Psi from THERM

[Peter Gunning]

Thanks to Peter Warm and his THERM settings and libraries, I now have my first working model which appears to be correct and putting our sensible results.

I am now in the PSI from THERM pw30 excel spreadsheet and have successfully copied out external and internal U-factors into S and T 16 which have transferred to the appropriate boxes. What do I put into U 16 to give me L2D ?

I am hoping that once L2D is in place the rest of the sheet will self fill but would be interested to know whats next if this is not the case.

Any help or advice would be appreciated.

[Ruairi]

Hi Peter

Well my first advice would be to do Pete’s Therm course if you haven't done already....But, what kind for junction you are looking at? It would also be useful to have a screen shot of your excel sheet completed so far and maybe the detail your are assessing?

R

[Peter Gunning]

Hi Ruairi

I intend to do the course at some stage but the next one is not until May. I am looking at a 90 degree corner in a timber frame wall.

I attach a screenshot from excel which has a drawing in it and the report from Therm. I have had a go at a mathematical calculation which brought out a psi of 0.00818 which I suspect is quite wrong so if a can work through the speradsheet I would be more confident in the result.

Peter

[Ruairi]

Hi Peter

Ok. You need to break down the basics of the  psi calculation into two parts. In this case the psi value is the difference between the total heat loss through your corner junction and the heat loss through sections of wall either side not including the corner. I will assume you have the correct settings in Therm with regard to boundary conditions etc.

First, you need to model a section of the wall without the corner in Therm,  1m long is the usual convention, call this wall A . Do not include any of the additional timber associated with the corner. Simulate in Therm to get u-factors, then go to > file >report. Copy and paste all the text from here, (check all U-factors are recorder as total length) into the first column in the spreadsheet, column S. You then repeat this process for the piece of wall the other side of the corner. As this is the same as the 1m section you just competed you can copy the results from the same Therm file into column T, call these u-factors Wall B. Now you construct a new file in Therm that contains the corner junction plus 1m of wall either side, call these u-factors wall C. This is the L2D2, simulate in Therm and copy the results in to column U.

The psi value  = wall C – (wall A + wall B).  Check through all the boxes in the spread sheet and the calculation should complete for the internal psi value.  Adjust the length to calculate the external psi value in the last box.

Hope this makes sense

R

[Peter Gunning]

Excellent, seems to make sense, thanks Ruairi.
I will give that a go.

[Nick Grant]

great manual Ruari!!

I'd echo your point about doing the course. I had been using Therm for a few years but still learnt a lot on the course.

Pete's sheet is great but it's good to do the long hand calculation (as Ruari has outlined) as well so you can get a feel for what psi values are about.

nick