Dew point - myth or real threat to walls? We figure out whether it is necessary to take this into account when insulating

In the comments, readers often write about a factor that must be taken into account when insulating a house - this is the dew point. In a nutshell, this is an area in the wall where condensation of water vapor is possible.

This process depends on two criteria: the concentration of water vapor (absolute humidity) in the wall and the temperature of the wall. The very process of moisture freezing after its condensation can lead to the destruction of the material. This is what they scare. It is even depicted by thermal calculators. For example, here is a graph in a wall of aerated concrete. We will take this material as an example in this article.

Inside the aerated concrete 30 cm thick in winter there will always be a zone 10-12 cm wide with moisture condensation (dew point). Therefore, it is not recommended to cover aerated concrete with vapor barrier insulation such as extruded polystyrene foam (EPS). So that moisture inside the wall does not accumulate and does not destroy the material.

But some self-builders deliberately insulate EPPS aerated concrete and when checking the walls a year later (in the cold season), there is no moisture under the insulation. How is this possible?

If the owner insulated the walls after drying (removing moisture) to a level of 3-5% of the block weight, then under no circumstances low temperatures, water vapor in the air bubbles of the wall will not be able to condense, much less damage block. There is too little water in the blocks, even for the formation of microlocks.

But what about the vapor permeability of aerated concrete? After all, moisture from the room goes out through the wall. I believe this is a myth. From the inside, plaster is applied, the walls are painted with paint or wallpaper. And in the bathroom - tiles. All this prevents the penetration of water vapor into the wall.

Let's take an example: let's say the room is 25 degrees and the relative humidity is 50% (it happens much less in winter).

In this case, the value of the absolute air humidity will be 11.6 g / m3. Logically, this moisture content will be found in the near-surface layer of the wall. With depth towards the outer surface, this value will decrease exponentially. And there will be even less moisture at the dew point than in the air outside the wall. But this is if the block was completely dry.

In fact, it contains 3% water by weight. If the block 40 cm in thickness, it is 1.2 kg of moisture. It turns out that the walls should humidify the air in the room. In the first year after construction, it turns out that way (the mass of water up to 30% from the mass of the block). Sometimes moisture even falls out in the corners of rooms.

The fact that blocks of aerated concrete through themselves remove moisture from the room to the outside is also a myth. For this to happen, the walls must be moistened with water, and only then will moisture seep out through moistening. And it should be up to 30-40% of the mass of the block (it will not absorb any more). Only under this condition will moisture exit to the zone of negative temperatures at the outer surface of the waterlogged block.

And then, such a waterlogged state will not immediately lead to the destruction of aerated concrete. Any material has frost resistance. This is the number of freeze-thaw cycles that the material can withstand. For autoclaved aerated concrete, this is F100. But if this material hibernates for several years in a row in a waterlogged form from contact with water, then this may also happen:

A block humidity comparable to that of the air will never produce such results. Therefore, the presence of a condensation zone (dew point) in the block will not affect the masonry in any way, even if the walls are insulated with EPS (Penoplex).

Although, in my opinion, it is better to insulate aerated concrete with a minelite. Not only because of vapor permeability. Make a wet facade in both cases. In terms of heat engineering, the miniplate is not much inferior to EPS. The main advantages of the miniplate are that it does not burn and it is an inorganic material, more durable.

If someone needs authoritative opinions, then you can familiarize yourself with the opinion of Gleb Grinfeld (the main one for aerated concrete in the Russian Federation): https://youtu.be/3664kLotnhY

His summary: condensation (at the dew point) in the wall is not equal to moisture accumulation. Moisture does not accumulate in the aerated concrete wall, even under EPS, during the cold season.

My conclusion from all this information: the dew point is not terrible for aerated concrete (in a state of natural humidity of blocks), just as it is not terrible for other wall materials based on concrete.

If someone has other ideas about the dew point - write your reasoned opinion in the comments.