ᐅ Roof Insulation for Summer Heat Protection

Dear community,

I would greatly appreciate expert advice on my upcoming personal project.

Project location:
Bedroom in the attic with a sloped roof structure

Current situation:
Existing external roof insulation with the following layers: tiles, battens, counter battens, bitumen membrane, sheathing, rafters, insulation, vapor retarder, ceiling.

One side of the room has a sloped roof with exposed rafters.

Planned measures:

1. Attach a wooden substructure about 3-5cm (1-2 inches) thick to the rafters between the exposed rafters.
2. Then fix clay building panels about 2.2cm (0.9 inches) thick onto this substructure.
3. Fill joints and paint with a hemp/clay/lime mixture.

Goal:
To create a delayed heat release into the room during summer by utilizing the air gap between the ceiling and the clay panels, thus avoiding unbearable heat.

Question to the community:
Are these measures sufficient to keep the room below 23°C (73°F) in summer, or would additional steps be necessary?

I am hesitant about using hemp insulation or clay insulation batts, as that would require placing a vapor retarder or vapor barrier between the insulation and the ceiling to avoid shifting the dew point too much. Is that correct? Or would hemp panels be better suited than clay panels for this purpose? However, hemp panels handle moisture less effectively.

The indoor humidity levels are generally 30-40% in summer and 15-30% in winter.

Thank you very much in advance!

Best regards
Michi

It may sound logical at first, but it isn’t. Air is a good insulator, but it must not be allowed to move, because convection can transfer heat through air very effectively.

In your cavity, the air on the roof side will warm up, start circulating between the roof and the clay board, and quickly transfer heat from the roof into the room. Moisture can also easily form in this cavity, since the expected temperature differences can easily cause the dew point to be reached. Your cavity is not vapor-tight, so the air inside cannot hold moisture.

Mineral wool, expanded polystyrene, aerated concrete, or similar materials trap air in small pores, preventing air circulation and heat transfer.

All in all, this is quite a bad idea, and that is why no one actually does it in practice.

Michi2026 schrieb:
Question to the community:
Are these measures enough to keep the room below 23°C (73°F) in summer, or would additional steps be needed?

You can add as much insulation to the roof as you want. Eventually, the insulation reaches its limit, and the attic will still get warm to hot. Just like in winter, when no amount of insulation can completely replace heating, in summer you need to remove heat from the room. The only effective way to do this is with an air conditioning system, which also helps regulate indoor humidity.
A photovoltaic system on the roof can provide some relief by preventing direct sunlight from hitting the roof tiles. Plus, the excess energy from the solar panels can be used to run the air conditioning during the day.