New windows do not always eliminate morning condensation. Sometimes the amount of water on the glass even increases after replacement, although the room has become warmer. It would be premature to conclude that the window must be defective. It is necessary to distinguish where condensation forms, when it appears, how much moisture is in the air and whether a clear air supply route remains. These questions help select an appropriate check, rather than immediately buying additional equipment.
Where exactly is the water?
First establish whether the water is on the room-side glass surface, on the outside or between the panes. These are different situations. This article concerns moisture forming on the room side. Moisture between the panes or visible water leakage around the frame requires a check of the window and its installation. Additional ventilation does not repair a leaking glazing unit or a defective wall detail.
Indoor condensation forms when the surface is cold enough for the air's moisture content. The EPA moisture-prevention guide recommends responding to condensation and addressing its causes. In practice, this means assessing both the moisture content and the surface temperature. The reading on the room thermometer alone does not reveal the condition at the window edge.
Why did the air route change?
The old window may have allowed outdoor air through its joints. The new one has reduced that unplanned route. If air is extracted through a natural ventilation shaft or a fan, a planned way for replacement air to enter is still needed. The PNNL air-movement explanation relates airflow to routes, resistance and pressure. Improved window airtightness must therefore be coordinated with ventilation.
Check that air inlets are not closed, painted over or obstructed by finishes. If there are none, that does not mean that any proposed opening should be installed blindly. First, an overall layout is needed: supply to occupied rooms, transfer between them and the intended extraction. In a heat recovery ventilation system, it is important to check the existing supply rather than bypass it on your own initiative.
Everyday activities create moisture
Showering, cooking and drying laundry change room air humidity. Building materials may also still be drying after renovation. These sources should not be lumped together as one permanent characteristic of the home. Record what happened before condensation appeared and how long it lasted. This helps distinguish a short spike from a problem persisting around the clock.
Also inspect whether curtains and the windowsill change air movement beside the glass. Conditions behind a dense curtain may differ from those measured in the middle of the room. However, moving furniture does not replace repair of a cold installation detail if that defect is identified. Avoid one convenient explanation for all windows and all rooms.
A simple multi-day log
Measure room temperature and relative humidity with a suitable instrument in accordance with its instructions. Take notes in the morning, in the evening and after a substantial moisture-producing activity. Show the exact area of condensation in a photograph: the whole pane, an edge, a corner or the frame. Include outdoor conditions, the number of people and the door position. One photograph of a water-covered pane does not provide that context.
The CO₂ trend can supplement observation of air exchange, but cannot establish every cause of condensation. HSE CO₂ guidance emphasises the measurement location and the sequence of readings. Do not put the device directly beside an open window or a person's face. This guidance explains a measurement method rather than determining the legal assessment of a particular Lithuanian home.
Illustrative example: condensation in only one room
Suppose all the windows in the home have been replaced, but condensation remains only in the bedroom. Two people sleep there, the door is closed at night and the room is little used during the day. The fact that the living-room glass is dry is not enough to rule out a ventilation issue. Room loads and airflow routes may differ greatly, even though the windows are the same model.
The check compares the patterns of moisture and temperature, the supply and transfer routes, and the condition of the window edge. If it is established that air does not reach or leave the closed room, the system is corrected. If a defective detail makes the surface too cold, that defect is addressed. Sometimes both actions are needed. Buying equipment before making this distinction may only increase costs.
What is not a final solution?
Wiping the glass every morning reduces the water present on the surface, but does not remove its cause. A dehumidifier may form part of a solution to a particular situation, but does not replace the supply of outdoor air. The presence of an air conditioner or air purifier also does not in itself prove adequate air exchange. The functions of these devices must be assessed separately.
There is no need to keep overheating the room merely because of condensation without looking for the cause. Heating can lower the relative humidity figure even though the water vapour has not yet been removed. This distinction matters when interpreting the result. Different heat distribution beside the window can also affect it, so the check must cover actual local conditions, not merely the thermostat setting.
Heating information matters too
If there used to be a radiator below the window and it was removed during renovation or covered by furniture, local heat distribution has changed. This does not mean that one heating system is inherently bad, but it explains why the same window model can behave differently under new conditions. Show this alteration to the specialist alongside the ventilation information. This reduces the risk of repairing only the window mechanism when the real problem lies in a combination of factors.
Also inspect where the water collects: is condensation soaking into a timber windowsill, the wall finish or the floor? Recurring moisture must be removed and vulnerable surfaces protected while the cause is investigated. Do not cover a damp area with new finishes simply because the water on the glass has disappeared at that moment. The state of drying and the correction of the defect must be assessed separately.
What to discuss with specialists
Provide the window replacement date, model, installation documents, home plan and observation log. State whether doors, heating or extract equipment changed after renovation. Ask for a confirmed defect to be distinguished from a hypothesis and for an explanation of what must be measured after the correction. The final result will then go beyond temporarily drying the glass.
After the work, compare similar conditions of use and weather. If one morning was dry but the problem returned later, this does not mean that all the work was worthless, but further assessment is needed. The general system principle is explained in the heat-recovery article. In your enquiry, provide the information gathered, not just a request to 'replace the window'.
Information sources
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