Ψ Values - Thermal Bridging

[Mark Siddall]

Is anybody aware of any software that will assist with calculating Ψ Values /thermal bridges?

There's a whole raft of guidance, see below, but what is really needed is a simple user-friendly tool to assist the everyday designer, rather than just the researching building scientist. Furthermore, the U-value software I have come across does not assist with non-repeating thermal bridges it simply focuses upon an elemental repeating thermal bridge.

Guidance includes:
EN ISO 9346 (1996) - Thermal insulation - Mass transfer - Physical quantities and definitions
EN ISO 10456 (1999) - Thermal insulation - Building materials and products - Determination of declared and design thermal values
BS EN ISO 10211 Thermal Bridging Pt1 + Pt2
EN ISO 13789 (1999) - Thermal performance of buildings - Transmission heat loss coefficient - Calculation method
EN ISO 14683 Thermal bridges in building constructions. Linear thermal transmittance. Simplified methods and default values.
BRE IP 106 Assessing the effects of thermal bridging at junctions around openings
BS EN 1745: 2002: Masonry and masonry products. Methods for determining design thermal values (London: British Standards Institution) (2002)

Mark

[David OLIVIER]

Mark

Have you tried THERM? Free from the USA, thanks to their enlightened Dept of Energy (Lawrence Berkeley Laboratory, Univ of California).

I used to use FRAME - free from Canada (written by Enermodal Engineering for the govt.) - but may have been overtaken by THERM. Was very user-friendly though.

David.

[Mark Siddall]

David,
I had a play with THERM a while back, very user friendly. I was surprised. The trouble with the software is that it does not calculate the Ψ Values/fRsi in accordance with ISO 10211, or any of the supporting, or related, documents. I contacted LBL about this that they gave the following response: -

“First, we don't anticipate adding the ISO 10211 calculation to THERM in the near future (i.e., the next few years), but I will add it to our features list.

Second, even though THERM doesn't report these numbers directly, THERM can be used to perform the simulations to calculate these numbers. For the linear thermal transmittance (psi value), you need to perform 2 separate simulations, one with a foam panel in place instead of the glazing system.
For the Condensation Risk, I assume that all you need are temperatures are certain points.”

I’m not sure how to use the data from the “2 separate simulations” to derive the Ψ Values. I was, perhaps lazily, hoping for a European version. A point worth noting is that THERM is really designed for designing windows, as a result the library of materials is not so great.

I have investigated the range of software from Physibel (Bisco, Trisco etc.) but each package costs about £2-4.5k, which is a little steep for the use it would get. Physibel did work on Eurokobra (a digital atlas of thermal bridging detail) I have had close a look at this software as well and found that:

·   Though you can customise existing details to some extent it does not allow you to construct new details and then test them. This is a great limitation (this also relates to a number of the subsequent points.)
·   Many of the details offered by the software are more suited to domestic use than to larger developments.
·   A good number of the details are continental i.e. they rely upon concrete frame structures, whereas in the UK the predominant primary structure is steel frame.
·   We tend to find that many contractors prefer to over board secondary steel with cementicious board, or similar, rather than blockwork, again this is not accounted for in the database of details. No details for this form of construction are available. Again this relates to the software not reflecting aspects of the UK construction industry.
·   It is a shame that it does not calculate U values in accordance with BS EN ISO 6946 - Building components and building elements (noted in the User Manual).

(Oh, and by the way if you visit the Eurokobra website, the BRE doesn't distribute the software anymore, EURISOL the insulation trade organisation may still do so but last I heard it cost £250, which means, to my mind at least, it isn’t worth the money.)

I’ll investigate FRAME, though I suspect that like THERM it will not calculate Ψ Values/fRsi in accordance to ISO 10211. If anyone else has any pointers they would be appreciated.

Mark

[Alan Clarke]

I've used Frame, and although it is fairly friendly, to be honest I can never work out how to get started each time I use it again.  I think this is because it is aimed at window analysis and I'm using it to model bits of wall etc as if they were components of a window.

It doesn't do the Ψ values in accordance with ISO 10211 - I'd be grateful if anyone could tell me what standard method this means in practice. 

I have calculated some sort of Ψ values by simply measuring the heat flow per unit internal area of a combined construction such as a corner and comparing it with the heat flow through the two adjoining elements with modelled with adiabatic boundaries - the difference being the extra heat flow due to the configuration of the corner.  I assume the ISO tells you how far away from the junction to model, as that will affect the absolute result.

Alan

[Mark Siddall]

Alan,
You are using a similar rule-of-thumb to me at this time. I calc the U-value for the weakest zone multiply by the internal area that the weak spot covers and estimate the losses. Adiabatic boundaries are assumed. I then tweak the elemental U-value to compensate for the losses.
The biggest problem is that such calculations do not allow for geometric heat losses i.e. window position within wall plane, external corners etc. It is here that really I get flummoxed.

If I recall correctly a thermal bridge impacts upon the isotherms within a region of 0.5m about the thermal bridge. As I say I'm not sure how to include for this distortion in the isotherms.

BRE IP 1/06  gives shorthand version of the calcs in ISO 10211.... but I still get lost....

Mark

[Mark Siddall]

Stamford Brook: Useful papers by Leeds Met which partly discuss Ψ Values and  Thermal Bridging
http://www.aecb.net/forum/index.php?topic=783.0

[Mark Siddall]

It turns out that Europe does have its own THERM/FRAME programme its called WIS.

It has evolved because this year the European Union is introducing CE Marking on insulating glass.  The calculation method for the above program uses a globally recognized ISO formula. However, the EU has decided that a European Norm - EN673 - shall be used for the purposes of the CE Marking.
Therefore, a new program must be used.
It can be found here: http://windat.ucd.ie/index.html

Apparently WIS uses exactly the same raw data (spectral data files) for each glazing layer as THERM. 
WIS is a uniform, multi-purpose, PC based European software tool to assist in determining the thermal and solar characteristics of window systems (glazing, frames, solar shading devices, etc.) and window components. The tool contains databases with component properties and routines for calculation of the thermal/optical interactions of components in a window.
I’ve not had a play with the software so I can’t comment upon whether its user-friendly or whether it can perform RSI calcs and Ψ value calcs, but at least Europe now has a tool that can be used to assist designers in assess cold bridging. In the worst case we just need to get them to develop the software to accommodate the above.

Mark

[Roderick Williams]

Second, even though THERM doesn't report these numbers directly, THERM can be used to perform the simulations to calculate these numbers. For the linear thermal transmittance (psi value), you need to perform 2 separate simulations, one with a foam panel in place instead of the glazing system.
For the Condensation Risk, I assume that all you need are temperatures are certain points.”

Many thanks for finding this software.  I've constructed models of a number of the thermal bridges on my project and I think I've figured out how to calculate the psi values.  As it may be helpful to someone else I've put all the THERM models, a report and the spreadsheet used to tie it together at:

http://www.rjaowilliams.plus.com/tmp/thermal-bridge-calcs/

There is also a model of a jamb combining a Masonite wall with a basic model of a fiberglass window frame, similar to those produced by Thermotech.  Using this its possible to see the interplay between the thermal bridge in isolation and when combined with a frame to look at condensation temps.

[Mark Siddall]

Great work Rod. Thanks for posting that up for review. Looks like you've had hours of fun ;-)

Have you looked at placing windows at different locations within the thickness of the wall? One of the Stamford Brook .pdf files suggests that the optimum position for the window is in line with the insulation (between 100 and 150mm offset from the face of the masonry wall.)*

How do you get THERM to give you the U-value? Its a while since I used the software and I don't recall the feature. Is it just on a simple pull down menu?

Mark

* Paragraph amended 30.04.07

[Chris Herring]

Just coming back to this topic.  Therm can be used to calculate psi values, by doing 2 calculations, one with glazing unit, then one with insulated panel.  Think this is in EN 10077 which I am just trying to come to grips with, also in another paper I found online.  If useful will try to clarify once I have got to grips with it.  Not time to look at Roderick's calcs...maybe he has sorted this?  Will also take a look at the European software, Mark.

[Roderick Williams]

Have you looked at placing windows at different locations within the thickness of the wall? One of the Stamford Brook .pdf files suggested that the psi value diminishes the closer the window (or what ever) element is located towards the center of the insulation plane.

How do you get THERM to give you the U-value? Its a while since I used the software and I don't recall the feature. Is it just on a simple pull down menu?

I looked at a couple of different reveal depths but didn't graph the results.  I was interested in this because with a deep reveal the thermal bridging results seemed fairly high which appeared at odds with the surface temperature staying above 18.9  degrees all the way to the frame  I didn't look into it in detail but guess that this was due to deep reveal adding approx 230mm of additional surface area.  If someone else wants to  model some of the window details from European inward opening windows, with additional insulation on the outside of the frame, I would be interested in seeing the results.

I have already ordered windows from Thermotech with an integrated fitting flange, which dictates the position of the window in the wall so I focussed on how we could get the details with the lowest bridging results.  The airtightness approach my builder uses is to externally wrap the building and then connect onto the door/window frames.  The fitting flanges are designed for this purpose. 

Therm calculates the U values automatically but you need to create multiple models in order to find the difference caused by introducing the thermal bridge.  This is the reason I created baseline models for each junction.  To find the U value for a model  you need to:
* create the model,
* click on the BC button to assign boundary conditions to the model
* the first time you need to select the exterior boundary, by clicking on the beginning and ending edge, press enter and assign the exterior boundary condition
* do the same for the interior but as well as selecting the boundary condition also set the U factor surface, I typically used frame but I don't believe this is critical
* run the simulation
* click on the show U factors button
* then plug the U values from both models and the junction length into the first page of the spreadsheet which will give the thermal bridge  [It took me a while to realise that Therm was working out the energy lost though the model and then scaling this up to a U-value.  Ie if the model was only 500mm long it would multiply the results by 2.   But as the thermal bridges are quite localised this process gives a higher answer than it should.  I confirmed this by extending one of the junction models to 1m long and the U-value was lower even though the junction remained the same. ]

I'm going to try to get the ISO document from the library and see how closely I can follow its approach using Therm. 

Chris:  I'd skip the calculations and just have a look at the report, there are two pages of text and 8 pages of detail drawings with overlaid isotherms. 

http://www.rjaowilliams.plus.com/tmp/thermal-bridge-calcs/thermal%20bridge-report-03-04-07.doc

Rod

[Mark Siddall]

Rod,
Interesting what you said about the Thermotech window and its fixing detail.  I think that this has something to do with the R2000 standard, if memory recalls, concerns about cold air gathering on the cill and ultimately causing a down draft, thus undermining comfort and greater condensation risk.
I'll see if I can find information on this and report back at some point.

Mark

[Chris Herring]

Mark
Had a fairly quick look at the WIS software.  Doesn't seem to do what Therm does, but one can take results from Therm and plug them into it.  Or have I missed something?

[David OLIVIER]

The Thermotech fixing detail is aimed at more standard North American walls which are quite thin. One wouldn't ideally use such a detail in a house with thicker walls designed to minimise thermal bridging. I did my modelling about 10 years ago using FRAME.

David.

[Mark Siddall]

Chris,
I have just had a quick look at WIS myslef. WIS is run through MS-Access so it seems that it's more of a calculation tool than 'modeling' tool. As you say you can import THERM models.
I was informed by a chap from Soutwall Technologies that WIS was similar to THERM, perhaps he ment this is just with regard to calculations rather than modeling....
Oh well, back to the search for the ultimate Psi value calculation tool....it seems that Rod's got the best methodology to date.

Mark