This blog series by Sustainable St Albans trustee Catherine Ross follows her journey, figuring out how to get off gas heating and the steps to consider before getting a heat-pump in her 1930s semi-detached home. Blog 1 introduced the context. This post will share the process of heat surveying, air tightness testing. Our final blog will share the resulting action plan along with indicative costs.
The thermal imaging survey
Once we had decided to see if our house was thermally efficient, it was on to the tests… On one level, the thermal imaging survey just reconfirmed things we already knew, but seeing it all laid out really brought it home.
In the photos, the orange shows warm areas, and the blue cold areas; these are where heat is being lost.
On the day of the test, we needed to turn our central heating up high for several hours beforehand, to ensure a big temperature difference between indoors and outside.
The front door showed evidence of substantial heat loss.
The side door, with cat flap, is even worse
We’ve insulated the loft, but not the loft hatch
The old double glazing at the back shows substantial heat loss where the seals are no longer good.
At the front, the new windows are doing well, but the dark purple patch shows lots of heat loss because of a cold ‘bridge’ where the internal wall meets the external one.
We must have missed some bits when we insulated the loft. The blue patch shows an awkward neglected spot, where the roof slopes right down and is hard to access.
This was a professional survey, carried out by the architect. You can do the same yourself by borrowing the thermal imaging camera from Sustainable St Albans. The cameras are available from Oct-March each year (when the outside temperature is low enough for the cameras to work well).
Once we had these images, the next stage was an air tightness test, to help the architects determine a set of recommendations for how to create our thermally efficient home.
The air tightness test
As the name suggests, the airtightness test helped us identify how leaky our house is.
The testing equipment was installed on the main door; a big fan inserted in an adjustable fabric frame which blocked the whole door-frame. Air was pumped out to depressurise the house, causing air to flow in through the cracks and openings of the house, due to the pressure difference. (A similar test can be done by pressurising, rather than depressurising the house. This tends to exacerbate leaks, however, because any gaps are stretched wider. With our depressurising test, gaps tend to be drawn closed.)
The equipment measured how many air changes there were per hour, with the doors and windows shut.
Before the test, the worst culprits were sealed up; the chimney, the cat flap, the shower fan. This means the results are over-optimistic.
What number are we aiming for?
- Modern building regs require a new house to be no more than 10 m³/h/m²@50pa. (10 full changes of the air in the house, per hour.)
- The Passivhaus standards require a figure equivalent to less than 0.6 m³/h/m²@50pa. But we don’t want to go that low; we’re not aiming for a Passivhaus, just for one that will be comfortable, heated by a heat pump.
- For this, the architect said we wanted the results to be somewhere between 3 and 5.
How did the house actually perform?
- The airtightness test revealed a result of 6.3 m³/ h/m². Not too bad!
- However, remember that the main door, chimney and all the obvious vents and cat flap were sealed. Therefore, our house is only really a ‘6’ once those problem areas are addressed, in the action plan.
Our next and final post will share the results of the full Recommendations report . We really hope our experience can be useful for other people. Once we’ve installed everything, and gone through a winter with the new heating system, we’ll write another blog to let you know how it all went!