Build a Thermos Flask

A Thermos flask keeps your tea hot all day because the vacuum it creates stops heat conduction. By achieving airtightness, you can do the same for your house. Imagine what your heating bills would be like if you lived inside a Thermos.

Part-L energy regulations stipulate how airtight new houses must be. In June (2022) the leakiest they could be reduced from 10 to 8 m3/(h·m2)@50Pa. In English, that means a new house built to 2022 regulations must be 20% more airtight than one built to the previous regs. In one hour, for every square metre of roof and wall surface area, you’re allowed to lose eight cubic metres of warmed air through leaks in the building fabric. For a new Passive House you’re only allowed to lose just over half a cubic metre, 0.6m3/(h·m2)@50Pa to be precise.

Part-L also now insists EVERY new house is tested instead of a handful of each archetype. Previously, a crafty developer could build 100 houses and only need to test 10 of them. Those 10 could be tested then fixed, tested then fixed, until they scraped through. Meaning 90% of the houses on the estate were untested and quite often left leaking like a sieve.

A 200m2 property with 2.4m ceilings (480m3) is currently allowed to lose around 3,840 cubic metres of warmed air every hour. Most of the time that air is warmed by burning fossil fuel, either directly as gas/oil, or indirectly by generating electricity. It sounds ridiculous because it is. If it were a Passive House, it would only be allowed to lose around 288 cubic metres of air

The cost of air leakage

Ask anyone who really knows the energy calcs what they think of them, and they’ll probably tell you most are not fit for purpose. SAP calcs are inaccurate because they still penalise you for heat recovery*, EPC calcs are inaccurate because they’re based on SAP and they assume older houses still meet older regulations, and ‘MCS-compatible’ heat loss calcs don’t credit you for airtightness or heat recovery. For anything like accuracy, you need to use the Passive House Planning Package (PHPP).

PHPP is the most accurate energy modelling tool in the world. It factors in the property location, orientation, amount of shading, fabric u-values, glass, wind speed, heat recovery efficiency, and most importantly of all – airtightness. To illustrate the benefit of the latter we’ve produced four PHPP models for the same 290m2 family home. All factors are the same with one exception, the airtightness achieved.

Model 1

Airtightness to meet the Passivhaus Enerphit standard,  i.e. it’s very airtight. With air permeability of 1.0 the predicted annual heating demand is 19,391kWh.

Model 2

Airtightness considered pretty good and an ideal candidate for MVHR. With air permeability of 5.0 the predicted annual heating demand is 25,615kWh.

Model 3

Permissible under pre-2022 regs but considered leaky to those in the know. With air permeability of 10.0 the predicted annual heating demand is 33,402kWh.

Model 4

Exactly the same as model 3, only with a higher wind speed to simulate an exposed location. With wind speed set to 20mph the predicted annual heating demand is 95,724kWh.

The conclusion

If you pay a little more attention to detail during the build and manage to achieve air permeability of 5.0 instead of 10.0, it’ll save almost a third of your heating costs forever. Unless the wind picks up, in which case it’ll save almost two thirds. At today’s rates for this house that’s about £100/month saved on gas if the house is sheltered, or £300/month if it’s exposed. Same house, different airtightness.

* Regarding SAP 10.2 – The BRE can’t test MVHR units with ducting greater than 150mm, i.e. units suitable for large properties. People normally end up with two systems when one bigger unit would suffice, saving money and energy (and carbon). SAP also encourages the use of branch ducting instead of the more efficient, lower pressure radial ducting. In our experience, SAP is responsible for many of the overheating issues we face because it encourages designers to specify heat-absorbing glass on the wrong elevations. It’s a limited compliance tool at best, not a tool for designing a low energy properties.

Avoid guesswork

There are two ways to approach airtightness. One way is to fit cheaper insulation products like wool, cavity batts, or recycled newspaper, then use expensive membranes, tapes, grommets, and sealant to make it airtight. The other way is to use expanding foam insulation that seals every available square millimetre, providing airtightness by default.

The difference is the first option has insulation that deteriorates over time and an air barrier 0.5mm thick. The second option has insulation that doesn’t deteriorate, and an air barrier the same thickness as the insulation, because the insulation is the air barrier. Thin air barriers may stop heat convection, but not heat conduction.

But whatever the airtightness strategy, the best ‘bang-for-buck’ energy investment you’ll ever make is an intermediate air pressure test mid-way through the build. Don’t leave it until the end to find out you have leaks, test and retest before you start plastering so you know you’ve already passed. Most of the leaks are behind the plasterboard anyway.

Theory is one thing, but nothing beats measurements.

Now our insulation will make sense
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If you don’t achieve your design Air Permeability at the final air pressure test your SAP calcs fail, meaning you may not get sign-off. That’s why we test airtightness mid-way through the build before the leaks are hidden behind plasterboard
Eliot Warrington
Founder and MD
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