Why New Extensions Get Condensation
Condensation occurs when warm, moisture-laden air contacts a surface below the dew point temperature — the temperature at which the air can no longer hold its moisture in vapour form, causing it to condense as liquid water.
New extensions are particularly prone to condensation for several reasons:
1. Construction Moisture
New extensions contain significant moisture from:
- Wet concrete foundations curing
- Screed drying (a 75mm screed releases large amounts of moisture)
- New plaster drying
- New mortar curing
This moisture must evaporate into the air and then be exhausted from the building. During the drying-out period (typically 6–12 weeks), interior humidity is significantly elevated.
2. Thermal Bridging
A thermal bridge is a point in the building fabric where heat passes through more easily than through the surrounding insulated area, creating a cold spot. Cold spots are where condensation forms first.
Common thermal bridges in London extensions:
- Steel beams that penetrate or connect to external walls
- Uninsulated lintel positions
- Window and door frame junctions with blockwork
- The concrete slab edge where it meets the external wall (ring beam/edge condition)
- Corners (where the surface-to-volume ratio is unfavourable)
3. Poor Airtightness
An airtight building envelope prevents warm, moist interior air from reaching cold structural elements hidden within the wall build-up. A poorly airtight envelope allows moist air to penetrate wall cavities and roof structures, where it can condense on cold surfaces — causing interstitial condensation that is not immediately visible but can cause long-term structural damage (timber rot, plasterboard degradation).
4. Inadequate Ventilation
Modern extensions are better insulated than the original house. This is beneficial for energy efficiency, but means the building does not "breathe" through the accidental ventilation (draughts) that older properties rely on. Without adequate mechanical ventilation, moisture from cooking, showering, and occupants accumulates in the interior air.
5. Occupancy Patterns
An open-plan kitchen/dining/extension creates a single large air volume that the kitchen fills with steam during cooking. Without extraction directly over the hob and adequate air change in the space, moisture builds up rapidly.
How to Prevent Condensation in a New Extension
During Construction and Drying Out
- Heat the extension gently with space heaters during the screed and plaster drying period
- Ventilate as much as possible (open windows and doors when weather allows)
- Do not lay floor finishes until screed moisture content is below the floor manufacturer's threshold (typically below 75% RH for moisture-sensitive floors)
Thermal Bridge Design
- Specify continuous insulation at the floor slab edge to break the thermal bridge at the junction of slab and wall
- Use thermal break elements at steel beam penetrations
- Ensure insulation is continuous at window reveals
- Check the Psi values (linear thermal transmittance) of key junctions in the building specification
Airtightness
- Seal all service penetrations through the external envelope with appropriate air barriers and expanding foam or dedicated sealing collars
- Ensure continuity of vapour control layer (where specified) behind plasterboard
- Seal the junction between new and old building carefully — this is a common air leakage point
Ventilation
For an open-plan extension with kitchen:
- Install a correctly sized extractor over the hob, vented externally (not recirculating)
- Provide background ventilation to the extension — trickle vents in windows or a mechanical supply
- Consider a Mechanical Ventilation with Heat Recovery (MVHR) system for the whole house if the building is heavily insulated and airtight
Checking the Building After Construction
If condensation appears on walls or windows:
- Is it on cold mornings only, clearing through the day? This is likely surface condensation during the building's drying-out period and should resolve over 3–6 months.
- Is it persistent? Investigate for thermal bridges or inadequate ventilation.
- Is it black mould in corners? This indicates a ventilation deficiency and may require mechanical extraction.
RCB Design & Build
RCB designs extensions with thermal bridging, airtightness, and ventilation as integrated considerations, not afterthoughts. We advise clients on the drying-out period and provide guidance on managing the building during the first months of occupation.