external wall thickness for slab R-value

[Albrecht Stoecklein]

Nathan recently asked a common question about slab R-values:

When you calculate the construction R-value for concrete floor what it mean by "THE EXTERNAL WALL THICKNESS' . Is it the thickness of the stud of the wall or the entire thickness of the wall Example . if you use veneer brick wall with 40 mm cavity and 90X45 stud Is the external wall thickness be 90 or 200

Have a look at the diagram on the Design Navigator http://www.design-navigator.co.nz/DesignFAQs.html page for question "Why is it important to insulate the slab perimeter?". The heat finds the shortest path to escape to the outside. The distance of that path is determined by the thickness of the wall, i.e. in a 600m strawbale house the shortest escape path is 600mm.

So it comes down to the question whether the cavity between the brick and the insulated timber wall framing can be considered closer to internal air temperatures or external temperatures. Considering the low R-values of brick (and probably even for aerated concrete panel systems), it is more realistic to assume that's outside air. So that means for brick veneer systems the wall thickness should be taken as 90mm rather than 200mm.

Sunny regards

Albrecht

[Steve Netwriter]

Hi Albrecht,
I've been playing with:
THERM http://windows.lbl.gov/software/therm/therm.html (FREE - it looks like it is only for windows, but is not)
and
HEAT2 http://www.buildingphysics.com/index-filer/heat2.htm (Not free - but there is a non-time limited reduced number of nodes version which is WELL worth getting. IMO it is a VERY cool program).

Both far better than BASECALC IMO.
If I understand correctly, HEAT2 includes user defined specific heats, and so can model non-steady state situations, whereas THERM is restricted to steady-state modelling because it does not have user defined specific heats.

I'm still on a steep learning curve, trying to fully understand the boundary conditions and other factors that affect my results, trying to create something which matches what I think is reality.
I thought this plot would help on this thread. I am a little surprised that the heat loss through the cavity is so low, but I guess concrete, brick & soil are better conductors

Using HEAT2:

[Albrecht Stoecklein]

Hi Steve,

Very interesting.

The Heat program is the software BRANZ is using. They have an extended version which from memory also does three dimensional calculations.

It seems there is something going wrong in the air gap (q=0 means that there is no heat flow at all). I suspect that it has something to do with the fact that the tool calculates conductive heat flows. Maybe you have to "simulate" the air gap using an R-value of about 0.18 (Two layers of still surface air). That would give a more realistic result, I guess.

Sunny regards

Albrecht

[Steve Netwriter]

Hi Albrecht,
Yes, I noticed BRANZ listed in one of their documents.
There is a HEAT2D and a HEAT3D (2-D and 3-D modelling). If you want extra nodes (the extended versions), you pay a little extra over the normal price.
I'm using the FREE versions, so on 2D I'm limited to 25x25 nodes, hence my rather small drawing.

I'm sorry, I should have explained a little. The "q" is the heat flow through the boundaries. The top one is adiabatic, hence 0.
Here's a better picture, with isotherms and temperature, so you can see the gradient across the cavity.



What I'm really interested in is the heat flow through the concrete up into the brick veneer (as well as the others of course).
I'm trying to quantify the effects of my different designs using THERM which doesn't have the limited number of nodes, so these are only useful to see the effects.

Here's one with just external vertical edge insulation added. You can see how the heat flow goes down through the soil and up through the brick veneer.


This one has polystyrene under the brick veneer to remove that thermal bridge:


Just to show that THERM can do the same sort of thing

Heat flux, like those from BRANZ you posted:


My latest, with deep soil at 10C, and a radiator in the room to get nice temperature gradients in the air and slab:


This is all work in progress. I'm learning about how to create the most realistic model.
I highly recommend THERM (specially as it's free), the only down point is that most of the documentation refers to modelling windows instead of general building design, but I have found a few reports on the use of it for foundations.

[Albrecht Stoecklein]

Very nice, Steve.

A great set of research tools.

I like the radiator modelling, but you have to be a bit careful whether it account for convective heat flow. I would have expected a higher temperature near the ceiling.

Cheers and thanks again for posting this.

Albrecht