ᐅ Reinforce insulation of the ceiling between floors to reduce heat transfer to the upper floor.

Hello everyone,

We built our house in 2016, and the floor/ceiling construction between the upper floor and the attic is as follows:

• 180 mm (7 inches) concrete
• PE foil as a vapor barrier
• 160 mm (6.3 inches) EPS 040 insulation

This assembly achieves a U-value of about 0.234. The attic is therefore an unheated roof space. Not a fully conventional cold roof, since there is also a vapor-permeable underlay membrane beneath the roof tiles, but close to it.

This U-value is clearly worse than the U-value of the walls on the upper floor (about 0.15), but so far everything is fine: no moisture issues, and adding more insulation would hardly pay off in terms of heating costs.

However, in extreme cases the upper floor can reach temperatures up to 26°C (79°F) in summer, and we have considered installing air conditioning. We will likely do this, but only on the ground floor, as it would be a huge construction effort on the upper floor.

A rough estimate shows that about two-thirds of the heat gain into the upper floor in summer comes through the ceiling from the attic. This is due to the approximately 92 m² (990 ft²) of surface area and the temperatures above 55°C (131°F) up there. (At 55°C, the LCD thermometer display turned black, so it may well get even hotter.)

The plan is now to reduce the heat transfer from the attic to the upper floor by improving the insulation. An additional 120 mm (4.7 inches) of EPS 035 insulation is planned, resulting in the following total assembly:

• 180 mm (7 inches) concrete
• PE foil as a vapor barrier
• 160 mm (6.3 inches) EPS 040
• 120 mm (4.7 inches) EPS 035

This should produce a U-value of 0.130 for the ceiling. This lowers the heat flow from the attic to the upper floor from about 650 watts to 350 watts, which accounts for approximately 30% of the total heat gain.

I hope this will slow down the temperature rise on the upper floor and reduce the maximum temperature from 26°C (79°F) to 25°C (77°F) or better, 24°C (75°F).

Question: Could this work, or am I missing something fundamental?

Thanks and best regards,

Andreas

andimann schrieb:

I expect this to slow down the temperature rise on the upper floor.

It’s not enough for more. The heat will enter the house anyway; maybe it will take one more day in summer until the upper floor becomes as warm as it was before the insulation was installed.

Most of the heat in summer comes through the window areas. Even when shaded, this only reduces the radiant heat by several kilowatts per day. To remove this amount of heat, only an air conditioning system or a construction style like in the Mediterranean helps (windows open onto an inner courtyard with views of water/fountain and plants) providing natural cooling.

We have recently had several calculations done regarding roof insulation. It becomes clear that at some point, you reach your limits, and adding more insulation only makes a minimal difference to the overall result. Heat or cold will still enter the building through other areas, as already mentioned. Therefore, it would only make a difference as part of a comprehensive approach, including windows, timely shading, walls, personal behavior, and so on. Nonetheless, heat will eventually get inside the house.

In my opinion, using an air conditioner is the only effective way to manage it if the temperature really becomes too high.

Why don’t you insulate the roof? This way, you wouldn’t heat up the airspace in the attic, which probably has very limited ventilation.

Adding more insulation does not improve summer heat protection.
The most important factor is shading, followed by cooling: from simply opening windows at night to using air conditioning.

Aside from that, it is quite odd why the roof—which is responsible for the largest heat losses—has a lower U-value chosen than the walls, especially considering there are almost no additional costs.