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Heat Loss Form Factor?! H.E.L.P!

This is a summary of Elrond Burrell's excellent article explaining why complex building forms make it harder to achieve the Passive House standard when compared to a simple and compact form.

The Heat Loss Form Factor is one way of measuring the efficiency of the surface area of the thermal envelope. The thermal envelope being the main area through which a Passive House building loses heat, one can start to see why the efficiency of the form factor can come into play.

The Heat Loss Form Factor is the ratio of thermal envelope surface area to the treated floor area (TFA). This is essentially the ratio of all surfaces that can lose heat (the thermal envelope) to the floor area that gets heated (TFA).

In other words, the Heat Loss Form Factor is a useful measure of the compactness of a building. And the more compact a building is, the easier it is to be energy efficient. Conversely, the less compact a building is, the more insulation will be required for the building to be energy efficient.

The Heat Loss Form Factor is a measure of compactness and an indication of how much insulation will be required to achieve the Passivhaus Standard.


Another easy analogy to remember is that buildings lose heat like bodies do. The second law of thermodynamics states that heat flows naturally from an object at a higher temperature to an object at a lower temperature. Always hot to cold, regardless if its a building or a body, the laws of physics stay the same.

Heat inside a building will gradually make its way to the outside, when it is colder outside. The heat moves out through the thermal envelope, particularly in a Passivhaus building where the ventilation system recovers the heat that would otherwise be lost through ventilation.

The purpose of the thermal envelope, therefore, is to reduce or slow down this heat movement as much as practical. However, the more surface area the thermal envelope has, the more surface area there is for heat to escape through.

We all know instinctively to wrap our arms around ourselves when we are cold. This instantly reduces the surface area of our “thermal envelope” that is directly exposed to the cold.

In water safety there is a specific term for the right posture to adopt: the Heat Escape Lessening Posture a.k.a H.E.L.P. This is effectively the same as Heat Loss Form Factor, but with a more memorable acronym.

If a building resembles a human body with four limbs spread out, the heat loss surface area is maximized. This kind of building will be more difficult (i.e. expensive) to make energy efficient.

If a building is compact and closer resembles a human adopting the Heat Escape Lessening Position, the heat loss surface areas are minimized. This kind of building is easier (i.e. cost effective) to make energy efficient and to meet the Passivhaus Standard.

It’s not just about the overall shape of a building either. A building can have a fairly simple massing, but if it has a lot of recesses or protrusions in the thermal envelope, the surface areas soon add up.

Heat Loss Form Factor = Heat Loss Area / Treated Floor Area

“Heat Loss Area” is the total surface area of the thermal envelope. This is the sum total of all the areas of insulated and airtight floors, walls, soffits and ceilings or roofs. It is measured at the outside face of the thermal envelope (ignoring ventilated cavities and rain screen cladding). This is more conservative, and therefore, more accurate than measuring to the inside face of the thermal envelope.

“Treated Floor Area” is the floor area of the rooms within the building that are heated. It excludes the areas of internal partitions, doors, stairs and unusable spaces.

The Heat Loss Form Factor is a number generally between 0.5 and 5, with a lower number indicating a more compact building. Passive House buildings aim to achieve 3 or less. Once the Form Factor is over 3, achieving the Passive House Standard efficiently becomes noticeably more challenging.

It is important to note that it is the thermal envelope surface area that matters; this might not be the same as the building envelope. For example, if the thermal envelope is at ceiling level this reduces the Form Factor compared to if the thermal envelope follows the roof pitch. However, it is also worth bearing in mind the complexity of the different options. Insulating and making the roof airtight might be easier than the ceiling, especially at the eaves junctions with the walls.

Heat Loss Form Factor is a good design indicator of how energy efficient a building will be.

To achieve a form factor below 2.5 on a stand-alone residential building is quite difficult. As you can see from the above illustration, with a TFA of 200m2, a compact square 2-storey building still has a form factor of 2.9. With larger buildings, however, it is possible to get an even better form factor.

Heat Loss Area and U-values have a linear relationship

If the Heat Loss Area is doubled, the required U-value will be half (twice as stringent) and vice versa. In simple terms, if the Heat Loss Area of one option is double that of another option, the insulation will need to be twice as thick!

A lower Heat Loss Form Factor requires an exponentially higher (i.e. less stringent) U-value of insulation in the thermal envelope.

The Compounding Effect of the Form Factor

Heat Loss Form Factor actually tells you more than just where the building energy efficiency of a design is heading. It also gives an indication of where costs are going.

If the Form Factor is high, thicker (or high performance) insulation will be required. And if the Form Factor is high, the thermal envelope has more surface area and will require more insulation. So an inefficient form needs thicker insulation and more of it and becomes more expensive.

It doesn’t stop there, though. Thicker insulation can require different construction detailing and more complex (i.e. expensive) structural solutions. In turn, these can lead to increased thermal bridging, which in turn requires more insulation to overcome…

The good news is that it works in reverse also! If the Form Factor is low, thinner insulation is required and less of it, along with positive knock-on effects in the construction and structural detailing, and costs.

The Heat Loss Form Factor is a key opportunity for architects and designers to understand the energy and cost implications of fundamental design decisions. A quick check on the Heat Loss Form Factor will quickly reveal the difference that the form and massing choices of a design can make.


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