Larch Corner Is a Passivhaus Wooden Wonder That Shows How We Should Be Thinking About Carbon

©. Mac Eye Projects

Mark Siddall of LEAP measures and calculates everything, thinks about it, and then calculates it again.

There are architects who can design and build but can’t write; there are architects who can write but can’t design or build very well. Mark Sidall of LEAP (a Lovingly Engineered Architectural Process) writes and designs, so we get a better explanation of his new Larch Corner project than we usually pry out of architects, and without a bit of jargon-filled architectese.

Then there are Passivhaus architects who design to hit the numbers but would insulate with baby seal fur if it did the job, not really caring about the sustainability of the materials used. The Passivhaus criteria are designed around results and are open-minded about what materials you use to get there. But since the Passivhaus standard was created, there has been an increasing understanding that Upfront Carbon Emissions, the carbon dioxide released in the making of building materials (and which I think is easier to understand and measure than how much carbon is “embodied”), are as important as the operating emissions.

Larch corner interior

© Mark Siddall

Mark Sidall understands UCE, and has built Larch Corner almost entirely out of natural, regenerative materials.

A celebration of the best modern timber engineering techniques, Larch Corner is a timber lover’s paradise that lies in the heart of the English countryside. Almost every fibre of this contemporary 3-bedroom single storey home has its origins in sustainably sourced timber – not only reducing emissions during processing and manufacture but curtailing carbon emissions during use. In a time of climate breakdown, Larch Corner demonstrates the diversity of timber and its uses. From structure to insulation, cladding to light fittings, it not only shows how environmental damage can be minimised but offers a clear indication of how it can contribute to more restorative actions, whilst letting the human spirit soar.


Larch Corner living area

© Larch Corner living area/ Mark Siddall

When talking diversity of timber, this house has it all. The structure is made from Cross-Laminated Timber, the ceilings from spruce, the wall insulation from 17 inches of wood fibre, and of course the exterior is clad in wood, Siberian larch. Both to meet Passivhaus standards and to eliminate the risk of rot, the house has to be really airtight, and it is:

By careful design, using CLT as the air barrier and exemplary workmanship the Air Permeability is 0.041 m3/hr/m2@50Pa. This, the UK’s most airtight home, is 244 times more airtight than Building Regulation requires. Gather all leaks together and the Equivalent Leakage Area is 196mm2 - an area that fits on a 1p coin [bigger than a US penny, smaller than a nickel].

There has been some pushback recently regarding wood construction, questions about whether it is as wonderful as its promoters say it is at avoiding Upfront Carbon Emissions. For instance, wood is burned to kiln-dry the lumber used to make the CLT, but burning wood has usually been considered carbon-neutral. I have never agreed with this, as it took decades to sequester that carbon and we are releasing it in one big carbon burp by burning it. Mark acknowledges this, and that that the subject area is “messy.”

When it comes to accounting for carbon emissions timber products are complex. This is because trees store carbon within them, which of course remains sequestered once the lumber becomes construction grade timber. To my mind this sequestration, important as it is, is a byproduct of forestry - not construction - so needs careful consideration ...yet some people play a numbers game.

In the end, Mark adjusts his calculations of carbon emissions because “the age of processed trees is unknown and premature felling negates the benefit of sequestration.” I have never heard of anyone doing this before, and yet the results are still impressive.

Building with wood may not be as perfect as the industry has been saying (which is why I have made the case that we should be designing to use as little of it as possible, and question whether Mark should have used wood framing instead of CLT) but using renewable, regenerative materials is still better and greener than the alternatives.

Comfort, comfort and comfort.

Larch Corner Bedroom

© Mark Siddall

I often quote Elrond Burrell about how the three most important things about Passivhaus are comfort, comfort and comfort. But getting it right is a challenge, and there are worries about overheating in summer. Mark has engineer Alan Clarke on the job, so the odds are in his favour.

There are two critical times of the year that need to be considered, summer and winter, and there are a range of factors that influence your perception of comfort, which include air temperature, surface temperature and drafts. When you are designing any super insulated low energy home one of the most critical considerations is summer comfort - if you get that wrong, you’ll create a pressure cooker. Meanwhile incorrect design, specification or workmanship can cause an energy performance gap.

We all have to learn from Mark Siddall.

Larch corner corners

© Mark Siddall

Mark Siddall reminds us that “in 2018 the IPCC stated we have 12 years to limit climate breakdown; therefore, a building’s lifecycle carbon emissions, including embodied carbon in materials, construction and maintenance are very important.”

That means we can forget about Life Cycle Analyses that talk about the 50 or 100 year amortization of Upfront Carbon Emissions. It is what we are doing RIGHT NOW that matters. That’s why Larch Corner is such an important project; Mark Sidall is measuring everything, both operating and upfront, then he is questioning and adjusting his calculations to account for the latest thinking. He is writing about it, sharing it, getting us all to think about it and question it.

Realistically, this is what every architect and engineer has to be doing with everything that we do. It is not easy, and we may not get everything right, but it’s the only way we will actually make a difference.

If you have suitable low-carbon transportation that can get you to Warwickshire, UK, you can visit Larch Corner during 2019 Passivhaus Open Days on 29 and 30 June, with registration closing on 23 June.

UPDATE: Mark advises that "if you miss the Passivhaus Open Days you can always watch the 3-part documentary series on another of this projects - the award winning Steel Farm. Simply go to" I will be covering Steel Farm shortly.