Scaling R value in passive solar.

Factual Questions
1

Hi, I’m not an engineer or architect.

I’d like to draw up plans for a scalable passive solar test structure to record temperatures in. I can scale down volume, square footage, glass area, overhangs, thermal storage, etc. My intuition says I would *NOT *scale down insulation R value. Is this correct?

Is there simple software I can use to plug these numbers in?

Thanks, Vini

2

There is some software available here.

3

Be careful: If you build an exact 1/10 scale model of something, you’ll have 1/100th of the surface area, but 1/1,000 of the volume. Which means, for instance, interior temperatures will change 10 times more quickly in the model than in the full scale.

4

I plan to calculate down from the volume and let the dimensions flow from that.
Thanks for the tip about rate of temperature change.

5

Do you have Mazria’s book?
If not, get it and read it before you start. If you can find the “professional” edition, there is a lot more detail in it.

6

That’s only one example, though; you need to think carefully about all your parameters. The faster rate of temperature change is really a by-product of the relatively much lower heat capacity (scales by volume) compared to radiation input/output (scales by area). Lots of other things might depend on a balance between something volume-related and something area-related (or length-related), and these will be out of whack in a scale model. Especially, if there are any sources or sinks other than radiation, you need to decide how they will affect things, since you can’t scale them both to the volume and to the surface (and of course, if you scale a heat source in your 1/10 scale model down by only 1/10, you’ve just screwed the results up even more).

I’m not saying physical scale modeling is never useful for quantitative data, but for a lot of things, it might be more accurate and easier in the end to just test-model each parameter (How much heat does a 1 sq foot section of wall lose?) separately, then calculate the result for a full-size structure, along with all the other parameters.

7

So, are there ratios or algorithms somewhere I could peek at?

Or better yet, is there some software I could enter location and structure variables?