, what seems to be an issue is whether addtional insulation over and above this is energy or cost efficient. I have seen research by contributers to this forum, comparing lifetime energy costs for addtional insulation in walls and lofts, perhaps some enterprising soul could do some sums for insulation under ground contact slabs using these figures rather than the steady state analysis heat loss figures.

It also seems clear that edge and perimeter insulation is vital to detail well, to prevent cold bridging at areas where the slab is losing heat quickest.

Other contributing factors will apply of course: perimeter to area ratio, soil type, and effects of thermal mass from incident solar radiation.

Overal it seems to me that in terms of low heat loss passive solar strategies, the argument for suspended timber is weak on energy terms in situtations where a ground contact slab would be possible because of the benefits of thermal mass (from incident solar radiation) and the thermal buffering provided by ground contact. In fact it would be interesting to see life cycle energy comparisons for suspended timber vs ground contact with in light of this research.

This would of course also bring in the issue of what material the slab is made of as well as the insulant.

However with so many potential variables, we should in the words of the article 'Beware the blanket approach' .

J
- CommentAuthorfostertom
- CommentTimeSep 24th 2007 edited
Posted By: tonybut too much is getting out of the sides of the slab isnt it?
Not if the wing is wide and thick enough or the downstand is tall and thick enough.

Either way, if the insulation is thick enough, heat that's being lost from the interior into the subsoil, particularly via the very edge of the slab, is forced to take a long path to the outside air, which is the source of ever-renewed cold. Note that the subsoil isn't itself a source of ever-renewed cold - it takes a while but once the interior has warmed it up, i.e. soaked up its initial store of cold, it is no longer itself a source of cold, but should be viewed as an insulator - OK a poor insulator, but used in enormous thickness. That thickness is the path length that heat is forced to travel, from the edge of the slab, downward, then outward, past the edge of the downstand or wing insulation, then upward to eventually reach outside air at the ground surface. The downstand or wing insulation has to be thick enough to prevent the heat from shortcutting through it to the surface/air.

Given all that, the subsoil that's contained within the downstand fence, or that lies under the wing, as well as acting as an insulator, is also a thermal mass available to the interior as a heat store. So you need more wing insulation compared to downstand insulation, as far as insulation is concerned; but then wing insulation brings in a lot more, as far as thermal mass is concerned.
- CommentAuthortony
- CommentTimeSep 24th 2007
I am with you on all that but it is crucial that the edge of the slab and fill above ground is sufficient well insulated -- better than normal for sure.
- CommentAuthorfostertom
- CommentTimeSep 24th 2007
I see - you mean the wall insulation has to connect up with the downstand or wing insulation.
- CommentAuthorfostertom
- CommentTimeSep 24th 2007
Another thought - with deep strip founds, cavity wall carried down, it's very easy to fill the cavity with insulation and/or lay insulation against inside and/or outside face before backfilling - hence downstand insulation wins. If, as we shall all have to as conc (incl blockwork) becomes increasingly expensive because fuel-intensive, it's being done by piles and ground-beams, without deep strip excavations, then wing insulation looks easier.

Compared to downstand insulation, wing insulation does increase the volume of massiveness available to the interior - but that works best for small buildings or buildings with a high perimeter-to-area ratio. For larger buildings, where the extra area added by a perimeter wing becomes insignificant, that advantage disappears.

Compared to downstand insulation, wing insulation doesn't create such a long path for the heat to travel, from the edge of the slab to outside air/ground; hence a wing has to be wider than a downstand is deep; it also has to be thicker as the temp gradient across it is greater, being so close to the surface. Plus the wing has to be protected from gardening. Gets complicated.
- CommentAuthorJeff Norton (NZ)
- CommentTimeSep 24th 2007
Here is a good Green detail for wing insulation, note the recommended distance of 20' (A fall 12 months of thermal travel)!

http://www.thenaturalhome.com/earthtube.htm

For a lightweight solar heated building it is important to have the ground in contact with the underside of slab floor and only the edges thermally broken.

I am using the stratagy of active solar heating the middle of the floor (and ground under) first with the highest temperature fluid and as tony mentioned create a bubble but with the middle being the hottest and the incrementally cooler to the outside of wing insulation.
- CommentAuthorfostertom
- CommentTimeSep 24th 2007
20ft - blimey!

If the slab plus wings is say 18m across, any thought of injecting the glut of summer solar heat say 9m deep, so the 'bubble' takes quite a while, like months, to reach the underside of the slab? Assuming no groundwater movement to convect it all away.
- CommentAuthorJeff Norton (NZ)
- CommentTimeSep 25th 2007
Obviously placing pipes under a house by 9m is a bit costly but there are some techinics that use that philosify of heat travel/time in the dry ground, rule of thumb a metre a month!
http://www.earthshelters.com/Index.html
Tom, I think you gave me this link before.

See figures 1 and 2 of pdf attached
- CommentAuthorjon
- CommentTimeSep 25th 2007
Hi Jeff

Very interesting. I have also been researching this and found that the commercial software was unsuitable so had to write my own software to work out losses. For the UK, I could not get the arrangement to work economically as the input energy, given the ground loss, appeared to be too high (particularly if trying to annualise energy loads). However would be interested in any results you get.

Jon
- CommentAuthorfostertom
- CommentTimeSep 25th 2007
Jeff, I hadn't come across this source but they're really got it sorted! I do have a EnvEng friend who has I think done work for these guys so I'll tap him up.

I see your drift now - can the same principles - floor exposed to subsoil, big umbrella around - be applied to non-buried buildings? maybe with some panels-and-pumps assistance in getting the heat into the ground, as the walls (and roof) aren't doing that role, just the floor.

Getting heat into the ground 9m deep - obviously not possible with a horizontal array of tepid water pipes but once (soon) it becomes economic to replace the wet panels with PVs, then heat wd be transferred into the ground by a single high-temp resistance element at the bottom of a borehole, heating the rock by radiation, so not requiring thermal contact, thus withdrawable for maintenance.
- CommentAuthorfostertom
- CommentTimeSep 25th 2007 edited
Paul in Montreal, if you're there, this is relevant to our little ding-dong debate.

The Rocky Mountain Center philosophy is to drive unnatural amounts of summer heat deep-and-wide into the ground so that the whole environment, enlarged deeper - house, subsoil, even somewhat the surface soil, maybe even slightly the air, adopts something above the natural annual average air (or sol-air?) temp. That's what keeps the house at even-ish temp, sitting passively in the middle of an even-ish elevated temp environment.

This contrasts with the ground source heat pump approach, which similarly uses deep-ish soil heat, but scavenges it mechanically from there all winter and drops the heat into the house, from where it's dissipated. What's missing is the deliberate summer replenishment of that heat, from the house-as-collector. The belief is that either a) deep-ish ground heat is an inexhaustible supply (and not because of earth-core heat rising from below, which, though small, plays a vital role, which GSHP knowledge ignores); and/or b) deep-ish ground heat is naturally replenished by the sun on the ground all summer (only true, beyond the topmost 1m, if local groundwater happens to be carrying the heat down deeper). However, neither of these is true and over a period of years, depletion of the deep-ish heat will occur, i.e. the deepish rocks will be steadily cooled. In principle, though it may take some years to become evident, GSHPs, far from creating the Rocky Mountain-type year-round warm cocoon, actually refrigerate the environment in which the house sits.

Below 1m, ground temp naturally stabilises at the annual average air (or sol-air?) temp, which shows that it takes much more than 1 year for surface soil heat gain/loss and consequent seasonal surface soil temp fluctuations to reach to any depth. If the climate suddenly changed (nah, not possible ...... but anyway) so the seasonal average air temp suddenly increased by 6oC, how long would it take for the subsoil at 1m, 3m, 6m deep to near-fully adjust? At a guess, I'd say 2yrs, 6yrs, 12yrs, or maybe only 1yr, 3yrs, 6yrs. By the same token if GSHP coils have cooled the subsoil 1.5m down by 6oC by the end of the winter, will the winter sun succeed in replenishing it back within the 6months max that it has? I'd say no - in other words GSHP progressively depletes soil heat, faster than it can be naturally replenished from the sun - or from below. This is partly masked by subsoil heat being drawn in sideways, from a widening area - but if your neighbour are doing the same, that wouldn't happen.

Paul, you mention using GSHP coils to dump summer air conditioning heat, which has the effect of subsoil replenishment, but this isn't seen as an essential part of a cycle.

Beyond all that, the Rocky Mountain approach aims to achieve temp stability by passive means without machinery. Surely with those hot summers that could work even in central Canada?
- CommentAuthorPaul in Montreal
- CommentTimeSep 25th 2007 edited
Posted By: fostertomThe belief is that either a) deep-ish ground heat is an inexhaustible supply (and not because of earth-core heat rising from below, which, though small, plays a vital role, which GSHP knowledge ignores); and/or b) deep-ish ground heat is naturally replenished by the sun on the ground all summer (only true, beyond the topmost 1m, if local groundwater happens to be carrying the heat down deeper). However, neither of these is true and over a period of years, depletion of the deep-ish heat will occur, i.e. the deepish rocks will be steadily cooled.

Tom, you can state your opinion as fact as many times as you want, but it does not make it fact - it is merely your conjecture. GSHP systems have been in use since the 1920s and I know of no cases (where the sizing is done correctly) of permanent refrigeration of the ground. There are some citations for ground heating in the southern US where the ground loops are either undersized or in poor thermal contact with the ground (dry sandy soil) or both, but I have seen no references to the ground being refrigerated.

As for using the house-as-collector, in pretty much everywhere except the UK, GSHPs are reversible. We have hot summers here and so need airconditioning - if the daily average temperature is 25C, no amount of passive nighttime cooling will achieve an interior temperature below that value. I've had visitors from the UK complain that it is far too hot inside at night (even with the air conditioning on). How many people in the UK are comfortable sleeping when the temperature is 22C overnight? Heck, that's warmer than most people in the UK ever have the temperature with the heating on! Whether running the GSHP in airconditioning mode is seen as essential or not is irrelevant over here in North America - the systems always run in that mode in summer.

Posted By: fostertomBeyond all that, the Rocky Mountain approach aims to achieve temp stability by passive means without machinery. Surely with those hot summers that could work even in central Canada?

Central Canada average air temperature is around 4C so a purely passive approach is difficult. The Drake Landing Solar Community in Alberta ( http://www.dlsc.ca ) uses solar collectors and pumps to store heat in boreholes in rock beneath a vast sheet of insulation. It is pseudo-passive in that it doesn't need a heat pump to extract usable heat - the ground rocks are targetted to reach 80C after three years of "charging". The Rocky Mountain Center approach works OK in a warmer climate where the annual average air temperature is much closer to "comfortable" but I doubt it would work in most of Canada (parts of BC may work as the average temperature is more like 14-15C).

Paul in Montreal.
- CommentAuthorfostertom
- CommentTimeSep 25th 2007 edited
Posted By: Paul in MontrealTom, you can state your opinion as fact as many times as you want, but it does not make it fact
I do accept that your experience is different - I just want to provoke someone to explain why - how that can be - because the rest of it, leading to the timespan reqd to recharge, seems to hold up.
Posted By: Paul in Montrealif the daily average temperature is 25C, no amount of passive nighttime cooling will achieve an interior temperature below that value
Wouldn't the Rocky Mountain Boys disagree?
Posted By: Paul in MontrealCentral Canada average air temperature is around 4C so a purely passive approach is difficult
The Rocky Mountain Boys (my new heroes - can you tell?) aim to bias the u/ground + house part of the local environment to near-enough stabilise at a temp higher than natural average. How much higher than natural average can their passive system provide? Can it be pushed further by boosting it by mechanical means - e.g. PVs supplying u/ground heating element?
Posted By: Paul in MontrealThe Drake Landing Solar Community in Alberta ( http://www.dlsc.ca ) uses solar collectors and pumps to store heat in boreholes
It's good - but at obviously vast capital (and maintenance) cost, to squeeze useable heat storage density out of the tepid (80oC absolute peak) thermofluid that they're stuck with. Roll on PV - absolute simplicity!

Thanks, Paul for your continued challenge - this forum truly is a university of the first quality.
- CommentAuthorPaul in Montreal
- CommentTimeSep 25th 2007
Posted By: fostertomWouldn't the Rocky Mountain Boys disagree?

Mountain climates with dry air have a much larger diurnal temperature variation than the climate I'm in. Passive cooling works well if it's cool and dry at night. When it's warm and humid (this summer we had humidex values of >30C all night on the worst days) it's much more difficult.

Posted By: fostertomIt's good - but at obviously vast capital (and maintenance) cost, to squeeze useable heat storage density out of the tepid (80oC absolute peak) thermofluid that they're stuck with. Roll on PV - absolute simplicity!

PV systems have a peak efficiency of around 20% right now - so even if you can make the ground hotter than 80C, it's fairly pointless. Heating works well with the transfer medium at around 50-60C (i.e. water :) ).

If you read the Drakes Landing Solar Community initial report, you'll see that they're running at around 96% efficiency - it will be years before PV reaches that level (if ever). Cold climates need heat in winter, not electricity (which is why it's sad that CHP (Combined Heat and Power) plants seem to have been un-remembered. In central Manchester, Bloom Street power station used to provide millions of kg of steam to the local area for heating - so the overall plant efficiency was much closer to 100% than the 30% the electricity portion only provides).

Here's the link to the PDF for the DLSC initial startup analysis - it makes very interesting reading.
http://dlsc.ca/reports/EPD_March_April_2007.pdf

Paul in Montreal.
- CommentAuthorfostertom
- CommentTimeSep 25th 2007
Posted By: Paul in MontrealPV systems have a peak efficiency of around 20% right now
Efficiency will no doubt improve - but maybe doesn't matter - of interest is kWh/Â£ - so at the right price (i.e. not just yet), big PV area at low efficiency is fine, because it can be at the bottom of the garden or anywhere getting maximum sun, whereas wet panels have to be close to where the heat's reqd. Anyway, if you mean efficiency as in %age conversion of all incident solar radiation, don't tell me that the the Drakes Landing, or any, wet panels get anywhere near 96%, especially when straining to get highest possible thermofluid output temp.
Posted By: Paul in MontrealPV systems have a peak efficiency of around 20% right now - so even if you can make the ground hotter than 80C, it's fairly pointless
Given the above, what's the problem with making the ground hotter than 80o? And why would that be pointless?
Posted By: Paul in MontrealCold climates need heat in winter, not electricity
PVs readily produce heat, if that's what's reqd.

I promise to read the DLSC initial startup analysis - soon.

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Housing - New Build: Using floors as thermal mass