Perhaps not surprisingly, the house performed very well in the cooler months of autumn and winter. I have a low tolerance for being cold indoors, so I have been very happy with how the house has maintained an even and comfortable temperature. A quick review of the numbers in the consultant’s report, showed that the predicted heating demand and load were half that of cooling and dehumidification. In practical terms, this has meant that the house felt more passive in the cold months because it required minimal input for heating. Visit the Passive House Database for PHPP data.
The only regular source of introduced heat was the oven and a couple of heated towel rails. We used the towel rails for a couple of hours on most days, mainly to quickly dry the towels, and the oven on 3 or 4 nights a week. As a result, our average daily total power consumption dropped from 22.5kWh in summer to 18.5kWh in autumn (which included a very hot period in March) and 18.2kWh in winter. This was a welcome compensation for the decrease in power that was being produced by the roof-top solar.
The temperature in the house generally stayed between 18-21°c. This was as per the dehumidifier sensor upstairs. There were a couple of times when it dipped to 17°c. The week between the 15th and 21st of June was a good test. There were persistent overnight lows of 3°c to 4°c, with 1.8c° recorded on the 19th. Without heating beyond what I have described, the temperature in the house got no lower than 17°c. If there was sun available to heat the concrete slab during the day, temperatures were reliably between 18°c and 21°c.
The roof is insulated with R4.1 fibreglass batts. They are pushed up to sit immediately beneath the airtight membrane that runs over the roof structure. Above that are a ventilation cavity and the metal roof. The ceiling void is within the thermal envelope, meaning that there is no insulation sitting on the ceiling between the joists, which is where you find it in most homes. I have been amazed at how effective this seemingly simple system has been.
There have been a couple of occasions where ice has formed on the roof overnight. We usually know when this has happened because we hear a reasonable volume of water running through the downpipes, as it melts in the morning. My guess is that heat loss from the house is so low that the temperature of the metal roof drops below the dew point, which allows moisture from the air to accumulate and form ice. This seems to happen even if the overnight temperature and the dew point are a few degrees apart. We have had ice on the roof without any signs of a frost on the ground.
On one of these occasions, ice formed in the channels of the roof and created, what looked like iceypoles, as the sections that were higher than the gutter started to melt and slide down. The indoor temperature on that morning was 17°c. This has been a revelation to me.
Some of the above was reasonably predictable if you are setting out to build a passive house. What I didn’t anticipate was just how much it would improve our lifestyle. It is no longer difficult to get out of bed and have a shower on a cold morning. The bathroom is the same comfortable temperature as the rest of the house. Dressing gowns, winter pyjamas and extra blankets on the bed are a thing of the past; as are cold hands, cold bench tops, cold dinner plates and cold bed linen. If anything, we have tended to slightly open a couple of bedroom windows overnight to lower the temperature by 1°c to 2°c. The gentle ingress of cold night air has made it very comfortable for sleeping and did not impact the core temperature of the house.
I have spent most of winter in shorts, t-shirt and a jumper. Oftentimes, the jumper wasn’t necessary, but put on out of habit. We have all been more active in the house; less time spent on the couch under a blanket or in bed. I have also noticed a change in our diet. The rich and heavy comfort food, soups etc., while always welcome, have almost disappeared. I have enjoyed no longer being cold to the bone and not having to deal with multiple layers. All of this has been a huge positive for my mood and wellbeing.