10 most common PHPP mistakes

Posted by on Oct 25, 2012 in Guides | Comments Off

WARM have now certified 21 Passivhaus schemes. We like to see the designer’s PHPP as early as possible so we can perform an initial check. Unfortunately this can sometimes be a painful experience as PHPP offers so many ways to make a mistake, and is rarely forgiving when you do. So I thought I’d go through the top 10 most common (and significant) PHPP mistakes. If you have a project you think you’d like certified we’d be happy to quote for an initial PHPP check as part of our standard certification fee.

1. Entering multiple casement windows in one row

It’s ok to enter two identical window casements in a row with a quantity of 2. But, it’s not ok to enter a two casement window in a single row (with a quantity of 1). PHPP will assume it is a single casement, meaning the proportion of frame will be too low, and the heat losses and gains will be wrong. Every time you do this a kitten dies. Actually that’s not true, but it does spoil my day finding it.

2. Heat loss areas don’t match U values

Heat loss areas for Passivhaus are measured externally. On a section this is a continuous line running around the outside of the thermal envelope. The question is – what is the outside of the thermal envelope? The easy answer is – the outside of whatever is the outer element in the U value calculation. Take a timber frame wall with a ventilated cavity and rain screen, it is the convention to ignore the cavity and rain screen in the U value calculation – as the thermal performance is unpredictable. In this case the outside of the thermal envelope will be outside of the timber wall – not the outside of the rainscreen.

 

3. Altitude not entered

A proportion of the BRE climate data weather stations are based at between 0 -50m, so there’s a good chance your site is higher. Enter the altitude of your site as soon as you start a PHPP to avoid the dismay of adding a kWh to the heating demand figure when it’s too late to change the design.

 

4. MVHR external duct data incomplete

To take account of the effect of heat loss from the MVHR’s intake/exhaust ductwork PHPP needs to know the duct length, diameter, insulation thickness and conductivity. Designers frequently miss the entry of the duct details – which results in no account being taken of the heat loss. The new PHPP version 7 has made this mistake harder to make, whereas PHPP 2007 hid the ‘secondary calculations’ on the ventilation sheet off the right as if they didn’t really matter. If your MVHR’s effective heat recovery efficiency (big green box) is the same as the unit’s quoted efficiency you have a problem, typically it should be a couple of percent lower.

5. Conductivity entered is not the Lambda 90/90

Most SAP assessors don’t have the time or desire to check manufactures claims, but Passivhaus certifiers do – in fact we love it. So makes sure your conductivities are lambda 90/90 values see BBA’s explanation (pdf download). And question other claims too – from 0.55 U value doors (you know who you are), to tin foil insulation and ventilated cavities on the warm side of insulation. If you want to check something, give us a call.

6. Ventilation rates are too low

PHPP frequently calculates a ventilation rate of around 0.3ACH, and warns you if the rate gets over 1.5x the supply air rate. However, this is based on German occupancy numbers. For UK homes, at the smaller end of the scale Part F ventilation rates – even accepting that these are peak rates –average ventilation rates can be closer to 0.5ACH. A nasty surprise, if the first time you see this rate is on the commissioning certificate.

 

7. Multiple MVHRS are not entered correctly

This particular issue is a sneaky one. It caught us out a while ago with unpleasant results. PHPP allows you to assess multiple dwelling units within a single thermal envelope – i.e. a terrace or block of flats. This means that there are likely multiple MVHRs, with multiple sets of intake/ exhaust ductwork. Completing the ventilation sheet as if it’s a single MVHR will not take account of this ductwork properly. PHPP version 7 has added a new (and almost incomprehensible) sheet to deal with this. For those with PHPP 2007 you have to create a work-around. The long-winded version is to derive the efficiency reduction due to ductwork for each MVHR separately- with the correct ductwork lengths, insulation and flow rate. You can then do a flow rate weighted average of these efficiencies and use this for the MVHR with no further reductions.

We have made an add-in tab to do this for you – but haven’t quite polished it enough to release – if enough people ask we will get round to it.

10 most common mistakes in PHPP

8. Shading mistakes

It’s hard to know where to start with this one because I think it’s actually impossible to get the shading 100% ‘right’ in PHPP. It’s also hard to recommend always being very conservative and assuming lots of shading – because this can hide overheating problems. The answer is to read the manual carefully (especially the averaging of reveal lengths) and try to be reasonable, we are unlikely to disagree in this case.

 

9. Overheating nasties

The overheating check in PHPP is basic and shouldn’t be used on non-residential buildings or buildings which go too far outside the ‘standard dwelling’ model. Common sense should also apply. PHPP distributes gains across the floor plan evenly. If your building has high solar gains concentrated in one room there might be problem which is being averaged-out in the overall assessment. Furthermore, it is easy to apparently solve any overheating problem by assuming a high enough ventilation rate. Try a stress-test of:

• minimum user operated summer shading

• MVHR operating in summer at its background rate (HX bypass if fitted),

• no natural vent during the day

• 0.1ACH due to night ventilation.

 

If the design overheats frequently with these parameters it might be time to rethink or consider the future occupant’s behaviour carefully.

 

10. Over reliance on solar gains

Passivhaus designs which rely on large quantities of glazing and resulting solar gains are risky from two points of view. Firstly overheating is much likely.
Secondly, the design relies critically on the window sizes, glazing specification and shading. Small changes in any of these, such as finding out laminated glass is required, can give large increases in the heating demand – and large headaches squeezing the design elsewhere. Try adding our results sheet into your PHPP and checking (on the bar graph heat balance at the top) what proportion of total losses are offset by solar gains? If your solar gains are in the region of one third of the total heat losses then in our opinion, and overheating check allowing, this is fairly safe. If they are much greater, and especially if they are nearing 50%, you need to have a serious think about how healthy an approach this is.

10 most common mistakes made in PHPP

I hope you’ve found this helpful and have not found anything nasty as a result!

John Trinick – WARM: low energy building practice