Insulation is one of the most debated topics in the conversion world. Here's a rundown of the popular options for UK conditions:
Closed-cell foam (e.g. Celotex/Kingspan):
- ✅ Excellent thermal performance
- ✅ Moisture resistant
- ❌ Rigid — harder to fit in curves
Sheep's wool:
- ✅ Natural moisture regulation
- ✅ Great acoustic insulation
- ✅ Fits into irregular spaces
- ❌ More expensive
Spray foam:
- ✅ Perfect coverage, no gaps
- ✅ Adds structural rigidity
- ❌ Expensive to have done professionally
- ❌ Difficult to remove if needed
Reflective foil (e.g. ThermoWrap):
- ✅ Thin — saves space
- ✅ Good radiant barrier
- ❌ Less effective alone — best as a secondary layer
My recommendation: A combination approach works best. Reflective foil on the metal skin, rigid foam boards in flat areas, and sheep's wool in the curves and gaps.
What's worked best in your build?