Features of wall insulation of a wooden house with foam

Recommendation points

It is believed that it is not necessary to insulate a wooden house. Yes, if the house is made of a gun carriage, timber from 200 mm wide or logs with a diameter of 280 mm. Nevertheless, wooden houses are insulated. Is it worth it to do it with foam or choose another material – we will tell you about this today.

Features of wall insulation of a wooden house with foam

Why are wooden houses insulated?

There are several reasons for insulating a wooden house. For example, a blockhouse made of “wild” logs does not have the most attractive appearance (from the point of view of modern ideas about design). The same can be said about a house made of ordinary timber of natural moisture. Houses made of rounded logs or glued beams do not have such a shortage, but this affects the cost of materials.

Thermal insulation of a wooden house for siding

Therefore, in order to save money, many people choose an ordinary log or timber and sheathe the walls with a block house or imitation of a bar. This division of functions into building and decorative gives another advantage – you can save money by using a smaller section of a bar or log, and compensate for the lack of thermal insulation properties by installing a heater between the main wall and the cladding..

Thermal insulation of a wooden house under a ventilation facade

But even if you do not save on size, additional insulation will reduce the load on the heating system. The same argument works in the case of additional insulation of frame houses..

Features of the insulation of a wooden house

If we talk about the dew point, then external insulation with any heat-insulating material will not worsen its position. Due to the fact that the density of the insulation is less than that of the main wall materials, the dew point shifts outward. And this does not lead to the appearance of condensation on the inner surface of the walls and the formation of fungus on them (which is very important for wood). In this respect, foam and mineral wool occupy approximately equal positions. But the thermal conductivity of expanded polystyrene or polyurethane foam is lower, therefore, their insulating properties are better.

Expanded polystyrene

But there is also such a thing as moisture transfer. What does it mean?

On average, one person emits at least one liter of water per day in the form of steam (up to 5 liters during hard work). And then there is wet cleaning, personal hygiene, cooking, pets and plants..

Part of this moisture is removed due to ventilation (natural or mechanical), and part through enclosing surfaces (including through walls).

Insulation of a frame house with expanded polystyrene

If the enclosing surfaces are not porous and have a high density (concrete, glass, metal), then there is no diffusion of water vapor, and these structures do not get wet.

Porous materials literally absorb water vapor, so the balance between moisture and weathering (drying) is important for them. If this condition is not met, then the structure gets wet and begins to collapse. And the tree starts to rot.

Therefore, the vapor permeability of the wall, insulation and external cladding should be equal or increase when moving from the inside to the outside..

Water vapor permeability of insulated walls a – wet wall; b – dry wall; µ – vapor permeability of materials; 1 – wall; 2 – insulation

If you look at the vapor permeability of the main building materials, you can see that wood has unique properties. Across the fibers, this indicator is very low (less only for concrete), but along the fibers – the highest.

Table 1. Water vapor permeability of building materials

Construction material Density, kg / m3 Vapor permeability coefficient, mg / (m h Pa)
Autoclaved aerated concrete D500 500 0.20
Expanded clay concrete 800 0.08
Reinforced concrete 2500 0.03
Solid clay brick 1800 0.11
Hollow clay brick 1000 0.15
Solid sand-lime brick 1800 0.11
Wood (pine, spruce) across the fibers 500 0.06
Wood (pine, spruce) along the grain 500 0.32

It turns out that the breathability of wood is provided mainly along the fibers, and a significant part of the moisture escapes through the ends of the log or timber in the corner joints of the walls.

If we proceed from the vapor permeability of the insulation, then only mineral wool is suitable for wooden houses for this indicator..

Table 2. Main parameters of heaters

Material Density, kg / m3 Thermal conductivity, W / (m ° С) Water vapor permeability, mg / (m h Pa)
Mineral wool 50-200 0.040-0.070 0.49-0.60
Expanded polystyrene 33-150 0.031-0.05 0.013-0.05
Extruded polystyrene foam 45 0.036 0.013
PVC foam 125 0.052 0.23
Polyurethane foam 30–80 0.020-0.041 0.0-0.05
Polyurethane mastic 1400 0.25 0.00023
Polyurea 1100 0.21 0.00023
Roofing material 600 0.17 0.001
Polyethylene 1500 0.30 0.00002

Its vapor permeability is much higher than that of wood along the grain. And of all types, stone wool is the most popular – it is easier to work than with glass wool, better environmental friendliness and thermal insulation than slag wool.

Thermal insulation of a wooden house outside with mineral wool

The vapor permeability of foam is significantly lower than wood. The closest representative is non-pressed polystyrene foam PSB, consisting of individual granules. Its vapor permeability is 0.05 mg / (m h Pa), but this indicator is 20% worse than that of pine across the fibers.

Of course, there is also PVC foam. It has a fairly high vapor permeability and very high strength indicators. It belongs to structural polystyrene and is even used as thermal insulation for plastic hulls of small ships. But its cost is very high..

PVC foam

Features of warming frame houses

If the supporting structure is made of a wooden bar, then the frame house is considered to be wooden.

There are two main technologies: panel and frame. The fundamental difference is that in the first case, ready-made SIP panels are used, in the second case, the sandwich is assembled on a frame. Most often, OSB is used as the outer and inner shell of the sandwich, although there can be cement-particle boards and moisture-resistant plywood..

Insulation of a frame house with expanded polystyrene

The vapor permeability of a brand new OSB with a protective layer can be an order of magnitude less than that of expanded polystyrene or polyurethane foam (the main types of insulation), and reach 0.0036 mg / (m · h · Pa). Therefore, for frame houses, any kind of facade decoration is allowed – brick cladding, hinged facade systems using natural and artificial materials, plastering and a wet facade (including with foam as insulation).

In the latter case, the technology is the same as for stone houses: marking, installation of a starting plate, gluing foam plates to the wall, fixing with dowels, reinforcing with a mesh, applying plaster and painting.

Insulation of a frame house with expanded polystyrene

Conclusion: it is best to insulate a house from a log or a bar with mineral wool, and leave the foam for frame houses.

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