Reinforce floor slab insulation, reduce heat entry into the upper floor

Hello everyone,

We built in 2016 and in our solid house the ceiling between the upper floor (i.e. towards the attic) is constructed as follows.

[*]180 mm concrete
[*]PE foil as vapor barrier
[*]160 mm EPS 040

This construction achieves a U-value of about 0.234. The attic is therefore a cold roof. Not a completely true one, since under the roof tiles there is still a diffusion-open underlay membrane installed, but largely so.

This U-value is indeed significantly worse than the U-value of the walls on the upper floor (which is about 0.15) but everything is fine so far, no moisture, and the additional effort for more insulation would hardly pay off in terms of heating costs.

However, in summer it can get as warm as 26 degrees in the upper floor in extreme cases and we have already considered installing an air conditioning system. We will probably do this, but only on the ground floor. Installing it on the upper floor would be a huge structural effort.

A rough estimate now shows that about two-thirds of the heat input into the upper floor in summer comes through the upper ceiling. This is based on the over 92 sqm area and over 55 degrees and more heat up there. (At 55 degrees the LCD display of the thermometer turned black, so it could easily be even hotter).

The plan is now to reduce the heat input from the attic into the upper floor by improving the insulation. An additional 120 mm EPS 035 is planned, resulting in the following construction.

[*]180 mm concrete
[*]PE foil as vapor barrier
[*]160 mm EPS 040
[*]120 mm EPS 035

This results in a U-value of 0.130 for the ceiling. This reduces the heat transfer from the attic to the upper floor from 650 watts to 350 watts, which in the overall calculation accounts for about 30% of the total heat input.

I hope this will slow the temperature increase in the upper floor and lower the maximum temperature from 26 degrees to 25 or better 24.

Question: Can this work, or am I overlooking something completely fundamental?

Thanks and best regards,

Andreas

I hope to slow down the temperature increase in the upper floor

It’s not enough for more. The heat enters the house anyway; maybe it takes a day longer in summer until the upper floor is as warm as before the insulation action.

The majority of the heat in summer comes from the window surfaces, even when shaded it is only a reduction of radiant heat by several kW daily, to get rid of these kW only an air conditioner or a construction style like in the Mediterranean (windows open onto the courtyard with a view of the water/fountain with plants) with natural air conditioning will help you.

We have currently had various calculations made regarding roof insulation. It shows that at some point you reach your limits and adding more insulation only changes the result slightly or the heat/cold comes in elsewhere, as already mentioned here. Ergo, it would only be effective as an overall concept, including windows, timely shading walls, personal behavior, etc., yet the heat will eventually get into the house. In my opinion, only with an air conditioner will you be able to do something substantial against it, if it really gets too warm for you.

Why don’t you insulate the roof? That way you wouldn’t even heat up the airspace in the attic, which is probably hardly ventilated.

More insulation does not help with summer heat protection. The most important thing is shading, followed by cooling: from simply opening the windows at night to using air conditioning. Apart from that, it is strange why one chooses a lower u-value for the roof, which is responsible for the biggest losses, than for the walls, with almost no additional cost.