Summer overheating risk
The risk that indoor temperature climbs beyond a tolerable level in summer because solar gains exceed the building’s ability to shed heat.
Why does a well-insulated house overheat?
The same envelope that keeps heat in during winter keeps it in during a heatwave. Insulation and airtightness reduce heat exchange in both directions, so in July the enemy is not heat loss but the solar gain that has already entered and has no way out. A low-energy or passive house with a large glazed wall can therefore be hotter inside than an old masonry house with small windows, which is a result clients find genuinely surprising.
Summer behaviour has to be designed rather than assumed. STN 73 0540 Part 2 contains a criterion for the highest daily air temperature in a critical room, meaning the room with the largest glazed area facing west or an unshaded south, usually a living room or an attic bedroom. It is checked by calculation at design stage, while the glazing and the shading can still be changed at the cost of a drawing revision rather than a building site.
Which levers work, and in what order?
The order matters, and it is not the order clients expect. Most arrive asking about air conditioning, which is last on the list, and few arrive asking about the size of the west window, which is first.
| Rank | Lever | What it does | Cost of changing it late |
|---|---|---|---|
| 1 | Glazing area and orientation | Determines how much energy can enter at all | Very high once built |
| 2 | External shading | Stops the gain before it enters the house | Moderate, if fixings were anticipated |
| 3 | Thermal mass | Shifts and flattens the temperature peak | Very high, it is the structure |
| 4 | Night ventilation | The only route by which stored heat leaves | Moderate, needs secure openings |
| 5 | Active cooling | Removes what the other four could not | Low, but it runs forever |
Reading the table from the bottom up explains most overheating complaints in Slovak houses. Cooling was treated as the answer, so nothing above it was designed, and the cooling load that results is far larger and more expensive to run than it needed to be.
Why are east and west windows harder than south ones?
Because of the sun's altitude. The summer sun over east and west is low in the sky and shines almost straight into the glass, whereas the July sun in the south is high, so a horizontal element blocks it. That is why a large west-facing living-room window is the most common cause of overheating here, and why the same area of south glazing with a modest overhang causes no trouble at all.
The design consequence follows directly. A fixed element such as an overhang or a loggia works on the south, blocking the high summer sun while admitting the low winter one, which is exactly the seasonal behaviour a house wants. On the west it fails, because the sun arrives at a shallow angle beneath any overhang, so a movable solution is needed instead: external roller shutters, venetian blinds, sliding panels or deciduous planting.
Why does external shading beat internal blinds?
External shading catches the energy in front of the glass, so it never enters the interior at all. An internal blind absorbs the radiation once it is already in the room and then re-radiates it as heat, warming the air even while it is drawn. The difference is not marginal, and no internal product closes it.
The second lever at the window is the glazing itself. The solar factor, or g-value, states what share of the solar energy passes inwards, and it is a genuine trade: a lower value cuts the summer gain but also removes winter gains and daylight. It is therefore set together with the daylighting assessment and the glazing build-up, not chosen in isolation from a catalogue. Triple glazing is not by itself a summer measure, and specifying it without addressing shading solves the winter problem while leaving the summer one untouched.
What does night ventilation need in order to work?
Thermal mass removes nothing; it only shifts and flattens the peak, and only if the house can be purged at night. That means the heavy layers must be reachable by the night air, so not buried under suspended ceilings and thick carpets, and a cross flow has to exist between two opposite sides of the house.
- Secure night openings designed in from the start: windows opened behind a locked shutter, ventilation flaps, or restricted top-hung sashes.
- Insect screens on every opening intended to stay open unattended.
- A path for air to cross the plan, which usually means an internal door or transfer opening that is not closed at night.
- Exposed mass on the flow path, typically a screed floor or a heavy internal wall.
Without night ventilation the mass charges up over the course of a hot spell and stops helping, usually by the third or fourth day, which is precisely when the occupants notice. A heat recovery unit is not air conditioning, and its airflow is a fraction of a night purge, but it does need a summer bypass to take the heat exchanger out of the loop when it is cooler outside than in.
When is active cooling the right answer?
| Situation | Verdict |
|---|---|
| Large west glazing that cannot be shaded externally | Cooling is likely to be needed, size it after fixing everything else |
| Attic bedroom under a lightweight roof | Insulation with heat storage and external shading first |
| House with exposed mass and cross ventilation | Usually unnecessary in a Slovak summer |
| Occupants absent all day, house sealed | Automated shading before cooling |
| Bedroom on a noisy street, windows shut at night | Mechanical night cooling is a legitimate answer |
Active cooling comes last, and once the earlier levers have been used its required output is small, its running cost is modest and it can often be met by a reversible heat pump already installed for heating. Reaching for it first produces the opposite: a large unit, a large bill, and a house that is still uncomfortable the moment it is switched off.
Frequently asked questions
- Why do modern low-energy houses overheat?
- The same envelope that keeps heat in during winter keeps it in during a heatwave. Insulation and airtightness reduce heat exchange in both directions, so in July the problem is not heat loss but the solar gain that has already entered and has no way out. Summer behaviour has to be designed, not assumed.
- Which window orientation causes the most overheating?
- West, and east after it. The summer sun over east and west is low and shines almost straight into the glass, whereas the July sun in the south is high and can be blocked by a horizontal element. A large west-facing living-room window is the most common single cause of overheating in Slovak houses.
- Is external shading really better than internal blinds?
- Substantially, and the reason is physical rather than a matter of degree. External shading catches the energy in front of the glass so it never enters the room. An internal blind absorbs radiation that is already inside and then radiates it as heat, warming the air even while it is drawn.
- Does thermal mass prevent overheating?
- It shifts and flattens the temperature peak; it removes nothing. Mass only helps if the house can be purged at night so the stored heat leaves again. Without night ventilation the mass charges up over the course of a hot spell and then stops helping altogether, usually on the third or fourth day.
- Can heat recovery ventilation cool a house?
- No. Its airflow is a fraction of what a night purge moves, and it is not a cooling device. What it does need is a summer bypass, which takes the heat exchanger out of the loop when the outside air is cooler than the inside air, so the unit stops recovering heat the house is trying to lose.