Why does a new, well-insulated house overheat?
Heat reaches a room by three routes and then has nowhere to go. The first is sun through the glass. The solar heat gain coefficient measures how much heat the glass transmits. The second is internal heat gain from people, cooking and appliances. The third is the building's capacity to store heat. The recast Energy Performance of Buildings Directive lists thermal capacity among the thermal characteristics that the calculation methodology must consider (Annex I point 4).
The overheating risk grows when large glass, light construction and no external shading meet in one house. The article on large glazing in a low-energy house covers the glass side.
What indoor temperature is acceptable in summer?
Decree 259/2008 gives ranges, not one limit. For rooms of long stay the decree asks for optimal conditions in both the warm and the cold season. The building design should create them. Technical equipment steps in only where the design cannot (§ 2 ods. 3). Table 2a of Annex 2 sets the warm-season operative temperature by activity class, for standard light clothing.
| Activity class | Example in the decree | Optimal, °C | Permissible, °C |
|---|---|---|---|
| 0 | Lying at rest, relaxed sitting | 25 to 28 | 20 to 29 |
| 1a | Sitting with minimal movement, work at a computer | 23 to 27 | 20 to 28 |
| 1b | Sitting or standing with some movement, cooking | 22 to 25 | 19 to 27 |
| 1c | Standing with constant work of arms and legs, cleaning | 20 to 24 | 17 to 26 |
The permissible range applies where the optimal one cannot be created, except on extremely hot days (§ 2 ods. 4). Table 2a also sets relative humidity at 30 to 70 % and a maximum air speed of 0.2 to 0.3 m/s depending on the class. For a bedroom the relevant row is class 0.
In what order should you tackle summer comfort?
The Directive gives priority to measures that improve the thermal performance of buildings in summer. It names shading, sufficient thermal capacity and passive cooling techniques (recital 70). It also says air-conditioning raises peak load and the cost of electricity. This supports an order in which the building comes first and the machine last.
- Glass area and orientation. Decree 259/2008 treats a total opening area of more than 40 % of the perimeter structures as a special case. In that case you cannot skip the measurement (§ 2 ods. 14). The window-to-wall ratio costs nothing to change on paper. The article on orienting a house explains the sides.
- External shading. The same paragraph requires external sun shading on sun-facing openings as one of the conditions.
- Thermal mass. Thermal mass stores heat. It does not remove heat.
- Night ventilation. Night purge ventilation removes stored heat when the outdoor air is cooler.
- Active cooling. Size it for what remains after steps 1 to 4.
Which shading works?
Decree 259/2008 names external shading, not internal blinds, as the condition for sun-facing openings (§ 2 ods. 14 písm. d). External shading stops sun before it passes the glass. An internal blind stops it only inside the room. The US Department of Energy buyer guide lists exterior overhangs or sunscreens and interior blinds or shades as options.
For a calculation, ask the manufacturer for the shading data of the real assembly. The energy assessment under decree 364/2012 records the solar gains for each orientation. It uses the solar transmittance of the glass and a shading factor (Annex 4, table 1). Examples of external devices are an external venetian blind and a fixed brise-soleil. Dynamic shading with automation covers the hours when nobody is at home.
How do you use the sun height and the orientation?
The SIEA brochure on ventilation and cooling gives the noon sun angle in Slovakia. It is 65 degrees in summer and 18 degrees in winter. Suitable shading therefore keeps the rays out in summer. In winter it lets them reach deeper into the building, where the heat is stored. Shading should not shade needlessly in winter and in the shoulder seasons. A main facade that faces south allows winter solar gain.
A roof, a balcony or a structure that projects over the window gives natural shading. Canopies, pergolas and retractable awnings also help. For windows that face west, the brochure recommends a larger roof overhang and external blinds. A solar study, which is a computer simulation of the sun path, shows the shape and length of the shadows on chosen dates.
The glass helps too. The lower the solar factor of the glazing, the less heat the glass lets through and the more it reflects. The brochure gives an example. At g = 0.6, 60 % of the solar heat passes inside. At g = 0.4, 40 % passes. A lower g also cuts winter gain, so choose it by orientation.
What do thermal mass and night ventilation do?
Thermal mass delays the rise of the indoor temperature but adds no cooling. Heat stored during the day must leave later. Night ventilation can move it out if the outdoor air is cooler than the indoor air. The recast Directive names sufficient thermal capacity and passive cooling among the measures that avoid overheating (recital 70).
A sliding door or a door does not count as a ventilation opening under decree 259/2008 (§ 3 ods. 3). Plan the openings for night air with the designer, and do not rely on doors.
How should you ventilate in a heat wave?
The SIEA brochure advises against ventilating all day in a heat wave, because the interior overheats needlessly. Keep daytime ventilation to a minimum. Ventilate at night and early in the morning, when it is cooler outside. This also pre-cools the rooms. Cross ventilation through windows works best. Among the drawbacks of natural ventilation, the brochure lists draught, noise from outside and weaker protection against break-ins.
A mechanical ventilation unit with heat recovery has a bypass damper that bypasses the heat exchanger. It works automatically and serves, for example, for night ventilation in summer, when cooler outdoor air lowers the temperature of an overheated interior. The brochure still recommends pre-cooling the house in summer by natural ventilation through windows. The article on heat recovery ventilation in practice explains heat recovery.
Do the roof and the wall insulation help?
The SIEA brochure says that heat enters the interior directly through unshaded windows and indirectly by heating poorly insulated structures. So insulating the roof and the walls shrinks the indirect route. The brochure lists insulation among the measures against overheating and calls it the most expensive one. In a new house you decide on it in the design.
According to the brochure, a green roof also prevents overheating effectively. It also retains rainwater and improves the thermal insulation of the roof.
When is active cooling needed, and what size?
Decree 364/2012 says a technical standard sets the calculation of the annual heating and cooling need. Its footnote names STN EN ISO 52016-1 (Annex 3, note c, footnote 1). This standard covers energy needs for heating and cooling, internal temperatures and heat loads. A cooling load calculation works room by room. It shows whether a room needs cooling at all and what capacity it needs. Selling a unit by floor area alone does not answer this.
The decree limits the assessment. Suppose only some rooms are cooled and ventilated mechanically, and their floor area is under 80 % of the building's. Then the building is not assessed for cooling and ventilation energy (Annex 3, note b).
With radiant cooling the surface must stay above the dew point. Decree 259/2008 allows no visible mould, damp or condensation on the ceiling, walls or floor of rooms where people stay (§ 2 ods. 11). Ask the designer how the controls keep the surface above the dew point.
What should you cool with: air conditioning or a radiant system?
The SIEA brochure divides cooling into circulating and radiant. Circulating units, that is air conditioners, cool with moving air. They are the most common and use an air-to-air heat pump. A radiant system runs water at 16 to 18 °C through pipes in walls or ceilings, and those surfaces take heat from the room. The pipework is built into the structure, so a radiant system is usually installed in a new house or a major renovation.
Ceiling cooling suits living rooms best. The brochure does not recommend it in bathrooms and kitchens, because condensate can form on the ceiling. In a building with large unshaded glazing, its output may not be enough. The cold source can be a heat pump that takes energy from the outdoor air, the ground or groundwater, or well water. With a ground-to-water heat pump, the heat pushed into the ground in summer helps to regenerate the borehole or the ground collector.
The brochure advises deciding on cooling only after you have used the other building measures. Cooling always uses energy, so allow for running and service costs. It names a combination with photovoltaic panels as an option.
What should you not overlook when you cool?
- Service. Cooling equipment is not maintenance-free. A service technician should see it at least once a year, ideally before summer. The user can clean the surface and change the filter as the manual says.
- Placement. The indoor unit should not blow directly on places where people stay long. The outdoor unit needs air supply and must not bother neighbours with dripping condensate or noise.
- Air. Keep the windows closed while the cooling runs. Cooling dries the air, and the brochure gives 35 to 60 % as the optimal humidity.
- Capacity. It depends on the heat gains. They come from the orientation and size of the room, the windows and their shading, the number of people and the appliances. The designer does the calculation.
What should you take away?
A new house overheats when heat gain exceeds what the building can store and remove. Decide glass area and orientation first. Then decide external shading, thermal mass and night ventilation. Size cooling for the remainder. Ask for a summer calculation before you ask for a quote for air conditioning. The wider context is in the overview passive house from A to Z.