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graeme m

Passive Certification?

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Jack. Here's where we are. (sory I haven't had time to read all the replies yet.)


I've done several renovations, large and small extentions with and without contractors.

The house has been designed by a Passive experienced Architect.

MBC to supply the passive foundation and the frame, the designs are currently with MBCs engineers. Work on construction Mid to late Nov (I believe).

Roof is standing seam zinc as is first floor cladding.

Ground floor walls render or brick slips or something that won't turn green (we are 100 metres from the shore, losts of moisture and SW winds).

Solar roof panels.

The above is pretty much a given.

Quotes from pasive window manufacturers and those close or better but without certification.

Heating built into MVHR or I have a two year old Valiant condesing boiler from the demolished house.

The plans are posted elsewhere on here.


Sorry pushed for time this morning.



Edited by graeme m

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On 09/09/2017 at 16:58, JSHarris said:

The problem with blind adherence to a particular performance standard, as if it may be the saviour of the planet, is that some models fundamentally work to such a fine level of performance detail as to be nonsensical in terms of how people will actually live in such houses, or how other local factors may affect real-world comfort and performance.


@SteamyTeas point about the neuroscientist working to a 5% confidence level is apt, except that I would say that we probably don't actually even need that sort of confidence level for a theoretical house performance modelling standard.


I would argue that a house performance model doesn't really be very accurate at all, as there are so many factors that any model like this will fail to model well, and making the model ever-more complex (which is exactly what has happened with PHPP over the years) is just an exercise in futility, in practical terms.  My view is coloured by having produced a very simplistic heat loss model, and then having been surprised as to how close it comes to predicting real performance, or, dare I say it, how close it comes to matching PHPP heat loss predictions (as others here have found).


Take our build as an example.  I used a couple of models, plus the mandatory SAP thing.  Both SAP and PHPP have an over-heating risk model, neither showed an appreciable risk of over-heating for our local climate data.  I went to some pains to get accurate local data, too, looking at the plots on the Met Office historical data site and interpolating to get what I thought would be the best fidelity I could from the available data.


In reality, it quickly became clear that both SAP and PHPP are very deeply flawed when it comes to dealing with a house that is partially set back into a South-facing hillside, and near the bottom of a steep valley with much lower than typical wind speeds for the area.  Luckily this first became clear during the ground works, when we had temperatures up towards 40 deg C in the corner where the big retaining wall was being erected.  At that point I made the decision to re-check the data and the models, and realised that the temperatures we were seeing that July were much hotter than the maximum predicted by the Met Office data.  The impact this had on the house was large, it massively increased the effective solar gain and decreased the heat losses.  I made some on-the-fly changes, but was restricted to only being able to change the MVHR from a passive system that had originally been planned to a system with a built-in air-to-air heat pump, to provide cooling.  I also found that we needed to add additional measures after the house was completed in order to reduce the heat gain still further, by fitting heat reflective film to the outside of some of the glazing and incorporating cooling into the slab.


How big was the error in the PHPP prediction?  Around 30% or so.  I should stress that PHPP as a model is fine, as far as it goes, but that the unforeseen microclimate we created made much of the modelling in such fine detail open to question.  Why model something to, say, 5% confidence, when a simple and unforeseen external climate difference can create a 30% error?


On the plus side, the microclimate effect is a definite positive in the heating season, as we get more winter solar gain than anticipated and slightly warmer local air temperatures, too.  I've now had two PHPP certified people look over the house and data, and both have taken on board the sensitivity of the modelling process to very local conditions.  Both admitted that they were unaware of the extent that microclimate could have on modelled performance.

Micro Climate isn't something I'd considered. The south coast Bognor to Littlehampton is quite unique. The Isle of wight seems to effect the cloud mass and send half west to Chichester and half East to Littlehampton, it can be raining at both but yet in the middle from Bognor to Elmer it's Sunny. I think better speak to the PH designer.

By the way has any one heard or dealt with "The Healthy Home"?

Edited by graeme m

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