The blackout-proof home: Passive House with high time constant

(leggi l'articolo originale in italiano)

Last week, our region (Emilia, Italy) was pounded with what locals are calling "The Big Snow". With it came very tangible proof of just how fragile our infrastructure is, particularly our electrical grid.

When my husband wrote this article, originally in Italian, FOUR DAYS after the February 6th Big Snow, more than 1000 homes in the province of Reggio Emilia alone were still without electricity, leaving 4,000 people to brave the cold, many without sufficient heating. How can we fix this problem?

In a blackout, the first thought that comes to many people's minds is "Ok, I could fix this problem by adding PV panels and a storage battery, in order to make my home more independent from the grid".  This reasoning is not entirely incorrect - this scenario would grant you more independence from the grid. However, when designing and installing a PV array and storage, you generally design it to be able to optimize energy consumption under normal circumstances (i.e. when you are connected to the grid), not during unpredictable and severe weather that cause blackouts.

During a blackout or in the case of power failure, most electrical storage systems will allow for a limited number of autonomous hours, therefore not constituting a comprehensive solution to the problem.

We'd like to take this opportunity to explain in non-technical terms an important characteristic of the homes we design -

the time constant.

Before being tested, as in this most recent, blackout, how often did you ask yourself:

If I shut off the heating, how many hours my house will stay warm?

This depends on two factors: (1) how much heat is stored in the structure of your home, and (2) how that heat disperses, in other words the thermal losses.

(1) Available stored heat is basically that which is stored in the first 10cm of your structural components (walls, slabs, etc), measured from the inside. (I say "basically" because if the house is insulated from the inside, for example, only the drywall or plaster inside the insulation would count.)

(2) Thermal losses are all point where heat disperses from the house, through walls, slabs, windows, cracks, etc.

The ratio of the (1) available stored heat and the (2) thermal losses gives us the "time constant" of the building.

What's the best time constant for a building? Depends on the final use of the building.

For a building that is in constant use, such as a home, it's advisable to have a high time constant (for example a masonry home, capable of storing a lot of heat, and well insulated from the outside).

For a building only used on the weekends, like a vacation home, it could be better to have lower values (in other words, have a structure that is well insulated but stores little heat on the inside, so it is quicker to heat up).

In order for the mass of the building to supply heat in the case of a winter blackout or power failure, that mass needs to be in direct contact with the environment that needs to be heated. (This is why it's best to have the insulation on the exterior of the wall in masonry or reinforced concrete structures, whether they be new constructions or renovations.)

If the mass of the building is separated from the inside by a layer of insulation (as is the case in many wooden houses or reinforced concrete structures with double formwork in polystyrene), the mass is not capable of releasing the stored heat to the environment and, therefore, the time constant remains low.

In a blackout or power failure, the longer the TIME CONSTANT, the longer the house stays WARM.

In addition to the storage ability of the mass of the building, another fundamental necessity in seeking a high time constant is the reduction of heat losses - avoiding the dispersion of heat to the outdoors via the walls, roof, slab, and more commonly the windows, doors, and cracks in the building envelope. Depending on the building, air leaks through cracks and holes in the building envelope can account for 10% to 30% of the total heat losses in a typical building (source: Passivhaus Institute + CasaClima).

To have a building that maintains a comfortable temperature, even in the case of power loss, it is necessary that it is well insulated and free from air leaks. One international standard that represents this achievement in construction is Passive House, which we explain more in this article.


Let's take a look at an example, two Passive Houses currently in construction phase in Cavriago, Reggio Emilia, Italy:

If I shut off the heating in January, with an external average temperature (day + night) of 0°C, how long does it take the house to go from 20°C to 18°C? 

  • In a typical house, not well insulated: 3.5 hours;
  • In a wooden Passive House: 10 hours;
  • In a masonry Passive House: 20.5 hours.

These times are based on a stripped down simulation that considers mechanical ventilation with heat recovery (85% efficiency, with incoming air at 16.3°C), but does NOT consider in any way heat gains from any internal source (such as people or appliances) or from any solar heat gains. So, it's really comparing apples with apples as much as possible.

Translated to a real life scenario, a Passive House remains warm many days longer, thanks to the free heat gains from people and the sun, and the fact that it loses hardly any of those gains.

This shows that, beyond the obvious advantages in terms of comfort and energy savings, a Passive House is also a guarantee of reliability in exceptional cases of emergency. This standard covers both new buildings and renovations / deep energy retrofits, as long as the necessary precautions are evaluated according to the specific needs of the project.

Other benefits of a building with a high time constant are:

  • The heating season is shortened considerably: for a Passive House in masonry, in climate zone E (Reggio Emilia), the total days of heating are approximately 140 rather than 183 (the heating needs to be turned on approximately November 10th, instead of October 15th);
  • The internal temperatures are more constant throughout the year, thus dramatically decreasing the demand for cooling in summer (provided that the windows are shaded sufficiently).

Mariana Pickering

Emu Systems, 6145 Broadway, Denver, CO, 80216