Passive design

Now I will come clean here and say that during the initial planning process I only intended to build a well insulated extension to our existing chalet bungalow. I designed a scheme where the roof would come off the bungalow, and with an extension to one side would add an extra floor on top and re roof. The whole house would be timber clad to cover over the poor quality and mixed brickwork on the ground floor and give a cohesive appearance so it wouldn’t look a mess. This was going to double the useable space inside the house without increasing the footprint too much.

Planning permission was granted and I set about further costings and detailed design. Building control and the structural engineer were happy that the existing foundations were ok to take the extra floor as it was going to be a light weight structure and we were all set to go.

It was at this stage that I decided that due to the compromises in the layout caused by reusing the existing ground floor, massive compromise with the thermal performance of reusing half a 1950’s built house with little insulation and of course the extra 20% I would be spending on VAT I decided to change plan and get permission to knock the house down and rebuild. To keep things simple with planning I decided to keep the outside appearance of the new build almost exactly the same, and rework the internal layout with minor adjustments to the windows. I put in for planning and without too much drama got permission for my replacement dwelling. The council gave me a condition for 10% renewables, which no one in planning seemed to actually understand. 10% of what? I wasn’t too worried as I had already planned to build a house far in excess of building regs with at least some solar pv on the roof which I was sure would far surpass the condition.

My original scheme that I had planning for had already followed the basic guidelines of window placement for passive houses, but I was aware I had far too much glazing to the west which would be taking in views over adjacent fields. I was not willing to loose these views with my new build to meet the passive house criteria so needed to look at approaches to mitigate any overheating risks, especially in the early autumn when the sun is quite low, but can still be quite strong. More on that another time.

Constructing a passive house

There are a number of ways to go about constructing a passive house, but the easiest and most commonly used route is some kind of timber frame structure with a cladding of your choice on the outside, mounted on a super insulated foundation. I have built and project managed timber frame, timber clad constructions before and really like the aesthetic so decided timber frame with timber cladding was the route for me.

The most common method of timber frame construction is to factory build framework with rigid insulation fitted between the studs and then assemble on site. This is what I had done previously, but to gain the levels of insulation and airtightness needed it would required further insulation fitting on site and a level of attention to the air tight membranes that is beyond most frame erectors. After speaking to a number of timber frame companies it was clear that most were not interested in putting in the extra effort required at first fix stage as it would slow them down to much.

I also looked at SIP panel construction where insulation is sandwiched between 2 sheets of osb board and then craned into place on site. This is a good construction method for gaining good airtightness, but I would still need to site fix further insulation internally to meet the required standard.

Both of these methods rely on large amounts of oil based rigid foam insulation, which is great for keeping the heat in, but due to its light weight nature can allow spikes in temperature and overheating, which can be an issue in super insulated houses. People talk about ‘thermal mass’ to regulate heat, which it seems is quite an over simplification of the subject, but a foam house definately has very little.

The issue seems to be not just ‘thermal mass’, but  Decrement delay and thermal buffering which is further explained here. What I needed was a construction type that would allow the transfer of heat to happen more slowly, evening out any spikes in external temperature.

So where too next. I didn’t want to live in a foam box with a high chance of overheating on a summers evening so some more research was required.