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The myth of breathing walls, or how to get rid of moisture.
“Please, tell me, will the wall breathe after it has been insulated by your system?” Similar questions, especially during the insulation season, are very-often asked by construction consultants all over Poland.
The myth of breathing walls has had quite a career. Repeated over and over, it has finally become a dogma, which boils down to the belief that thermal insulation with low diffusion resistance is a prerequisite for ensuring satisfactory humidity inside a building, by drawing the excess water vapour away.
What is the reality? It is worth taking a closer look at what the water balance looks like in a typical apartment. Let's assume that this is a facility with an area of 60m2, occupied by two adults with a child. Each of the mentioned people is a potential water-vapour generator.
In order for this model to be credible, we must take into account additional factors, such as plants, and the activities carried out by people, for example cooking, cleaning, doing laundry, and others, as additional sources of water vapour.
Referring to the estimates of the mass emissions of water vapour as published by Marek Łojewski in “Murator” magazine (2/1997), the following values can be assumed.
- Bathing for all 3 people - 3,300g of vapour
- Sleeping (50g per hour) - 1,200g
- Cooking (2 hours) - 1,500g
- Doing laundry - 300g
- 5 plants - 1,200g
- Other housework (4 hours) - 400g
- Other intensive work (2 hours) - 350g
This way, we have obtained a round number of 10,000g of water vapour produced by the family during the 24 hours of using the apartment.
Following the previous assumptions, our family occupies a flat with an area of 60m2. Let's assume that two of the walls are façade walls, and that they are the ones transferring water vapour. Based on our model, let's assume that their total area is 40m2.
Now, let's estimate the mass of vapour transferred in winter conditions, i.e. when such transfer is the most intensive.
In the example given, let's assume that the following conditions apply inside the apartment
- Temperature +20oc
- Humidity 55%
The outside is frosty
- Temperature -10oc
- Humidity 85%
The mass of the transferred water vapour will be proportional to the difference in the separate pressures inside and outside the building. On the basis of such conditions, 4 types of wall will be discussed.
Example 1 - a 55cm-thick ceramic-brick wall, covered with 2cm-thick cement-lime plaster on both sides
Diffusion resistance 61
Example 2 - a 35cm-thick concrete wall, covered with 2cm-thick cement-lime plaster on both sides
Diffusion resistance 24
Example 3 - a 30cm-thick cellular-concrete wall, covered with a 2cm-thick layer of cement and lime plaster, additionally insulated with a layer of mineral wool
Diffusion resistance 24
Example 4 - a 30cm-thick cellular-concrete wall, covered with a 2cm-thick layer of cement and lime plaster, additionally insulated with a 10cm layer of foamed polystyrene, and finished with a thin layer of acrylic plaster
Diffusion resistance 109
If we collect all the given data and substitute it in the formula*, we will have an answer to the question about the amount of water which can penetrate through the presented wall models within 24 hours:
- Brick wall (example 1) 443g
- Cellular-concrete wall (example 2) 1,440g
- Wall insulated with mineral wool (example 3) 1,536g
- Wall insulated with foamed polystyrene (example 4) 211g
Comparing these values with the total mass in the apartment (10,000g), we will reach the following conclusion: the amount of water vapour which will “escape” through the walls oscillates within the range from 2% (insulation with polystyrene foam) to 15% (wall insulated with mineral wool).
As the calculations clearly demonstrate, the impairment resulting from the so-called “breathing walls” is, in essence, only a popular myth. The ventilation system we use, and the air-tightness of the windows have a much greater contribution to maintaining the proper level of humidity in the house than the construction of a barrier.
Assuming that a properly designed and constructed ventilation system should ensure 9 air exchanges per day in living rooms, and 13 air exchanges per day in such rooms as the kitchen or bathroom, the effects of building barriers (walls or roofs), and their permeability to water vapour in the successful discharging of its excess, can be safely assumed as negligible.
*m = (Δp x S x t)/R, where m = water-vapour mass, Δp = partial pressure difference inside and outside the building, S = barrier area, t = transfer time (24 hours, in the presented example), R = the diffusion resistance of the barrier.
