ᐅ Floor insulation for a heated sunroom over an unheated garage?

Hello,
We are planning to renovate the old conservatory, which is built on top of an unheated garage.
The base (24cm (9.5 inches)) was constructed with bricks, and the floor currently has untreated wooden floorboards without insulation.

My idea would be:
- Level the floor with a self-leveling compound
- Glue Styrodur or XPS insulation panels on the inside of the base
- Install about 20cm (8 inches) of Styrodur/XPS insulation boards on the floor
- Lay 22mm (7/8 inch) OSB boards floating on top

On the finished floor, we plan to install a 18mm (3/4 inch) thick underfloor heating system (Fermacell-like panels with recessed grooves for the pipes, then covered with a leveling compound). We have been using these panels for years in the house on the old floorboards.

Thanks in advance for any tips or advice.

Hello Commocruiser,

your plan for the new conservatory sounds well thought out!
We also spent a lot of time considering the right floor construction for our conservatory in Munich, so here are a few tips based on our experience:

• Floor insulation: 20cm (8 inches) of Styrodur or XPS is an excellent choice – especially essential for a conservatory above an unheated garage. Make sure to use pressure-resistant material (at least XPS 300) to keep the floor stable.
• Plinth and edge insulation: Great idea to apply XPS to the plinth as well. Additionally, using an edge insulation strip is recommended to avoid sound and thermal bridges.
• Vapor barrier: Since your conservatory is on top of a garage, a vapor barrier under the insulation is advisable to prevent moisture from rising.
• Overall build-up height: With insulation, OSB boards, and underfloor heating (e.g., Fermacell system), there will be a considerable height buildup – so please check the connection heights for doors and thresholds.
• Underfloor heating on OSB: Ensure that the build-up panels are suitable for wooden subfloors and securely fixed to avoid any creaking or movement later.

By the way, we also planned for retrofitting shading on our conservatory in Munich – it can get quite hot in summer otherwise.

All in all, your concept is very well thought out! I wish you great success with your project – a cozy conservatory really is a wonderful addition to any home.

The overall construction sounds solid, but there are a few things you should still consider:

• XPS with sufficient compressive strength (at least 300 kPa) is essential; otherwise, the underfloor heating may cause dents later on.
• Vapor barrier or vapor retarder should be installed below the insulation to prevent moisture from rising from the garage – especially important when using wood and OSB.
• 20 cm XPS (8 inches) + underfloor heating + OSB = considerable build-up height – check door clearances and possibly the railing height as well.
• If the Fermacell boards are not explicitly approved for wood substrates, clarify this beforehand – there are different versions for various substrates.

Otherwise: good plan, it should work like this!