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

[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

[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

[Mark Siddall]

This link from elsewhere on the forum has a few interesting notes on permissible/realistic Ψ Values (courtesy of Nick, David O and Peter Warm).
https://aecb.net/forum/index.php?topic=602.msg2060

NOTE: The AECB Standards don't appear to offer any advice on minimum Ψ Values (at least I haven't spotted it). As a consequence I've done some digging on the the internet. The results suggest that the Ψ Value for a Passive House, and therefore presumably Gold standard, has to be =< 0.01 W/(mK)! (In the UK the convention is measure heat loss based upon the internal area rather than a specific weakness in the thermal design. I'm not sure whether the same method of measurement is used in the PH standard. As this will have an impact upon internal/external corners etc it would be helpful if someone could clarify how the PH standard operates with regard to this matter?)

As there are no AECB standard details available as yet the thermal modeling of details, in order to derive the Ψ Value, seems to be critical. Not only because its the only way to be sure that energy performance will be achieved, but also to ensure that the designs can be achieved in a suitably economic fashion i.e. using UK Robust Detail Ψ Values could result in additional costs due to additional “un-required” (though never the less useful) insulation being specified.

To this end can anyone verify Rod's Ψ Value modeling and calculation methods?

Mark

P.S. Just had a look through some of my library. My earlier posting on the 0.5m isotherm zone was a little off beam. The 0.5m dim relates to how you mimic thermal bridging in ESP-r.
BS EN ISO 10211 Thermal Bridging suggests that a 1m plane of symmetary should be used at the respective detail i.e. the legs of the “L” formed by the ground floor/wall junction should be 1m.

[Mark Siddall]

It would appear that the required PassivHaus Psi value of 0.01 W/mK relates to external locations rather than internal locations. This is (as I understand it) contrary to UK methods whereby the convention is to measure heat loss based upon the internal area rather than a specific weakness in the thermal design.

Having contacted the BRE they have confirmed that they will be looking into this over the coming months as they prepare technical guidance on Passive House construction.

A new version of BRE's Information Paper IP01/06 was updated in February 2007 and states the reasons why internal locations are used, but there is currently no guidance on if there is a direct relationship between the two values (so that we can work out a conversion factor) – this is something which they anticipate will be tackled in the near future.

In the meantime within PHPP the BRE are using external Psi values where available (as this is what the package has been designed around), if thermal bridging information is not available then they assume accredited construction details values and whilst this does not give true benefit for the construction type (you have to work a bit harder elsewhere) it is considered to be a pragmatic approach in the meantime, but obviously something which needs remedying.

Mark

[David Olivier]

The AECB is preparing both.

PHPP says that if you measure areas using external dimensions (defined) you may ignore small linear non-repeating thermal bridges with a psi value of < 0.01 W/mK (many listed by PHI).

David.

[SimmondsMills]

Peter Warm has worked out a conversion method for turning psi internal into psi external – used for the Silver Detail design guidance document (soon to be published!!!)

[David Olivier]

The method for doing this is also set out in the PHPP Manual and one can use PHPP to convert one to the other.

David

[Nick Grant]

I've split this topic and moved the excellent discussion of window installation details to here:

https://aecb.net/forum/index.php?topic=1015.0

[Mark Siddall]

Thermal Bridging Catalogue by Swiss Federal Energy Office:
http://www.lesosai.com/download/Warmebruckenkatalog_f.pdf

Mark

P.S. Helps if you can read French, though the diagrams and tables almost explain themselves. (If you don't do French then try Googling Bablefish.)

[Mark Siddall]

The thing that confuses me is these negative psi values. I appreciate that they are used in PHPP but why? I've been thinking about it and this is the best that I have considered to date: –

For example a “normal” external corner of a building is has both a positive internal psi and a negative external psi. In terms of building physics I appreciate the fact the external corner has a greater surface area, that as a consequence the average distribution of energy across this area therefore lower than through a planar wall and that for this reason the internal temp of the corner inside the building is, due to geometry, lower than the surrounding areas of wall. One could be forgiven for thinking that this imposes a heat load on the heating system. But does it? If the psi value is measured externally it appears that, relative to the corner, the insulation per m2 increases. On this basis you can appreciate that the corner will lose less energy per m2 than is suggested by the internal psi value.

Whilst the suggestion is that the a negative psi signifies additional insulation properties (reduced heat loss) if we consider a “negative” corner (corner flexed in toward the living space) we find that due to geometry the conditions have become the inverse to the description above.

This cursory description of the physics suggests that depending upon building geometry the heat load is either reduced when measured externally or increased when measured internally. It can be seen that these two methodologies for various geometrical conditions can lead to some quite considerable differences in thermal bridge assessments. As we tend to have more “normal” external corners on a home than “negative” corners, depending upon how the psi value is calculated, the calculated/predicted performance of a building can be affected quite seriously.

Whist appreciating that one should just use the methodology of the calculation system that is required PHPP or SAP/iSBEM. I can safwely say that this matter is most perplexing!

When handling a negative psi value in PHPP do you actually enter “zero?”
Does anyone with more experience have any comments?

Mark

[David Olivier]

Mark

They're negative simply because the actual heat flux is less than that calculated using external areas – i.e., the sum of all the UAs. By contrast, the actual heat flux is more than that calculated on the basis of just the internal areas.

If you measure externally, use good details and assume psi = 0.00 W/mK, you avoid a lot of work on a small building and you still get a precise enough result. To be exact, the result tends to be conservative – rather than representing a chronic underestimate of heat loss, as has been the case in the UK for several decades ever since we started using insulation.

Have you read the PHPP Manual?

David.

[Mark Siddall]

Thanks David.

Since my orignial post I heard that Chris Sanders helped to co-author BR497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors. As Chris and I were in contact a while ago so I emailed him about this positive/negative psi subject. He clarified matters in the form of calcs; see the attached.

Thought that it would be worthwhile to post his feedback here for others to see. Should help to avoid people having the same dilemas as me.

Mark

[Mark Siddall]

Here are new thermal bridge programmes from Building Physics, they are called Heat 2 and Heat 3 (2D and 3D.) They are both cheaper than TRISCO and SISCO.
http://www.buildingphysics.com

Mark

P.S. Heat 2 has was developed for and used on a PH Primary School in Riedberg. (I think that it was PHI people that wrote the software but not sure. PHI definitely seem to have played a part in its development.)

[Mark Siddall]

Some more thermal bridging software and tools:
http://www.kornicki.com/antherm/EN/index.htm

Mark

[Anonymous]

I have recently been looking into the thermal bridging issue and the THERM software. After looking at the excellent work done by Roderick Williams, I wonder why the THERM software is needed. Surely THERM is just a U-value calculator i.e. a 2D finite temperature calculation. Within a standard U-value calculation, the mesh size if sized at the material depth, as the temperature drop through the material is assumed to be linear. Therefore by using a “normal” u-value calculation through various different “slices” of the thermal bridge and weighting them according to their height, effectively enables the thermal bridge calculation to be resolved and then applied on a project. Obviously the limitation is with 2D, however THERM does not carry out 3D calculations either. I may be (and probably are!) missing something fundemental, however I believe you can work through Rodericks worksheets using a standard U-value calculator for each of the slices. Is this right?

[Mark Siddall]

Hi Nathan,
As I see it there are perhaps 2D conditions that could, more or less (i.e. within a reasonable level of accuracy), be assessed shorthand through the use of fractional areas, however there are many complex details that need to be assessed using tools such as THERM. Typical examples are corners (external, internal, eaves, ridge, wall/ground, basements, stepped party walls etc), and structural openings (head jamb, sill, threshold) where not only are fractional contributions play a part in heat loss but the geometric aspects of the building element also influence the thermal performance (positively and negatively).

For example, one boring Sunday I undertook a theoretical study considering the location of *where* you put a window within a wall. I found that if located at the front of the insulation the heat loss is “high”, if central it is “low,” if at the back of the insulation the heat loss is “high” again (forming a U-shaped curve). If the window encroaches beyond the insulation then heat losses begin to sore. I also learn that the adjacency of one material to another (or within a matterial) can have a suprising influence heat loss; another subtle geometric consideration. If you then factor in fractional areas then things begin to get very complicated very fast, consequently no shorthand calculation can really quantify these issues.

So using THERM or another modelling tool is a means to an end. After you've built the model and extracted the heat loss data then you can determine the psi-value with a nice shorthand calculation….but only once you have the heat loss data. I think that the apparent simplicity of the psi-calc is beguiling you into thinking that thermal modeling is not required in low energy buildings. It is.

Hope this helps,
Mark

[Nick Grant]

Hi Nathan

Mark is correct but I'd just like to make the point that it is possible to design even to Passivhaus without ever needing 2D heat flow models or psi value calculations.

The PH convention of using external areas means that as long as you use good details (see CLP and various sources of PH details) you can ignore most psi values.

Situations where I think Therm and commercial variants are useful are:

1. For geeks like Mark and I wanting to try and understand how heat flows thru building junctions.

2. Where refurbishment or other constraints prevent clean thermal 'bridge free' details being used and a fix needs to be found.

3. When struggling to meet the 15 kWh/m2 target for Certification under PH – in this situation some psi values can be demonstrated to be negative and so credit claimed (although the design remains the same).

4. For exploring innovative solutions such as optimising the installation of uninsulated triple glazing to achieve low heat loss with less expense as demonstrated by Mark and Chris Herring.

5. For making pretty pictures for reports!

Cheers

Nick

[Mark Siddall]

Hi Nick,
As Nathan enquired into the relevance of THERM, rather than thermal bridging calcs, I limited myself to that topic. I agree with you that you could design buildings using CLP/PH detail and avoid the need for psi-value calculations. However, whilst working on a PH scheme I have found that if it surprising how often, due to non-energy performance requirements (your “other constraints”), that CLP and PH details are not suitable (and sometimes even subtle changes can have surprising results). This is not to discredit the details, I am simply noting that client driven non-energy performance requirements can lead to some very different solutions that require bespoke assessment. (It is after all a little unfair to expect the CLP details to cover all situations…. even the well funded Accredited and EST details do not do this.)

Examples of details that have required bespoke assessment include:
a) Details resulting from the topography (terraced houses with terraced (that is stepped) levels at floor and roof (here the detail required to achieve zero bridging will pretty much change at each different level change…. We tried to standardise level changes where possible.)
b) When seeking to achieve level access and paving/patios adjacent to the perimeter of the properties (whilst also addressing NHBC requirements to have the sole plate 75-1500mm above ground level!)

As a consequence of the 13 details I examined:
5 of the details assessed were completely unavoidable i.e. they were site specific or were influenced by non-energy considerations.
1 was a result of poorly/undocumented detailing to assist with airtight construction (hybrid balloon frame)
3 helped to verify how the PH windows should be installed so that we actually achieve our goal (construction tolerances could be determined, and I learned about some subtle things about the installation details that are less obvious than the PH details sometimes suggest.)
4 were strictly developed to enable us to mine the design for energy performance. This was a result of changes to the design, namely requirement for laminated glazing to achieve Secured-by-Design, degraded energy performance (we lost 0.1W/m2K on the Ug). Mining the psi-values has helped to ensure that we keep the built costs down over the project as a whole….

In the end by assessing all the details we have achieved roughly a 35% improvement upon EST Best Practice (~70% improvement upon Accredited details). The SAP/Code Assessor on the project was staggered by how much improving the little appreciated Y-value could have such a significant impact upon performance.

Having learned the lessons we should be able to reduce the psi-calcs on future schemes. I agree that building refurb is one area where psi-calcs will pay for themselves many times over. ….I also agree that geeky interest helped spur me into undertaking the psi-calcs. 😉

Mark

[Tom Foster]

4. For exploring innovative solutions such as optimising the installation of uninsulated triple glazing to achieve low heat loss with less expense as demonstrated by Mark and Chris Herring

Sounds intruiguing! What's this?

[Mark Siddall]

Tom,
Have a look through the AECB Windows thread in the “Materials and products”
The concepts are discussed repeatedly.
https://aecb.net/forum/index.php?topic=36.0

Also have a look at “thermal bridging around windows with wrap around external insulation”
https://aecb.net/forum/index.php?topic=1621.0

Mark

[Nick Grant]

Thanks Mark, was looking for the threads for Tom when I spotted you had posted!!!

Absolutely agree with all your points re Therm, just wanted to make the point for general browsers that a cookbook approach is possible, especially if it is possible to keep things simple, which as you point out, is always a struggle!

Nick

[Anonymous]

Mark,
Many thanks for your reply. My concern is that THERM is only a 2D approach not 3D – therefore ground floor corners etc require a 3D software. A 2D approach looks at the construction in slices (inside to outside), therefore by breaking the bridging element down into slices, a simple hand calculation would provide the same detail as THERM. Unless THERM does carry out a 3D approach… I probably need to look into THERM in more detail. Actually, maybe the hand calculation is just a 1D analysis.

[Nick Grant]

Yep, hand calc is 1D!

Corners largely get dealt with (approximately) by using external measurements and I have personally only seen 3D used for columns going through a floor.

If you look at some of the graphical output from Therm you will see the heat flow is not 1D!

Typically when calculating psi values, Therm is used to work out the 1D heat loss thru the plain bits either side of the detail – eg I beam walls. Is already drawn so quick to do and ensures calculation is the same as for the 2D bit. This won't make much sense unless you have doe psi value calcs and I am too rusty to explain clearly!

Nick

[Mark Siddall]

Nathan,
As Nick says long hand is 1D and THERM is 2D (the “2D” bridges that I mentioned earlier are fractional bridges in 1D heat flow; they have actually 2D or even 3D affect upon the energy flow).

Yes a 3D corner, say two walls and a floor, requires 3D analysis for the most accurate of calculations. Luckily in this situation 2D analysis is sufficiently accurate for the most part and provided that you have the 2D analysis the additional heat loss/impact upon surface temps is marginal.

If you think that you can calculate 2D heat loss using your 1D methodology then here is the challenge. Take a 90 deg corner, internal dims 1m x 1m, one 300mm wall has a U-value of 0.1W/m2K the other is 250mm and has a U-value of 0.12W/m2K. Just to keep things simple there are no physical thermal bridges in either of the wall element. Using your methodology calculate the internal psi-value without using THERM.

I'll declare right now that I can not calculate the psi-value using a 1D method. Hats off if you can do it.

Mark

P.S. If you do manage please post the result and the methodology on the forum. I'd be very interested to see how you managed. (I'm not a physicist so would not have a clue where to start.)

[Anonymous]

Mark,

Sorry I had not seen your last post.

I was thinking (incorrectly) that the hand u-value calc was a 2D analysis, which it obviously is not. I have recently been looking at the THERM program in more detail using the suggestions above.

[Mark Siddall]

I recently observed that Internal psi-values result in slight geometric underestimation of the heat loss. This results from the fact that linear psi-values consolidate geometric and structural heat loss into a singular value.

In the case of internal psi-values the linear length of the psi-value is determined by the length of the internal datum, the result, say at a walls external corner, is a small geometric square of wall that is not accounted for. Thus when two internal psi-values meet, wall/floor junction, there is a small cubic volume of unaddressed heat loss. Conversely the use of external dimensions will account for this cube and may result in a small over estimation due to the overlapping linear geometries (erring on the side of safety).

All in all, at a building scale, this additional heat loss may be relatively trivial (depending upon geometric complexity) but this discrepancy would show up when 2D and 3D analysis of a wall/floor junction are compared. I've not managed to check this with a 3D tool. Has anyone else noticed/been thinking about this?

Mark

[Anonymous]

Hi Folks,

I have used Heat 2 & 3 but found out quite quickly that Heat 2 cannot cater for complex junctions. In fact, I have to find out the thermal coupling coeficient and refer to the BRE's calculation conventions in order to work out the Ψ factor.

Also, I found using Heat 2 and 3 a bit tedious. As it is so easy to move blocks without realising it. However, I have now quickly moved on to use Physibel software solutions – Trisco 2D.

Has anyone used any Physibel products?

Regards

Graham

[Author]

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