Moisture content of wood when building: KVH, measuring, storage and installation

Wood is not a dead material. Even long after it has been processed into a timber or a cladding board, it keeps absorbing and releasing water depending on the environment it is in. When building, it is therefore not enough to choose the right cross-section and wood species. Equally important is the moisture content at which you install the wood and how you handle it from purchase to installation.

What the moisture content of wood is

The moisture content of wood expresses how much water the wood contains compared with the weight of completely dry wood. It is given as a percentage. Freshly felled wood can have a moisture content well above 50 %, kiln-dried sawn timber only a fraction of this value.

Wood is hygroscopic. This means that its moisture content gradually balances with the surrounding air. In a dry, heated flat wood dries out; in a damp cellar or in the open air it absorbs water. The value at which the exchange stops is called the equilibrium moisture content. It is different for each environment and also changes with the seasons.

Why wood shrinks and swells

While water is only in the cell cavities, the dimensions of the wood practically do not change. However, when the moisture content drops below the so-called fibre saturation point (the literature usually gives around 30 %), water also starts to leave the cell walls themselves. That is when wood shrinks. When it gets wet again, it swells. The dimension changes most across the grain and only minimally along its length.

Why moisture content matters when building

If you install wood that is much wetter than the environment in which it will serve, it will keep drying out in the structure for a long time. This shows in different ways:

• Cracks and twisting. Uneven drying causes longitudinal cracks, and timbers can twist or bow.
• Loosened joints. Shrinkage changes the dimensions, so screwed and carpentry joints can lose their original tightness.
• Gaps in cladding and flooring. Boards that narrow after installation leave visible gaps between them or come out of the tongue.
• Problems with the finish. Oil or woodstain soaks in and adheres less well on wood that is too wet.
• Risk of mould and fungi. It is generally stated that wood-decaying fungi do not have suitable conditions when the moisture content of wood stays below 20 % in the long term. Wood that remains wet and cannot dry out is at risk.

The opposite case, wood that is too dry installed in a damp environment, leads to swelling. With cladding or flooring without expansion gaps, the boards can lift or bulge.

Structural timber and KVH

Ordinary construction sawn timber is sold with varying moisture content and is often not checked on delivery. KVH (from the German Konstruktionsvollholz) is solid structural timber with precisely defined properties. One of the main ones is kiln drying to a moisture content of approximately 15 % ± 3 %. This value is close to the equilibrium moisture content that wood reaches in a covered and insulated structure, so after installation it no longer deforms significantly.

Other typical properties of KVH:

• strength class usually C24,
• cut away from the pith (heart), which limits cracking during shrinkage,
• longer lengths possible thanks to finger jointing,
• two visual grades: NSi for non-visible structures (levelled surface, chamfered edges) and Si for visible structures (planed surface, stricter appearance requirements).

In our range you will find KVH structural timber beams in grade NSi in several cross-sections. They are intended for load-bearing and auxiliary structures such as roof trusses, posts, pergolas, canopies or frame structures.

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Indicative moisture values

• KVH on delivery: approximately 15 % ± 3 %.
• Wood for heated interiors (cladding, flooring, furniture): usually 8 – 12 %.
• The limit it is best not to exceed in the long term: around 20 %.

These are commonly used indicative values. For a specific product, the manufacturer's data always apply.

How to measure the moisture content of wood

Hand-held moisture meters are most often used on site. It is worth knowing how they differ:

• A resistance (pin) moisture meter measures the electrical resistance between two pins pushed into the wood. It is affordable and fairly reliable, but you need to set the correct wood species and take the wood temperature into account.
• A capacitive (pinless) moisture meter is simply placed on the surface and does not damage the wood. The result is affected by the density of the wood and it measures only to a small depth, so it is rather indicative.
• The oven-dry (weighing) method is the most accurate. The sample is weighed, dried in an oven and weighed again. It is used in laboratories and to check the calibration of instruments.

How to measure so that the result means something

• Measure several pieces from the delivery, not just one.
• Take several readings on each piece and do not measure right next to the end. The ends of the wood dry faster than the middle.
• With thicker cross-sections it is not enough to measure only on the surface. The surface can be dry while the inside is still wet.
• Set the wood species and temperature according to the instrument's instructions.
• Take the result as indicative and rather watch the trend: whether the values differ significantly between pieces and whether they approach the environment where the wood will go.

Storing wood before installation

Even well-dried wood can get wet again if it is stored badly. A few principles:

• Store it under a roof, protected from rain, snow and direct sunlight.
• Do not lay the wood directly on the ground or on concrete. Support it on timbers so that it is straight and does not sag.
• For longer storage, place spacers between the layers so that air can circulate around the wood.
• Do not wrap the wood tightly in airtight film. Moisture condenses under it. If you cover it, leave the sides open for ventilation.
• Do not store wood in a new building where the plaster or screed is still drying. The moisture from the wet processes would quickly turn it into wet wood.

Acclimatisation of cladding and flooring

Cladding and floor boards should "get used to" the room in which they will be before installation. Bring them into the room where they will be installed and leave them there for several days. At least 48 hours is commonly recommended, but the exact time is set by the manufacturer of the specific product. It is a good idea to open or spread out the packs so that air can reach the boards.

The room should have approximately the same conditions as it will have later. This means finished wet processes, dried plaster and, in the cold season, background heating.

What to keep in mind during installation

Wood cladding

• Cladding is installed on a substructure of battens, that is supporting timbers of smaller cross-sections. KVH timber is not used for the substructure.
• Leave space between the cladding and the wall for air to circulate, especially on external walls and outdoors.
• Leave an expansion gap around the perimeter, which a moulding will later cover. This allows the cladding to move without pressing against the ceiling or the adjacent wall.
• If you are applying a finish to the cladding, it is advantageous to apply at least the first coat before installation. This way, untreated strips will not show in the joints when the wood shrinks later.

You will find spruce and thermally modified wood cladding profiles in the Wood cladding and Thermally modified wood categories.

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Wood flooring

• The subfloor must be dry. With concrete or screed, this means a sufficiently long drying time and ideally also a check measurement of the subfloor moisture.
• Design the floor build-up, including a membrane against moisture from the subfloor, according to the manufacturer's recommendations and the type of structure.
• Leave an expansion gap at the walls, which a skirting board will cover.

Decks and exterior

Outdoors, wood will never have a constant moisture content. What matters is that it dries quickly after rain. That is why gaps are left between decking boards, the substructure is raised above the ground and the structure is designed so that water does not stand on it.

A short checklist

• Are you buying wood with a defined moisture content (for example KVH)?
• Is the wood stored dry, on supports and with access to air?
• Have you measured the moisture content on several pieces?
• Have you let the cladding or flooring acclimatise in the room?
• Are the wet processes in the room finished?
• Are you allowing for expansion gaps and ventilation?

Wood and advice in Kysucké Nové Mesto

In the Zrubko shop at Hviezdoslavova 1118 in Kysucké Nové Mesto we have KVH structural timber beams, cladding, decking boards as well as ESSVE fasteners and wood paints and oils in stock. If you are not sure which wood to choose for your building, call us on +421 911 869 777 or stop by. We are open Monday to Friday from 8:00 to 17:00 and on Saturday from 8:00 to 12:00.