DIY PHPP Assessment
Posted 31 January 2015 - 06:27 PM
I was happy enough with my modelling to not bother paying £1500 or more for a nice little certificate and plaque from the PassivHaus Institut, and it would be great if we could give others the confidence to do the same.
One reason I've invested in a thermal imaging camera is that I want to use it to show how badly built new houses are, and go around naming and shaming the big developers that build this rubbish. I also want to help people understand how heat escapes from poor design detail and what they can do about it, as well as do some free thermal surveys for anyone that is within reasonable travelling distance, as I'm sick and tired of every energy saving initiative being used as an excuse to create more jobs for "specialist consultants". For most people who own a suitable smartphone or tablet, buying one of these cheap thermal imaging camera add-ons is going to tell them more about their major heat loss areas for a lot less than they'd pay a consultant.
It's about time we put more of this stuff back in the hands of DIY'ers, and halted the growth in specialist consultants.
Posted 01 February 2015 - 03:48 PM
I know I have been selective in my quotations above, but I think they both point to what would be a very worthwhile exercise. Whilst there are many members who have both the technical knowledge and commitment to delve deeply into the world of detailed modelling, I think there are a large number of people who would welcome a generic and simple to complete model (especially if it saves the cost / time involved in doing a PHPP)
I've just been having a look at Jeremy's calculator (inputting known performance figures for my house) and the results come out just over 12% lower than SAP. Jeremy advised that his calculator / PHPP results were virtually the same, SAP being 20% higher. Can anyone else chip in with a % difference?
The interesting variable for me was air infiltration and mechanical ventilation. SAP assumes a mechanical ventilation ACH of 0.5 and I suspect this is where it really falls down. My house appears to have an ACH of 0.6 according to Jeremy's calculator . However, knowing the rate at which I mechanically ventilate my house, I can therefore work backwards and deduce the result of the air tightness test (not required when I built and (in hindsight very foolishly) didn't think it worth the bother / cost of doing an air test). In essence I have a high infiltration rate (calculator suggests 5m3/hr/m2) and low rate of mechanical ventilation (about 0.33 ACH). Ideally the infiltration rate should be lower (although variables such as how exposed the sight is need to be taken into account as I believe this can effectively double the infiltration rate in an exposed area / half it in a very sheltered area) and the mechanical ventilation rate higher (if needed) to get the benefit of heat recovery. The occupancy rate of the house is also important - the quality of air within a house will vary according to how many people are in the house - having tried it in our house, I know that I cannot reduce the ventilation rate / ACH any lower without encountering problems such as condensation and an atmosphere so stuffy (levels of CO2 I assume) that it gives you headaches. It makes me wonder whether these component parts of ACH should be separated in any calculator?
Posted 01 February 2015 - 05:11 PM
Did you balance your MVHR after commissioning?
Posted 01 February 2015 - 05:35 PM
Cost & complexity - the MBC package delivered the necessary u values & air tightness requirements at a very affordable price and minimal complexity (i.e. they design, deliver and erect all the superstructure and a majority of the interior - floors, walls etc).
An ICF build would have required more supervision and there would have been more detailing design (roof, floors etc) required and potentially additional insulation to meet the same u-values. It was also just a shell, another contractor would be required for internal floors (whether timber or concrete), walls etc. Also windows & doors can't be ordered until the apertures are complete vs a factory built system.
I think ICF has a lot of benefits but economically it didn't work for us and since we're PMing the build ourselves (and are no experts) there is more peace of mind with the MBC approach and less professional services (architect, PM, etc..) required.
Posted 01 February 2015 - 11:29 PM
Did you balance your MVHR after commissioning?
Ouch indeed. I was somewhat surprised by this and went back and double checked everything, including the mechanical ventilation rate - turns out I used the wrong figure (too low) - ventilation accounts for 0.45 ACH with the remaining 0.15 ACH infiltration, which suggests 3m3/hr/m2, which I think is closer to the mark / reality given our experience of the house. Interestingly, this is very near the figure I got when working backwards using SAP (2.75m3/hr/m2 infiltration, assumed 0.5 ACH ventilation). Lesson of the day - take care and double check the figures you input.
I've also played about with different levels of heat recovery in the calculator - my heat loss would drop 40% if I had MVHR as opposed to the 0% heat recovery of the exhaust air heat pump system I currently have. Yes, our system was balanced by the company that commissioned my system.