What’s an insulation material?

With more and more people talking about building energy efficiency and passive houses, it has become more and more common to hear about insulation.

In this article, we explain what an insulation material is, to try and shed some light on the topic, and bust some myths.

The energy efficiency of a building, as well as its comfort level, are based on the quality of its thermal envelope. This statement is valid for both winter heating, and summer cooling.

The best way to achieve that, is to decouple the indoor environment from the external conditions, so that the internal conditions (temperature and humidity) can be controlled with a minimum amount of energy.

Schematic diagram of an efficient thermal envelope, with the insulation layer (yellow) wrapping around the heated/cooled portion of the building

The compactness of the envelope remains of primary importance. Besides that, the decoupling of the inside from the outside is achieved by inserting one or more insulation layers within the structure of the envelope (walls, roof, openings and so on). The envelope needs to be airtight too.


In the first place, you need to understand what you what to insulate from.

The main goals for insulation are:

– protection from cold weather;

– protection from hot weather;

– protection from noise.

To improve the energy efficiency of a building, the structures that require insulation are the ones that define the thermal envelope, which enclose the heated/cooled part of the building.

Andrew Michler’s passive house near Fort Collins, Colorado

In this article, we address the first point of the list: insulation to protect from cold weather. We’re going to address the issue of protection from hot weather and from noise in our future articles.


Insulation materials can be grouped into two families: materials that insulate via conduction (that is, materials that delay the heat transfer by having poor conduction level), and materials that insulate via radiation (materials that have low heat emission via radiation). This article is dedicated to the first group.

Insulated glass units (double-pane or triple pane glass) work in a quite different way, and we’re covering them in a separate article.


Fondazione perfettamente isolata da isolamento continuo
Cellular glass insulates via conduction: as a panel (black, vertical), and gravel

In all materials that insulate via conduction, it’s not the material per se to insulate, but the air that is trapped in it (or the lack of air, in case of vacuum panels).

Overall speaking, an insulation material needs to be lightweight, because it has to contain a lot of air.

ANIT insulation
Heat transfer through a material, via a combination of conduction, convection and radiation (ANIT)

The “ability” of a material to insulate is described by its lambda value (which actually represents the opposite, that is, the ability of the material to conduct heat).

The insulative properties depend on the kind of material (cork, rock wool etc.), and on its density. This can range from 20 kg/m3 (1.25 lb/ft3) to up to 250 kg/m3 (15 lb/ft3), depending on the final use of the material.

Lambda and density
The lambda value of EPS depending on its density (MAICO)

The lower the lambda value, the more insulative the material. Keeping this in mind, here are some typical lambda values (*) for insulation materials, ordered from the most performing to the least one:

PUR: 0,024 W/mK (0,014 BTU/h*ft*°F);

EPS with graphite: 0,031 W/mK (0,018 BTU/h*ft*°F);

Rock wool: 0,035 W/mK (0,020 BTU/h*ft*°F);

Wood fiber: 0,039 W/mK (0,023 BTU/h*ft*°F);

Cork: 0,045 W/mK (0,026 BTU/h*ft*°F).

(*) Please note that the values listed here are declared lambda values. We’ll have a chance to explain the difference between declared lambda value and design lambda value in a specific article.


Generally speaking, a material is considered to be “insulation” if its lambda value is around 0,040 W/mK (0,023 BTU/h*ft*°F).

A common myth, widespread among non-professionals (at least in Italy), is that brick or stone masonry is per se insulative. With an equivalent lambda value of 1,4 W/mK (0,809 BTU/h*ft*°F), stone masonry “loses” 35 time more heat than an average insulation material.

Just to comply with current Italian energy requirements, the total thickness of such a stone masonry wall should be over four meters thick (13 feet); it should be over nine meters thick (29 feet) to meet passive house requirements.

With its very thick stone masonry walls, this prehistoric Nuraghe in Sardinia is probably one of the few stone buildings that can be considered “insulated”


Another common myth, quite common also among industry professionals, is that wood is an insulation material. With an average lambda value of 0,13 W/mK (0,075 BTU/h*ft*°F, for fir or spruce wood), wood conducts heat three times better than an insulation material.

Wood is not an insulation material.

As a secondary myth, many believe that structural elements made of wood do not cause a thermal bridge when crossing an insulation layer.

Thermal bridge wooden balcony
The thermal bridge caused by a wooden balcony. On the right, the isotherms

Can you see the isotherms, on the right? Can you see how they bend where the wood slab crosses the insulation layer? That’s a thermal bridge.

As pictured in the finite element analysis above, a wood element crossing an insulation layer causes a thermal bridge. Sure, the entity of such a bridge is several times lower than it would be if the element was made of metal or concrete.

In case of highly efficient buildings and passive houses, however, you need to take all these thermal bridges into account: the more efficient the building, the more precise the calculation needs to be. The thermal bridge above does not cause any problems as far as internal temperatures (fRsi = 0,94); however its PSI value (0,0247 W/mK) is quite substantial in a context of energy efficient buildings, and cannot be overlooked in the energy balance.


  1. Nice article. I’ve been enjoying reading your blog lately and like your very scientific feeling approach. In a industry dominated by “green” marketing its nice to read something that is educational and informative without needing to push a product. I also like how much you guys use and discuss therm.

    I would note however, that the idea that adding 30′ of stone masonry or concrete would get to passive house standards is a bit misleading. That only deals with the conductance of heat through the material. This does not take into consideration the specific heat capacity of the material. Which I think is the big flaw in how passive house is modeled, no consideration to thermal mass. The 30′ thick stone structure would equilibrate to the average of day and night temperatures over time, and to meet the passive house standard you may have to dump an extraordinary amount of energy into the masonry to keep the mean radiant temperature in a comfortable range. I would Imagine this to be true even in hot equatorial climates.

    1. Hello Jason and thank you for the support!
      I’m afraid I don’t fully understand what you mean about thermal mass. If you can explain your position better, I’m happy to reply.
      My position on thermal mass is that it does not influence the total heating demand, because the heat flow in winter is semi-constant, from inside to outside. It does however reduce sensibly the cooling demand, as the the direction of the heat flow changes over the time of the day. PHI has published a specific paper on this a while ago, and this is why thermal mass is a secondary parameter in ISO13790 (and PHPP). The more “dynamic” the heat flow, the more thermal mass counts. ISO13790 (and PHPP) have their strengths and limitations.
      That being said, I’m a huge fan of thermal mass, and that’s why we decided to use heavy brick masonry for the passive houses for my family that we are currently building in Cavriago.
      Another big advantage of thermal mass is that it increase the time constant of the building, making the house more resilient to extreme weather events.
      Our article on thermal insulation is at the beginning of a long series of definitions/explainations: we are going to go in depth about thermal mass, transpirability, comfort, windows, IAQ… you name it.
      About Therm, we use a different software, Dartwin, because it’s more user friendly and allows you to save a lot of time if you use it frequently. It does become attictive. Are you also in Colorado? We can play with it together sometimes.

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