NZ Passive House Forum

Steve Netwriter · Wed, 29/07/2009 18:41:10

Hi,
My aim is to create a large set of simulations for various possible concrete slab insulation designs, so that I can compare cost/complexity against both energy saving and comfort. Living in a fairly modern NZ house with slab on grade in the winter has demonstrated very well the importance of the temperature of that slab! It's not all about heat loss.

You can see my current list of possible designs here: http://neuralnetwriter.cylo42.com/node/1119
I would welcome comments on them. Both criticism and other suggestions, especially re NZ regulations.

I think my aim is slightly different to what I have read on here, in that I do not need to conform to any standard for my results. I want to create a model which as closely as possible mirrors reality, and gives me total heat loss through the floor, but most importantly the temperature of the floor.

For that reason I have created a frame cavity for the room, and set the boundary condition on the little inset on the right to 45C (to represent a heat source), and adjusted the film coefficient until the room temperature was about right.
The concrete has a layer of 10mm of felt to represent carpet.

The result looks like this:

To get the U-factor I used the "radiator" as the U-factor surface, and set the custom length equal to the floor length. This gave 0.1612W/m2C, or R = 6.2, which seems about right.
Of course there is also heat loss through the wall, so I'm really measuring both.
The outside surface (brick, concrete, soil) can't be used as there is heat flow from the deep soil (10C) to the soil surface.

So now the question. Does that sound sensible? Is there a better way?
As I explained, I'd like to get this base model right before creating all the others.

I'll happily post all my results when I get them

Last edited by Steve Netwriter (Wed, 29/07/2009 18:48:18)

Mark S · Thu, 30/07/2009 10:38:10

Steve,
You say that you "do not need to conform to any standard for my results." You almost certainly need to conform to some standards rather than make it all up. Please could you be more specific are you saying that you don't need to confirm to the PassivHaus standard or the procedures for calculating thermal bridging/surface temperatures (as described in the EuroNorms such as EN ISO 10211.)
Please note that thermal bridging calculations also consider surface temperatures.

You also say "I want to create a model which as closely as possible mirrors reality" does that mean a dynamic model that will allow for temperature fluctuations and time? Therm is not the tool for this. You may want to try some of the tools from Physibel or Heat 2D / 3D.

In terms of your model I can begin to appreciate what you are trying to do in some ways but can't say that I follow the direction that you are taking - sounds like you could be fiddling around with the model until you get the aesthetic that you desire rather than determining the reality from hard data. At this time I'm not sure where to point you - other than to the EuroNorm conventions.

Mark

Steve Netwriter · Sun, 09/08/2009 17:56:54

Hi Mark,
Sorry for not getting back sooner, I've been a bit swamped recently. Thanks for your reply.

I'm designing for NZ, in which no floor insulation is acceptable! So anything I add and how I decide what is the best solution in this case is entirely open to me.

I seem to have followed a path of ever increasing complexity and detail. I started with the simple aim of understanding the relative importance of each cold bridge of a slab-on-grade design, in particular I wanted to find out how important the bridge between concrete and brick veneer was and whether it was worth the effort of trying to get approval for extruded polystyrene under the brick veneer.

Since my post, and having read your reply, and after getting comments from other people, I've changed my model, and simplified it.

I would like to get into dynamic modelling, but at the moment I'm just considering steady state mid-winter conditions, and comparing heat flows for each design. I do have the free Heat2 & Heat3, and I'd love to get the full versions.

Having thought I was close to a result I have simply found yet more to investigate. I did not expect to become an "expert" on soil, which I have found is THE most important aspect of a slab-on-grade design, and I suspect a factor that is vastly overlooked by most designers.

I have read quite a few papers now, on modelling, measurement of real case situations, to the effect of salt on soil conductivity.

I have got some very interesting results, including some quite surprising ones, but I now have yet more variables to investigate.

As I said, my prime aim is to find the relative importance of each cold bridge, (which I now have a pretty good idea about), but I am trying to ground my model in reality by comparing my results with measurements taken from real houses.

I do intend to publish my results, which I hope will be of interest and use to others.

Mark S · Mon, 10/08/2009 07:55:46

Steve,
I'll return to my point you state again that you do not need to satisfy ANY standards. I think what you mean to say is that you do not have to satisfy any regulatory energy performance standards. I know that I'm being fussy but the point that I'm making is that the calculation procedures should follow a standard - or else you are reinventing the wheel for the sake of it. These standards have been developed because they have been found to have congruity with reality. Of the back of this it may, for academic purposes, be appropriate to prove whether the standards can be applied in NZ (given that the laws of physics do not change the answer will be "yes" though of course refinements may be determined.)

In light of my comments the following may be of interest:

This paper validates EN13370 against measured data (not PassivHaus):
Temperatures in and under a slab-on-ground floor: two- and three-dimensional numerical simulations and comparison with experimental data
http://www.sciencedirect.com/science?_o … abfe7389d4

Not read this paper but may be of use: Insitu measurements and numerical simulation of heat transfer beneath a heated ground floor slab
http://www.sciencedirect.com/science?_o … 95f10bb382

Mark

Steve Netwriter · Thu, 10/09/2009 11:34:58

Hi Mark,
Unfortunately I don't have access to those articles.
This is one I have read though:

con76: A Four-Year Site Measurement of Heat Flow in Slab-on-Ground Floors with Wet Soils H.A. Trethowen and A.E. Delsante
http://www.branz.co.nz/cms_show_download.php?id=78

This is quite good for putting the New Zealand situation into context:

The history of insulation in NZ - mentions Harry Trethowen, B102
http://www.branz.co.nz/cms_show_download.php?id=940

I've published my results so far. I've had to split it into lots of sub-articles.
This is the starting point:

Test Series 2: Introduction to Modelling Concrete Foundations with THERM
http://neuralnetwriter.cylo42.com/node/1810

Given your greater experience, knowledge and access to articles, I would appreciate it if you could let me know if I have made any obvious modelling errors. How does what I have done differ from the standards you quote when using a steady state 2D model?
I have compared my results to those from the design navigator, and they do differ in sensitivity to soil moisture.

My main aim was to gain an understanding of the importance of insulating under the slab and at the edge. I have certainly achieved that. I have gained a lot of knowledge, but also have many unanswered questions.

IMO the most important aspect I have discovered for myself is that soil type and moisture is probably the most important factor, and one that I have not seen mentioned anywhere near as prominently as I think it should be.

Anyway, when you get time, have a look at my article. You may or may not find it interesting/informative. Let me know
Steve

NZ Passive House Forum

#1 Wed, 29/07/2009 18:41:10

Getting realistic temperature gradients and U-factors with THERM

#2 Thu, 30/07/2009 10:38:10

Re: Getting realistic temperature gradients and U-factors with THERM

#3 Sun, 09/08/2009 17:56:54

Re: Getting realistic temperature gradients and U-factors with THERM

#4 Mon, 10/08/2009 07:55:46

Re: Getting realistic temperature gradients and U-factors with THERM

#5 Thu, 10/09/2009 11:34:58

Re: Getting realistic temperature gradients and U-factors with THERM

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