Allow for mistakes and changes in the course of the design (and build) process. That, in a nutshell, is possibly the best piece of advice we can offer to Passive House designers. It’s based on the many projects we either design, advise on or certify. In a perfect world, you’ll never need to use the following tips. But as is wildly evident, we’re not living in a perfect world, so adding safety margin is prudent. The following suggestions reflect what we have adopted as standard practice as a team.
Build in an extra 1.5 kWh/m2/year heating demand margin at the initial design phase. (We’re talking about an initial design that involves a designPH model and carefully constructed PHPP file. That’s not the same as an initial design made up of a few hand calcs on the back of a napkin.) As we work through the detailed design phase, we step down from 1.5 to 0.5 kWh/m2/year. Obviously all this margin is removed before submission for certification.
Carrying margin like this (usually stashed away as a thermal bridge allowance) means that small design changes that negatively impact on heating demand can be absorbed without rework. It makes the project much less stressful for everyone involved.
It is not sufficient to cover big mistakes or component specifications going awry. When that happens, we do have a few tricks up our sleeve: read on.
Don’t add insulation to the ceiling service cavity at design stage. This means that even late in construction, insulation can be readily added to improve thermal performance.
Similarly, if there’s a crawl space under a suspended floor, consider making it tall enough to allow for an additional 90mm of insulation if it were needed.
It’s not the easiest solution but we did rescue one building’s certification by adding wings onto the slab. These extend out past the slab-edge insulation, increasing the slab’s thermal performance. They are a common technique used on EnerPHit projects.
‘Mathing it out’ is our least favourite solution for a building that’s gone over the certification limits. Calculating all of the thermal bridges in detail, including the negative ones, often yields a bit more margin. This takes quite a bit of time, which costs money, all while not making the building perform any better in the real world. Better to avoid being stuck in this situation.
All in all, the best approach is to carry a bit of margin in the design so you don’t have to resort to applying these rescue remedies. It’s good to know they exist, but I hope you never have to use them.