How does thermal modelling in IES-VE compare to PHPP?

[Anonymous]

For the Energy in Buildings module of my MSc we've been asked to use IES to design improvements saving at least 30% energy compared to the baseline case for a domestic building.

Does anyone have a view on how IES and PHPP compare and what parameters drive any differences in the results?

Thanks
Steve

[Nick Grant]

Steve

This is a good summary of PHPP and comparison with dynamic models:

http://passipedia.passiv.de/passipedia_en/planning/calculating_energy_efficiency/phpp_-_the_passive_house_planning_package?s%5B%5D=phpp

I have not used IES but have worked with Alan Clarke on a number of non-domestic buildings now where we have used PHPP and the M&E consultants have used IES for Building regs and to check overheating. My crude assessment would be that IES performs precise minute by minute calculations on some crude assumptions whilst PHPP applies a tuned but simple degree day calculation to more carefully considered assumptions and building data. If one model is using 3W/m2 gains and the other 8 then the answer will be very different.

PHPP includes some very carefully considered assumptions and demands the information that has been found to have the greatest impact on energy balance. So window frames and glazing are entered in a lot of detail and there are cells to enter critical ventilation duct lengths and insulation.

What I really like about PHPP is that you can look under the hood and see what the equations and assumptions are rather than getting an answer from a black box. If you want to can look at the losses and gains from each element or each window orientation – even each window if you want. Also it is really quick to change say the window frame specification, glazing, U values etc and get an immediate result making it a great design tool.

As it is an Excel (or open office) file you can add sheets to describe building geometry or generate graphs of energy balance etc.

Whilst PHPP is not recommended for the checking of overheating in non-dom buildings which will have concentrated gains and complex cooling strategies, I have seen IES predict no problem when common sense and then evidence shows there is a problem.

The danger with any model is that it is easy to forget that it is just a dumb tool. Buying a set of expensive spanners doesn't mean you can rebuild a Ferrari engine.

[Tom Foster]

From my limited experience of Tas (IES equivalent) it may model the physics from scratch, which on the face of it seems superior and more versatile than the dumb spreadsheet that PHPP is. But in Tas it's not possible to input much complex detail – it's quite a broad-brush thing. Whereas PHPP i understand goes big on inputting all the significant edges and bridges that make all the difference. I'm inclined to believe that PHPP will be more accurate and versatile than Tas, within its defined limits, because within those limits it's a very well validated empirical calculator.

However, I understand, PHPP gets outside its limits with external shading, horizons etc. This is because PHPP isn't all that interested in useful solar gain esp in deep winter (when horizons are critical), because solar gain is seen primarily as a summer overheating issue, with only (almost accidental) account taken of winter use of the solar gain that remains after most of it's been shaded out. This is a big missing piece in PHPP, which seems to be because of ideology based on 20yr old research – things have changed.

Mind you, Tas's head office is also uninterested, but I'd say Tas is well capable of modelling horizons, by clunky workaround until head office rethinks it. I'm told IES is the same.

[Anonymous]

Thanks for the comments and the link.

IES seems to have a bewildering amount of detail in some areas and it is hard to see what's critical to get right and what can be left as defaults or estimated. Fortunately this is only a course assignment not a someone's dream home!

There's seems to be quite a lot of hacking/workarounds needed (but then we've only had minimal training on using this).

Examples
– I can override thermal bridging defaults, but only for broad classes of component (external wall, floor etc.)
– although IES supports adding inert gases to a double glazed cavity, the package crashes, so I fudged it by increasing the resistance of the cavity to produce the correct overall U value (adequate for modelling)
– IES applies a mandatory minimum infiltration rate of 0.25 ach, so to model better airtightness I'd need to inflate my base case to show the energy saving possible from more careful construction …

Anyway, I'm looking forward to using PHPP when I get the time. Only a month to go now on my course!

[Nick Grant]

Tom

I know where you are coming from and wish you luck but for the rest of us I think that whilst limited, PHPP does consider shading in some detail albeit using simple geometry.

It is possible to override this and put in your own shading value. The resultant gains help offset glazing losses in all orientation although only South gives a net useful gain. The summer shading considers reveals and overhangs which reduce useful gains but then factors in moveable shade.

I'd question how much precision is worth doing as trees grow and people build buildings that then shade our dwelling. What PHPP is good (enough) at is modelling the shading due to reveals etc which can inform design decisions.

[Lois Hurst]

Although many years later, I have the same question as Steve. Is there an updated link to the passipedia PHPP summary and comparison which Nick Grant posted?

[David Edwards]

Hi Lois,
Here’s an updated link to the Passipedia article cited by Nick:
https://passipedia.org/passipedia_en/planning/calculating_energy_efficiency/phpp_-_the_passive_house_planning_package

Regarding input of external shading, designPH 2.0 (expected later this year… 😉 ) will have a much more sophisticated system that will hopefully allay the misgivings that some people have about the PHPP shading inputs…

Dave

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