Thermal insulation with limited permeability

logo_smallWind is known to bring a lot of clean energy like shown in this document about wind of the European Environment Agency. Indeed, we have a lot of wind energy in Europe like shown in the following map.

wind map.png

But there is the less popular message that wind during winter also extracts a lot of heat from the houses due to their permeability. It is seldom realized than 1m³ air has a weight of 1kg. Heating 1m³ air takes 25% of the energy to heat 1l water, although the last one is a much larger concern for most people.

As an example, I made a calculation for a fictive passive house, build with high density mineral wool in such a structure that the mineral wool is fully exposed to the wind and inner climate. This is never the case but it shows the orders of magnitude of the wind effect.

We developed a simple permeability spreadsheet (green fields can be adapted), where we used the published  Air Flow Resistance (inverse of permeability) of mineral wool. We assume a thermal conductivity = 0.040 W/mK and used a thickness of 0.4m to reach U=0.1 for passive housing. We also assumed as default 120 kg/m³ density and a temperature gradient of 20°C. Width and length of the house are 10m, height is 5m and the wind blows on two walls.

We observe that with a wind of 5m/s (15 Pa wind pressure) that we extract 2000 W while only 800W is needed when there is no wind at all. This fictive situation shows that permeability, even at high density induces a heat loss 2.5 times the declared heat loss.

But if we had used 120 kg/m³ density GLAPOR cellular glass (0.050 W/mK), we should consume only 1000 W, even with much larger wind speeds. The price of both is comparable because GLAPOR foams recycled glass at 800°C and mineral wool involves a melting step up to 1600°C.

Mineral wool, even at high density, has to be used with a severe wind protection, while this is not needed for cellular glass. This is clearly mentioned on the Paroc website.

Wind induces draft over the thermal insulation on an attic. This was already shown in a previous post. Indeed, in “Convection in horizontal loose-fill insulation below an air layer”, we can read: “If wind velocity above the insulation is as much as 1.5 m/s, which is reasonable in an attic space, heat losses can increase by 10 – 30% depending on the permeability of the material.”  This is demonstrated in the folowing figures:

wind_atticwind_loss

The left figure shows the draft above the thermal insulation for a certain wind velocity outside. The right figure shows the extra heat transfer through the thermal insulation due to the draft, generated by the outside wind. It is clear that severe wind protection would help a lot.

Also in this case, GLAPOR cellular glass would have done the job without extra heat loss due to the wind outside.

 

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