The Surprising Benefits of Passive Houses

What it is…

It’s a machine for conserving space heat energy.

The building is a fairly modest two story house set into the south western slope of Edinburgh’s Corstorphine Hill in Scotland. Its precision timber frame was factory constructed offsite, and assembled on a thick aerated concrete slab. All penetrations (doors, windows, etc) are designed to eliminate thermal bridging. All joints and penetrations are taped and sealed to ensure airtightness. Ventilation is maintained by a PAUL Wärmerückgewinnung GmbH novus 300 mechanical ventilation system injecting pollen-filtered fresh air into bedrooms and public rooms, and extracting it from wet rooms for heat recovery and exchange. Windows in the public rooms and bedrooms are full height, full width, high efficiency sliding units. Wall and ceiling thickness increases are limited by the use of high performance insulation materials. Hot water is supplied by a gas fired on-demand high efficiency combi-boiler.

What it’s like to live in…

The most noticeable features of the home for me are air quality and air comfort. The ventilation system delivers several complete changes per hour of filtered and heated fresh air. It maintains average CO2 below 600ppm (well below the threshold we sense as “stuffy”) and humidity between 40% to 60%.

There is no heating system, although there is a small preheater fitted to the air injection system for when the house has been cold soaked after prolonged periods of absence. Under normal conditions, heat from occupants and cooking is conserved to maintain an air temperature of around 21 degrees year round, with any excess from solar gain rejected by the ventilation system.

A more subtle source of comfort comes from the fact that all interior surfaces, including windows, are warm. This reduces the effect of the skin’s blackbody radiation toward cold surfaces, particularly when we are wet, that can cause us to feel chilled even when the air temperature is relatively warm. As a consequence, the air temperature can be further reduced.

During the day, the large windows admit plenty of light. At night, lighting is provided throughout by low energy LED units.

What it costs…

I estimate construction costs after recovery of VAT to have been between 10% and 15% higher than a comparable structure built to conventional standards. Significant sources of additional costs included the large quantities of insulation, the higher required specification for the windows, and the mechanical ventilation system and ducting. These were partially offset by the lack of requirement for a heating system.

Renewable gas and electricity is sourced from PurePlanet. Electricity costs are about 50% of conventional—largely, cooking and lighting. Gas costs are about 20%—mostly hot water, and a small amount of heating top up after long spells of subzero weather.

Some surprising benefits…

Heating our home is the single largest non-discretionary energy demand in most people’s lives (certainly at Northern Europe’s latitude). Viewed as a technology, Passive House construction techniques' ability to reduce that demand by up to 80%, at relatively low cost, and with little dependence on fragile support systems, is disruptive, dwarfing the capacity and economics of current “renewable” technologies to supply the equivalent amount of energy. That disruptiveness has significant implications over a range of social policy issues.

Consider these three, for example.

The first is the simple net present value of the avoided energy costs over the life of the building. What’s particularly interesting is to work this out in terms of the annuity I would have had to purchase, say on retirement, to fund the energy cost avoided by the design. This provides a straightforward way of evaluating cost/benefit. On the conservative assumption of 60% energy reduction and 5% per annum energy cost inflation, I worked out I’d need an annuity of about £130,000. While that’s significantly more than the incremental cost of the technology, here’s the real benefit: the value of a house-as-an-annuity-to-avoid-escalating-future-energy-costs is realised the moment the house is complete. The value of a pension-as-an-annuity-to-pay-escalating-future-energy-costs depends—over decades—on (i) the reliability of the financial instruments generating the cash and (ii) the reliability, availability, and cost of whatever network will be supplying energy by then. Neither seem particularly certain to me.

Another interesting feature. By my estimates, it so happens that the cost of converting 20% of Scottish housing stock to this standard would have been about the cost of building its current wind farm stock, and that the energy demand avoided by doing so would have been about equal to the energy supplied by the turbines. A key difference, though: the ongoing maintenance requirements of the housing stock would have been almost nothing. The ongoing maintenance requirements of the wind turbines requires a hydrocarbon powered global industrial manufacturing system, and a fragile supply chain reaching back to tantalum mines in the Gobi Desert. Given the scarcity of capital, and growing propensity for global financial and political instability, there is a reasonable perspective from which its investment in wind turbines might be viewed as a misallocation.

Finally, much of conventional economic evaluation of various energy technologies, formats, and policies starts from the assumption that some form of energy is continuously available and must be provided, and compares the various costs of doing so. But this is a Passive House, and that enables a different set of policy assumptions. Even if Putin crimps the gas pipe supplying Europe in the depths of the next prolonged dark, windless winter, a Passive House occupant is relatively comfortable where others, worryingly, are not (current emergency policy in this scenario will be to shovel the vulnerable into village halls with space heaters). Economics offers no reliable way of pricing the utility of that.

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There are few silver bullets in our energy transformation journey. But I think the radical destruction of energy demand offered by passive housing technology in the single largest non-discretionary application of energy most people have in their lives must be a good candidate. I commend it to you.