Skip to content
Home » Blog » Getting heat-pump ready: Blog 2 – The thermal energy audit and air tightness test

Getting heat-pump ready: Blog 2 – The thermal energy audit and air tightness test

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.

Photographs of the front door so dark blue areas around the windows demonstrating extreme cold coming through into the home.

The side door, with cat flap, is even worse

Photographs of the side door show dark blue areas around the catflap and the base of the door more generally, demonstrating extreme cold coming through into the home.

We’ve insulated the loft, but not the loft hatch

Photographs of the ceiling side by side show the loft hatch, with the thermal image showing dark blue areas around the hatch edges demonstrating extreme cold coming through into the home.

The old double glazing at the back shows substantial heat loss where the seals are no longer good.

Photographs of the rear glass doors show dark blue areas around the base of the door demonstrating extreme cold coming through into the home.

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.

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.

Pictures illustrate the corner of the living room ceiling by thermal camera which is dark blue showing a small section not insulated, 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. 

Photograph from inside the home shows a red piece of fabric with integrated large fan structure, stretched over the door frame, with two men standing looking at the structure.

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!

4 thoughts on “Getting heat-pump ready: Blog 2 – The thermal energy audit and air tightness test”

  1. Interesting. We live in a 1930s’ semi in Tring and have just installed an air source heat pump successfully. We did not do an air tightness test; as I understand it, these are only really needed if you are hoping to achieve Passivhaus standards. We also didn’t use an architect at any stage! But all is working well and our house is now warmer than it has generally been in the 40 years we have lived in it. We did have some additional cavity wall insulation put in (in original external walls that had not been insulated by insulated extensions) and we also replaced all the uPVC double glazing, of various ages, with new metal-frame double glazed windows with much better thermal insulating values. We now only have gas for our cooker hob and are considering eventually replacing this with an induction hob – but waiting, a) because of the embedded carbon in a cooker that still works and b) to see whether there are power cuts over the winter, which might mean we are glad of the hob for some hot food (and the wood burner for heating the living room) – not ideal from an emissions point of view I know!

  2. Thank you, very interesting, I guess lots of us are looking to go down the same path. One pedantic point (bit of background) , the air leakage was measured as a function of the area of the external envelope (/m2), whereas an air change rate is a function of volume of the house (m3). . If the numbers matched , that was a bit of luck , on say a similar sized bungalow they then might not. Now onto your next blog, again thanks

  3. Realy interesting! I am thinking of going down the same road with my just post-war semi but also incorporating an MVHR system.
    As you have a gas hob I would assume there would be an air-brick in your kitchen? Was that left unblocked for the airtightness test? Presumably for safety (carbon monoxide, etc.) this would have to be maintained until you go for the induction hob. I’m in the same boat with my cooker, which is fine but also has a gas hob. I have cooker hood which vents outside so I’m thinking about converting to filtered internal instead.
    Would it be possible to put me in touch with the folks who did your air-tightness test?
    many thanks!

    1. helenburridgesustainablestalbansorg

      Hi there John. The author has shared the architect’s contact details: all tests were through the same organisation. Good luck with your work!

Leave a Reply