Many of the problems blamed on millwork after installation start with the building, not the shop. Gaps at mitres, doors that stop closing flush, panels that cup and countertops that lift at a seam usually trace back to the humidity the woodwork met on site. Below: the conditions the woodwork standards require, when millwork should arrive, and what a general contractor can check in the week before delivery.
Wood takes on and gives off moisture until it matches the air around it. As it does, it swells and shrinks, mostly across the grain. Solid wood moves the most. Veneered panels on MDF or particleboard cores move far less, and plastic laminate on a balanced core moves least, but none of them are immune. Cores that get wet can swell permanently, and a panel laminated on one face only can bow when the air dries out.
Millwork is built in a shop at fairly steady conditions. If the site is much wetter or much drier, the pieces adjust after they are installed, and the movement shows up as gaps, joints that open, or doors and drawers that no longer line up.
In Canada, architectural woodwork is usually specified to the North American Architectural Woodwork Standards (NAAWS), published by AWMAC and the Woodwork Institute; US projects often cite the ANSI/AWI standards instead. Both treat site conditions as part of the standard, not a courtesy. The care and storage section sets relative humidity ranges by region:
Holding a space in that range keeps wood at roughly 5 to 10 percent moisture content. Steadiness matters as much as the number: the standards limit how far humidity should swing, and NAAWS guidance suggests keeping daily and monthly variation within about 15 percentage points of relative humidity and 5.6°C. The standards say temperature matters mainly through its effect on humidity, so many project specifications add their own temperature range, commonly 16 to 32°C (60 to 90°F).
The same specifications usually say woodwork should not be delivered or installed until:
The standards do not set an acclimation period. Some specifications, and AWI’s own guidance articles, suggest letting woodwork sit in the conditioned space for about 72 hours before installation. That only helps if the space is already in range: acclimating finished millwork in a space that is too wet or too dry damages it before it is installed. Check the specification for your project, because the governing values are the ones it names.
Two things work against woodwork in this climate. In the heating season, indoor air gets very dry unless it is humidified, and relative humidity in a heated building can fall well below the range the standards ask for. In a new building, the opposite happens first: concrete, drywall compound, paint and plaster release a lot of moisture as they cure. A space can swing from damp during construction to very dry once the heat is on, which is the worst sequence for solid wood.
Temporary heat adds to the problem. Direct-fired propane or natural gas heaters put water vapour into the air as they burn, so a space heated that way during construction can be much more humid than it looks. Millwork should not be delivered into a space on temporary heat.
Material choice and detailing decide how much movement you will see:
The right time is later than many schedules assume. Millwork should follow the wet trades, not overlap them. A practical sequence for most interiors:
Storage matters as much as installation. Millwork should be stored flat or on proper racks, raised off the slab on dry blocking, inside the conditioned space and away from exterior walls and doors. An unheated trailer, container or loading dock is not a storage area for finished woodwork. If the site is not ready, it is usually better to hold fixtures at the manufacturer than to store them on site.
The woodwork standards split responsibility for humidity three ways: the designer for a design that suits the space, the general contractor for conditions during site storage and installation, and the building’s operators once the space is occupied. AWI guidance notes that when conditions are outside the required range, woodworkers may ask the general contractor to accept responsibility for dimensional change before they install. Warranties on architectural woodwork commonly exclude damage caused by conditions outside the specified range. On projects that carry an AWMAC Guarantee and Inspection Service (GIS) guarantee, agree early how site conditions will be recorded. Daily temperature and humidity logs are the record every party will ask for.
Retail fixture programs add a scheduling problem: each store has its own turnover date, landlord and HVAC status. On a multi-store program it helps to make site conditions part of each store’s readiness check, alongside blocking, power and dock booking. Our store fixture installation checklist covers the rest of that list.
DNS builds, finishes and stages fixtures in our Vaughan plant, including store-by-store kitting for rollout programs. We work with the project’s general contractor or installer, so deliveries are timed around site readiness. If a site is not ready, tell us early. Shifting a delivery date costs far less than repairing woodwork that was installed into a wet or freezing space. For planning ranges, see store fixture lead times, or send us your drawings and schedule.
For most of Canada, the woodwork standards call for relative humidity between 25 and 55 percent, held steadily by the building’s HVAC system (20 to 50 percent in the Prairie provinces). Your project specification governs if it names different values.
The standards do not set a period. Some specifications and AWI guidance suggest about 72 hours in the space where it will be installed, but only once the space is already within the required humidity range. Acclimating millwork in a space that is out of range does more harm than good.
It should not be. Temporary heat does not hold humidity steady, and direct-fired heaters add moisture to the air. Installing before the HVAC is operating is one of the most common causes of gaps and warping later.
Heated indoor air is dry. Solid wood shrinks across the grain as it loses moisture, so joints open and panels shrink in their frames. Humidification and good detailing, such as floating panels and reveals, reduce the effect.
The woodwork standards place conditions during site storage and installation with the general contractor, design suitability with the designer, and conditions after occupancy with the building’s operators. Woodworkers may ask the contractor to accept responsibility for dimensional change if they are asked to install outside the specified range.
