Garden Rooms · Problem & repair
Damp, condensation or musty smell in a garden room
Most garden room damp is not a leak. It is condensation - warm, moist indoor air meeting a cold surface, usually because insulation, ventilation or the vapour control layer is not doing its job.
Working out whether moisture is coming in or forming inside is the whole diagnosis.
How to tell this is your problem
- Water running down the inside of the glazing
- Black mould in corners and behind furniture
- A musty smell after the room has been closed up
- Damp patches at the base of walls
- Flooring lifting or cupping
Repair or replace?
Diagnose before you dehumidify. A dehumidifier manages the symptom; ventilation and insulation remove the cause.
Why it happens
Interstitial and surface condensation
Without a continuous vapour control layer on the warm side of the insulation, moist air reaches cold surfaces within the build-up.
Cold bridging
Timber or steel elements that pass from inside to outside conduct cold in and create localised condensation lines.
Moisture rising from the base
A slab or timber deck without a damp-proof membrane lets ground moisture into the floor build-up.
No ventilation
A sealed, well-insulated room used as an office or gym generates a lot of moisture with nowhere for it to go.
Repair options, cheapest first
- 01
Improve ventilation
Trickle vents, an extract fan, or a small heat-recovery unit remove moist air continuously. Often the single most effective change.
- 02
Correct the vapour control layer
A properly sealed VCL on the warm side of the insulation stops moist air entering the structure at all.
- 03
Upgrade insulation and remove cold bridges
Continuous insulation and insulated linings raise internal surface temperatures above the dew point.
- 04
Deal with the base
A damp-proof membrane, ventilated void or raised base separates the floor from ground moisture.
Cost and time on site
Indicative cost
Ventilation improvements are relatively low cost. Lining, insulation and base remediation are priced by area and access.
Typical duration
Ventilation is often a day. Re-lining and insulation work is typically several days.
Every quote is priced after seeing the site. We survey the build-up and ventilation before recommending anything.
Damp in a garden room: common questions
- Is condensation in a garden room normal?
- Occasional condensation on glazing in cold weather is common. Persistent running water, mould and musty smells are not - they indicate a build or ventilation problem.
- Will a dehumidifier solve it?
- It helps in the short term and is worth running while the cause is investigated, but it does not fix insulation, vapour control or base moisture.
- Does a garden room need a damp-proof membrane?
- Yes - any habitable structure needs the floor build-up separated from ground moisture, whether on a slab or a raised base.
- Why is mould only in the corners?
- Corners are the coldest internal surfaces and often the least ventilated, so they hit dew point first.
Services that fix this
Related reading: Garden Room Planning Permission in Surrey & Hampshire
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