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Humidity in a New Home: Why Your Floors Split, Your Drywall Cracks, and the 35 to 50 Percent Rule That Prevents It
Warranty and maintenance First winter From a builder
Humidity in a New Home: Why Your Floors Split, Your Drywall Cracks, and the 35 to 50 Percent Rule That Prevents It
One February I got three calls in a month from the previous year’s clients: floorboards splitting, drywall cracking, trim pulling away. In a new house? I went and measured. The highest relative humidity in those three houses was 18 percent. One was at 6. Six. You cannot keep a houseplant alive at 6 percent, never mind a hardwood floor.
Short version: Wood moves with moisture. Dry winter air at 15 percent relative humidity shrinks every piece of wood in the house: floors gap and split, doors stop closing, trusses lift and crack the corner of the ceiling, mitres open, cabinet doors check. Humid summer air swells it all back and, past 60 percent, grows mould and sweats the windows. Hold the house between 35 and 50 percent, measured with a $20 hygrometer, all year. In winter that means a humidifier (ideally one integrated with the furnace, heat pump or HRV) and not running the HRV and exhaust fans more than you need; in summer it means the HRV, the air conditioning or a dehumidifier. Some window condensation at minus 20 is the price of a floor that stays flat.
What dry air does to a new house
- Hardwood and engineered floors gap, and solid boards split along the grain; wide planks and radiant floors suffer first.
- Drywall cracks at corners and above doors, and nail pops appear as framing shrinks.
- Truss uplift: the bottom chord of a roof truss dries and bows up, lifting the ceiling off the interior walls and opening a crack along the top of the wall. It closes again in summer. The fix is floating corner clips, not more mud.
- Trim shrinks and pulls from the wall; mitre joints open.
- Doors shrink, latches miss, cabinet doors and panels check.
- Everyone in the house gets nosebleeds and static shocks.
What wet air does
- Hardwood cups and swells; doors stick.
- Condensation runs down the windows and into the sill and the wall below.
- Mould in the cold corners, behind furniture on exterior walls, in closets.
- Dust mites and a stuffy house. Mould is the same moisture story at a different stage.
The rule, and how to measure it
Thirty-five to fifty percent relative humidity, measured in the living space away from the kitchen and bathroom. Buy two or three cheap digital hygrometers and put one on the main floor, one upstairs, one in the basement. In a brand-new house the first winter is the worst: the concrete, the framing and the drywall are still giving up construction moisture in the fall (humidity reads high), and then the heating season starts and it crashes. Watch it weekly that first year.
Raising it in winter
- A humidifier. A whole-house unit on the furnace or air handler, or a steam humidifier for a radiant house with no ducts, is the right tool; portable units work for one room. Set it to 35 to 40 percent and lower it on the coldest nights.
- Turn the HRV down to its lowest continuous setting in deep winter; it exchanges humid indoor air for bone-dry outdoor air and is the biggest dryer in the house. Do not turn it off; a tight house needs the fresh air. An ERV holds more moisture than an HRV, which is one reason to choose it. HRV vs ERV goes through that.
- Use bath and kitchen fans for the job and then shut them off; a fan left running all day pulls dry air through the house.
- Let the bath water sit until it cools, cook without the fan when the air is dry, keep plants, dry a rack of laundry indoors.
- Radiant floors dry the floor structure from below; a radiant house needs the humidifier more than a forced-air house, not less. Flooring over radiant heat covers the floor side of that.
Lowering it in summer and fall
- Run the HRV normally; the exchange dries the house when outdoor air is drier than indoor.
- Air conditioning or a heat pump in cooling mode pulls water out of the air; a dehumidifier does in a basement.
- Use the fans when you shower and cook, every time.
- In the first fall after construction, expect high readings while the structure dries out; a dehumidifier in the basement for those months is cheap insurance.
The window condensation trade-off
At 45 percent indoors and minus 20 outside, the inside of even a good window will sweat. That is physics, not a defect. The choice is a drier house with dry windows and split floors, or a comfortable house with a towel on the sill on the ten coldest nights. Keep blinds and drapes a little open so warm air reaches the glass, run the ceiling fans, and lower the humidifier a few points for the cold snap. Triple-pane windows with warm-edge spacers push the sweating point much lower, which is one of the quiet arguments for them in how to buy windows.
Why the builder cares
Because the builder gets the call. Tarion’s construction performance guidelines allow for normal shrinkage and seasonal movement and expect the homeowner to keep the humidity in range; the warranty covers defects, not a floor that split at 10 percent humidity. We say this at the hardwood selection, again at the pre-delivery inspection, and again here. First-year maintenance has the rest of the first-winter list.
Related guides on this site
Sources and further reading
Questions people ask
What humidity should a new house be kept at in winter?
Between 35 and 50 percent relative humidity, measured in the living space with a hygrometer; aim for 35 to 40 in a cold snap to limit window condensation. Below 30 percent, wood floors, trim and drywall start to shrink and crack.
Why is my new house so dry in winter?
Cold outdoor air holds very little moisture, and the HRV and exhaust fans replace indoor air with it. Radiant floors and forced-air heat dry the structure further. A humidifier and a low HRV setting fix it.
Is the builder responsible for cracked drywall and gapped floors in the first winter?
Normal shrinkage and seasonal movement are expected and are not defects; Tarion's guidelines assume the owner keeps humidity in range. Builders typically repair the first-year drywall cracks once as a courtesy; a floor that split at very low humidity is on the house, not the builder.
Why do my windows sweat when I run the humidifier?
At minus 20 the inside glass is cold enough to condense air at 45 percent humidity. Lower the setting a few points on the coldest nights, keep air moving at the glass, and accept a little condensation as the cost of a stable house. Triple-pane windows reduce it a lot.
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Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
Absolutely! Some people experience respiratory symptoms in the summer months, when the weather is hot, and the air contains more allergens. Air conditioners and fans can circulate dry air through the room, and air conditioners remove any moisture from the air. A humidifier may be beneficial during this season.
You need to be smart and stop installing wood floors. If after all your preaching to clients, that you’re still the one held responsible for those floors then the answer is simple: stop installing them that way you won’t be held accountable for damages….it’s the only way to circumvent clients who won’t listen to you….