These are the patterns I've seen in the real world: on jobs I've been called to fix, on quotes I've been asked to
look at, and on builds I've seen slowly go wrong over time. Each one is avoidable if the structure, membranes and
detailing are thought through properly.
Use this alongside the Truth Lab and the Cost Engine
when you're comparing quotes - especially if one number is a lot lower than the others.
Common ways garden rooms fail
Tap each heading to see what causes it, how it shows up, and what I do differently in a SIP build.
What causes it:
- Vapour barriers on the wrong side, or missing altogether.
- No cavity where one is needed for the cladding system.
- Leaks around doors/roof that aren't allowed to dry out.
How it shows up:
- Musty smell, swollen skirtings, "mysterious" staining.
- Soft patches around corners or below windows.
What I do differently:
- Correct warm-side vapour control layer on SIPs.
- Breathable membranes and drained cavities where needed.
- Details that let any water that gets in, get back out again.
What causes it:
- Cold bridging through timbers and poorly detailed junctions.
- Insufficient insulation or gaps where warm air meets cold surfaces.
- No real thought about how the room will be heated and ventilated.
How it shows up:
- Drips on cold mornings, misting at ceiling level, mould in corners.
- Needing to wipe down surfaces or run a dehumidifier constantly.
What I do differently:
- SIP build-ups designed for real U-values, not just "some insulation".
- Attention to cold bridges at edges, openings and roof junctions.
- Heating and ventilation considered in the design, not as an afterthought.
What causes it:
- "Flat" roofs rushed in a day with minimal prep.
- Flashing kits improvised instead of designed for the system.
- No thought about where water actually wants to go.
How it shows up:
- Stains at the ceiling edges, drips near doors, swollen linings.
- Leaks that only appear in certain wind directions or heavy rain.
What I do differently:
- Roof build designed around the SIPs with correct falls and outlets.
- Flashing and trims thought through before we're on site, not improvised.
- Careful work around roof lights, wall abutments and parapets.
What causes it:
- Thin, interlocking timber cabins sold as "fully insulated rooms".
- Insulation added in odd places with no real understanding of dew points.
- Structures never meant for daily, year-round use.
How it shows up:
- Too hot in summer, too cold in winter, drafts and movement.
- Walls that never feel truly "solid" compared to a SIP build.
What I do differently:
- SIP structure from day one - designed as a room, not a shed.
- Clear wall, floor and roof build-ups in writing.
- Aim is a proper extension-like feel, not an upgraded cabin.
What causes it:
- Bases thrown in as an afterthought or left to "someone else".
- Uneven or undersized supports under a SIP floor.
- No allowance for ground conditions, drainage or access.
How it shows up:
- Doors catching, cracks, uneven floors, odd gaps opening up.
- The structure always feeling just slightly "off".
What I do differently:
- Bases treated as part of the structure, not a separate mystery line item.
- Ground screws or slabs sized and set out properly for SIP loads.
- Honest conversations about what's needed if the site is tricky.
What causes it:
- Huge expanses of south-facing glass with no shading.
- No thought about solar gain, cross-ventilation or blinds.
- Rooms sold on "wow" factor, not year-round usability.
How it shows up:
- Rooms that feel amazing in spring and unbearable in August.
- Clients avoiding the space when they need it most.
What I do differently:
- Glazing balanced against orientation and intended use.
- Discussion about blinds, opening windows and shading early on.
- Designing for long-term comfort, not just brochure shots.
How to use this when you're comparing quotes
You don't need to become a builder. You just need to ask the right questions.
When you read any quote - from me or anyone else - ask yourself:
"Which of these failures has this person actually thought about in writing?"
If the answer is "none", treat that low price very carefully.
If you'd like, you can send me a quote through the Truth Lab and I'll happily point
out where the risks might be, with no obligation.
Open the Truth Lab
