Choosing thermal insulation solutions for comfort in summer

03 June 2007 - Comments (0) Construction

Choosing thermal insulation solutions for comfort in summer When it comes to comfort in summer, the best thermal insulation materials are the ones that delay the entrance of outdoor heat indoor. We explained before that thermal phase lag is the parameter measuring that capacity. It is related to the thermal inertia (C), the thermal conductivity (λ) and the thickness of the material. In this article, we explain how the thermal phase lag affect the choice of insulating materials in the different parts of one house.

In which part of the house, will thermal phase lag make a difference ?

If we look at the thermal insulation of one house, we have basically 3 parts : the ground floor, the walls and the roof (excluding windows that are not concerned by this discussion).

Ground floor

The thermal insulation of the ground floor is never exposed to high temperatures, even in summer, because the temperature of the ground is pretty much constant. Thermal phase lag is therefore not an issue.


For the walls, thermal insulation being usually applied on some heavy construction materials such as concrete or clay blocks, the small thermal phase lag of the insulation material will usually be compensated by the high thermal inertia of the construction material. So, as long as the walls have some sort of thermal inertia, the thermal phase lag of the insulating material should not be an issue. If the walls are made of lightweight materials, such as lightweight concrete, wood or prefabricated panels, the resulting small thermal phase lag could lead to overheating and discomfort in summer.


Remain the roof, where the thermal insulation material will be exposed to the strongest heat in summer because of the direct exposure under the sun. Materials used for the roofing, such as clay titles, do not have much thermal inertia and do not offer much protection against heat.

The thermal phase lag of the insulating material used for the roof is therefore very important for comfort in summer. For materials with similar thermal conductivity (λ) and for equal thickness, it means you should prefer those insulating materials with the highest thermal inertia.

Of course, besides the choice of the thermal insulation material, other strategies will help to avoid oveheating in summer, such as ventilated roof, shading with vegetation and radiant barrier.

Phase lag for comfort in summer

In the hottest summer days, outdoor temperature is bearable in the morning, reach a peak early afternoon and becomes uncomfortable, then, become bearable again in the evening.

In order to maintain the indoor temperature bearable all day long, it is necessary to delay the entrance of heat long enough to pass those uncomfortable hours. In order words, the thermal insulation should have a thermal phase lag long enough to pass those hours.

How much phase lag do you need ?

A phase lag of 1 or 2 hours, that you would typically get with 10 cm of glass mineral wool, will not be enough to avoid most of the outdoor peak temperature to get indoor. A phase lag of 8 to 10 hours, that you would typically get with 20 cm of cellulose or wood fiber, will protect the indoor from reaching the peak temperature because by the time the heat wave crosses the material, the outdoor temperature has already dropped.

Temperature amplitude ratio

The attenuation of the peak value obtained for a given phase lag is called the temperature amplitude ratio. For instance if the peak of temperature outdoor is 35 degrees celsius, 90% amplitude ratio would give a maximum indoor temperature of 31.5 degrees celsius. The bigger the phase lag, the bigger the amplitude ratio will be.

Comparison of thermal insulation materials

Thermal phase lag being directly linked to the thermal inertia of thermal insulation materials, it is clear that, lightweight insulation materials, will be the least suited to protect from the hottest summer days.

The table below shows a comparison of various thermal insulation material.

 CelluloseRock woolGlass woolExpanded polystyrene
Density ρ (kg/m3) 20 to 100 30 to 90 15 to 50 12 to 20
Thermal conductivity λ (W/m.K) 0.037 0.035 to 0.040 0.035 to 0.040 0.035 to 0.040
Specific heat capacity c (J/kg.K) 1946 850 850 1210
Thermal inertia C (kJ/m3.K) 38.9 to 194.6 25.5 to 76.5 12.8 to 42.5 14.5 to 24.2
Thermal diffusivity a (m²/s) 13 90 90 26
Temp. amplitude ratio for 10cm thickness 77% 92% 95% 98%
Thermal phase lag for 10 cm thickness (in hours) 3.4 1.9 1.5 1

Comparison of various thermal insulation materials regarding behavior in summer. The values are coming from a manufacturer of cellulose so they should be taken as purely indicative (source Thermofloc).

The best protection will come from a thick layer (20cm at least) of dense insulation materials such as cellulose or wooden fiber, which are certainly not widely available if at all in Serbia.

The worst protection will come from polystyrenes: despite of the fact they have the smallest thermal conductivity, they also have the smallest thermal inertia.

A compromise could be found using 20cm of high density rock wool.


When choosing thermal insulation solutions it is not enough to look at the traditional parameters such as λ or U-value. Thermal phase lag has also to be taken into account, at least for the roof. It will determine the performance of the material to prevent peak of temperature outdoor to get indoor in summer.

Finding the best thermal insulation solution is therefore a compromise between small thermal conductivity (λ) and high thermal inertia (C). That best compromise will be obtained with cellulose, wood fiber or high density rock wool.

Of course, you can always buy an air conditioning system, but it costs money, it consumes electricity, and does not provide an healthy indoor climate. It is a complete nonsense to consume lot of energy to heat up the house in winter and again to cool it down in summer. Better design of the house and chosing better materials can reduce dramatically the energy needed in winter and stop the need for air conditioning in summer.

For comfort in summer, without air conditioning system, it is important that walls are made of materials with high thermal inertia and that the roof is insulated with a thick (20cm at least) layer of material with high thermal phase lag.

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