Options for edge insulation

[Steve Netwriter]

Hi Albrecht,
First a HUGE thank you from me. This is a superb website. It's been very helpful to me

I have so many questions, and I'm thinking about so many things. Maybe it's best for me to explain what I have been thinking about so far.
If you would care to look at this, you'll hopefully understand my thinking so far:

Concrete Foundation and Slab Insulation Options
http://neuralnetwriter.cylo42.com/node/1119

Having read in so many places about the importance of edge insulation I've found it very difficult to find out what the options are.

What I would like to be able to do is compare different designs for heat loss and the resultant temperature of the floor. As I see it, specially sitting here typing while my feet are resting on a floor slab that's at about 10C the temperature of the floor is a significant factor to comfort.

I've used your calculator, but it doesn't let me change the thickness of the edge insulation separately from the under-slab insulation. It also doesn't tell me what the floor temperature is likely to be.
I've tried BaseCalc, but it falls over when I select slab on ground, and appears to insist on the slab being below grade.

I see this topic as very important to NZ building, or should be, yet I get the impression that the options and cost/benefits are not well known in the building trade here.
Steve

[Albrecht Stoecklein]

Hi Steve,

Thanks for the flowers .

That's a very curly question you ask. The nature of the slab floor heat losses is really complex. In contrast to wall and roof insulation the heat flow paths are truly three dimensional and on top of that you have the time delay effects caused by the thermal mass in the slab.

Therefore there are no really simple solutions. I have been struggling with this question for a while already. There is a bit of information available (NZS4214, the BRANZ magazines, some international papers, etc.) and I want to incorporate that in the design navigator, but have not yet been able to implement it into a generic algorithm.

I guess to really understand this one has to use a CFD simulation tool. BRANZ has one of those and regularly publishes slab heat loss graphs in their BUILD magazine.



I love your drawings and sketches, especially the penguins. One helpful rule of thumb is that 100mm of concrete has an R-value of only R-0.06. So even small thermal bridges have big effects. But that means that even adding small amounts of timber or EPS/XPS will make a big improvement. Another thing to keep in mind is to keep the vertical polystyrene below the bottom plate, otherwise you have all sorts of water related problems.

I have a few links to other good detailing documents on the design navigator website (http://www.design-navigator.co.nz/OtherLinks.html). Most of them are - as expected - from overseas. Building with Insulated Concrete form blocks will allow some very simple and good solutions.



I'm also currently running a few workshops for designers and architects on energy efficient detailing, and slab insulation is always a hot - shall I say "cold" - topic with lots of opinions but few practical solutions.

But maybe we'll come out with a few practical solutions in the end.

Sunny regards

Albrecht

[Steve Netwriter]

Hi Albrecht,
Thanks for the reply.

Have you looked at BaseCalc:
http://canmetenergy-canmetenergie.nrcan ... ecalc.html

A menu-driven interface allows the user to quickly and efficiently describe how the basement or slab-on-grade is constructed, where insulation is placed, what type of insulation is used, and to select ground properties and weather. BASECALC then performs a series of detailed finite-element calculations—which may take several minutes to run—before presenting succinct, easy-to-read results to the user.
...
# The calculation core adapts a finite-element mesh to suit the user's input and performs a series of finite-element analyses -- all transparent to the user
# 2 steady-state and one transient 2-dimensional finite-element calculations are performed; BASECALC then accounts for 3-dimensional effects around corners and processes the finite-element results with weather data to predict energy and heat losses

Those BRANZ results are fascinating

I've searched for NZ solutions, and found this one from BRANZ:



which confusingly was from a B100 document, of which there are two!

Insulating slab-on-ground floors by Ian Cox-Smith BRANZ Thermal Testing Scientist, June/July 2007
http://www.branz.co.nz/cms_show_download.php?id=611

from here:

Build 100 - June/July 2007
http://www.branz.co.nz/cms_display.php? ... =1&pg=1698

This is the one I found first:

B100 Timber aids insulation of slab-on-ground floors, June/July 2007 - smaller version of B100
http://www.branz.co.nz/cms_show_download.php?id=629

Initially I thought that was a possibility. I then came up with various similar options with XPS, with various ways to protect it.
But then I realised that there is a problem with that idea. Differential settlement.
With different designs and different loads, the foundations and slab are bound to settle differently. Not so much of an issue except at the junctions of the outer wall and internal wall, where there will be issues, surely
The only way I see that working is if there is XPS under the slab and that rests on top of the foundation, but I haven't thought about whether he XPS would support the slab and internal walls if the slab settled more than the foundations, and won't solve the problem if the foundations settle more than the slab.

Thanks for the insulated concrete block forms info. I have looked at that, but dislike the idea because the great benefit of concrete is the thermal mass, which is useless when inside the insulation.
I have found a better solution: http://www.threedee.co.nz which has the insulation in the middle. The extra cost and lack of successful examples (as far as I know) on the south island where we are makes me a little less enthusiastic.

Unfortunately you're giving me the definite impression that this is ground breaking technology, which is a pain. I'd prefer a simple life, and to be able to just know what to do.

I've just found this one, which is interesting:



from: http://www.buildingscienceconsulting.co ... apolis.htm

This is well worth a read as well:

Sand Layers Should Not Be Placed Between Polyethylene Vapor Barriers and Concrete Floor Slabs
http://www.buildingscienceconsulting.co ... r_slab.htm

I have checked out some of your links. If I had more time I'd have checked them all out.

Cheers from cold and cloudy Christchurch.
Steve

[Albrecht Stoecklein]

Some really neat links. Thanks for them, Steve.

I think the sand layer/polythene foil is a bit different from the scenario where you have a layer of polystyrene instead. I think Joe's main issue is the water "absorption" of the sand. EPS is quite different and will not readily absorb water (Over long periods and when directly exposed, then yes, but not if it is only exposed over shorter times and to small amounts of moisture.)

I tried the Basecalc tool. Didn't have too much joy with it. I added a few insulation options and regularly got errors for the construction details. I guess it's one of those tools which have a very thorough technical basis but are a bit cumbersome to use.

Re those details: Once your walls are 140mm thick the whole issue becomes a lot easier, i.e. you can cantilever the bottom plate over the exterior footing insulation or over the interior thermal break and get some nice clean details without odd flashings or exposed timber. 140mm framing is getting a lot more popular and the added cost can be quite small, because it may reduce the need for closer stud spacing or dwangs.

Re. the threedee system. I personally think that thermal mass is a bit over-rated in NZ. Yes, if you have a well insulated house with good solar orientation and have most of the other solar design basics right, then thermal mass is a great feature to balance heat and to store surplus gains. But in most NZ climates I personally would always take better insulation over higher thermal mass. There are several other ways to deal with overheating (ventilation, shading...) but there are no alternatives to good insulation. The other thing is that slab floors are great for thermal mass and generally have sufficient mass so that you won't need additional mass in the walls. Having said that I think the threedee system is not bad and there are certainly worse systems out there.

Cheers

Albrecht

[nellyt]

I was intending on placing 100m of poly under our slab and to surround the slab with a timber insert as suggested in several BRANZ articles.

My builder however is insisting that he ties the slab into the footings every 600m with a reinforcing steel bar, that penetrates 400mm into the slab, passing through the timber thermal break.

I reckon this creates a significant thermal bridge.

He claims all slabs should be tied into the footings for stability reasons.

Neither the BRANZ articles or your calculator appear to take into consideration these ties into the slab.

Is this because they've been forgotten about or is my builder trying to do something that is not done these days?

[Albrecht Stoecklein]

Hi Nellyt

It's actually a very good question. My gut feeling was always that a few rods of steel should not be too much of a problem, but I just did some calculations and it seems the effect is quite major.

Below a few examples (Not sure what typical slab tie diameters are.) :

The timber by itself has an R-value of about R-0.38.

with 10mm reinforcement rods @ 600mm centers you get a total of only R-0.13
with 14mm reinforcement rods @ 600mm centers you get a total of only R-0.08
with 10mm reinforcement rods @ 1200mm centers you get a total of only R-0.19
with 14mm reinforcement rods @ 1200mm centers you get a total of only R-0.13

So you loose about 2/3 of the timber performance. I'm actually quite certain that the BRANZ calculations do not account for the ties.

BRANZ and a few other engineers confirmed to me that for single storey buildings there is no tie required. But I am sure that some engineers would feel un-easy about that.This certainly also depends on other factors such as soil type and moisture, earthquake zone, site slope, etc.

So in summary I think the external perimeter edge insulation option is better. Also have a look at the discussion tread http://www.design-navigator.co.nz/phpBB3/viewtopic.php?f=4&t=49

I hope this helps.

Sunny regards

Albrecht

[Steve Netwriter]

I've done a huge number of simulations on foundations. Unfortunately I've not had time to complete them and publish the results.

But one thing I've learnt is that the soil type and moisture is VERY important. I suspect that most people put the soil almost as an after thought, if they consider it at all.
But from my results the soil is actually THE most important factor !

If you have dry soil, then worry about insulating the edge more than underneath.
If on the other hand you have wet soil (especially with a high water table), then you can almost forget about the edge insulation, and instead worry about under the slab.

Basically the many many websites that just say "edge insulation is important" are omitting a very important factor.

Our local compliance guys insist on the reinforcement link. With earthquakes, wet soil, and the danger of differential settlement I'm not surprised.
Personally, unless you're on very sound rock, I'd put the insulation on the outside if you need/want it.

[nellyt]

Thanks for the swift replies and some people who actually understand the maths.

The reinforcing bars are infact 16mm diam. at 600 centers on my plans.

The effect is the same as the argument that its pointless filling your walls with too much insulation unless the windows are upgraded or pointless putting tons off roof insulation in only to puncture it with downlights everywhere. The thermal bridge dominates the result.

Steve : you say you have to have these links between slab and footing : where in the country are you.?
I'm near ChCh.

[Albrecht Stoecklein]

Hi nellyt,

16mm at 600mm is of course even worse. With 16mm reinforcement rods @ 600mm centers I calculate a total of only R-0.06!

I think we are really pushing the boundaries with these simple heat loss calculations and we may be out quite a bit for these specific cases. My calculations for example assume that the conditions for an isothermal planes approach are correct, i.e. that the heatflow *within* the concrete towards the reinforcement bars is not limited by the concrete R-value.

But having said all that I think it is safe to say that the effects of the reinforcement are significant.

Steve, I was wondering whether your ground heat loss analysis uses a dynamic simulation model. My gut feeling would be that if you use a static model it would over-estimate the moisture effect of the ground heat losses. For the edges the R-value and the thermal capacity is so small that the heat losses can easily occur over a 24h period. For heat losses through the center of the slab the heat capacity of the slab is so large that day/night heat flows might see-saw to a degree.

Not sure how you currently model this but recently I was thinking that for center-of-slab ground boundary conditions I would probably use constant T rather than Q=0.

Just a thought...

Sunny regards

Albrecht

[Albrecht Stoecklein]

Neil just spotted a stupid mistake in my calculations (mixing up radius and diameter...! )

So the correct values are:

with 10mm reinforcement rods @ 600mm centers you get a total of only R-0.25
with 14mm reinforcement rods @ 600mm centers you get a total of only R-0.19
with 16mm reinforcement rods @ 600mm centers you get a total of only R-0.17
with 10mm reinforcement rods @ 1200mm centers you get a total of only R-0.30
with 14mm reinforcement rods @ 1200mm centers you get a total of only R-0.26
with 16mm reinforcement rods @ 1200mm centers you get a total of only R-0.23

That's compared to R-0.38 for the timber itself. So the losses are not quite as bad as I initially thought, but still bad enough to worry about.