Timber engineered panels explained
To fully appreciate the many benefits of timer frame panel construction it helps to understand what will happen to the panel once it is in place. Wall panels are subjected to a number of pressures they need to withstand, and proper design is key to their success.
Thermal resistance
Walls act as a thermal barrier. At various times they are called upon to keep heat in or keep heat out. Insulation plays a key role. The strength of insulation is measured in thermal transmittance, or U-Value. The lower the U-Value the better the insulation. Timber frame panels are easy to insulate because they create a cavity into which insulation can be placed. It is important to understand that the insulation has a lower U-Value than the timber studs that surround it. In real life conditions, the timber can act as a thermal bridge, where heat can move more quickly through the timber than through the insulation surrounding it.
Thermal bridges create temperature differences on the surface of the wall that can lead to condensation. In a wall panel design, it is important that continuous insulation be applied to the surface of the wall to create a “thermal break.
Condensation in walls
Dewpoint is a vital concept in wall design. It is the temperature at which moisture in air of a specific relative humidity will condense into water. So, if the indoor temperature is 23oC and the relative humidity is 70%, the dewpoint is about 16oC.
Warm air holds more moisture than cold air and, as warm air cools down, moisture will drop out of it. The inside of a wall is at room temperature, say 23oC, but outside, it might be just 5oC so the surface temperature of the outside wall will also be 5oC. There is a thermal gradient inside the wall because temperatures inside the wall will drop 18 degrees from the inside to the outside. If there is an air leak
in the wall, the warm moist air inside will leak out, dropping 18oC in temperature as it makes
its way outside. Moisture will drop from this air inside the wall and one reason why air tightness is
very important.
Vapour diffusion
There doesn’t have to be a leak for moisture to get into a wall. A pressure difference can cause moisture to move into walls. Because warm air holds more moisture than cooler air, there is a higher concentration of water vapour in the warm inside air than the cooler outside air. Particles in an area of high concentration will move to an area of lower concentration, and will move right through vapour permeable materials, including building materials.
To prevent this, a vapour impermeable membrane, called a vapour barrier is installed. Cold surfaces are condensing surfaces and, if cold, a vapour barrier will actually capture moisture. So in the UK, vapour barriers are installed on the inside of the wall. If the vapour barrier is warm, no condensation will form on it, and water vapour cannot penetrate through it. In the summer, with air conditioning, the situation is reversed (the inside walls are cool), and water can condense on the outside of the vapour barrier (inside the wall), but this moisture will dry to the outside because a vapour permeable barrier has been placed there.
Air barriers
Air barriers are often misleadingly called “breather membranes.” In fact, they are more properly described as “vapour permeable air barriers”. While air cannot move through an air barrier, water vapour can. No matter how carefully a wall panel is constructed, there is a chance that some moisture will penetrate into the wall. To ensure there is an opportunity to dry, a vapour permeable membrane is installed on the outside wall. The water can then evaporate through the membrane to the outside, keeping the wall dry.
Wind washing
Factory engineered closed timber frame panels prevent wind washing. Wind washing is the movement of heat through an insulated wall, even one that is air tight, caused by improper insulation installation. If fibre insulation is badly installed in the wall cavity, it loses its insulation properties. This allows cold air from outside to cool down the interior of the wall and, though convection, the cold air is transferred to the inside.
Factory pre-engineering provides the highest quality of insulation installation to prevent wind washing and other forms of heat loss from within the building.
