Shading and summer comfort

A well-insulated house keeps heat in, and that works against you in July. Sunlight comes through the windows, people and appliances add heat, and the shell of the house holds on to it. Whether the house overheats depends on how much heat gets in, how much can leave, and how much the structure can absorb in between (its thermal mass, the heat the walls and floors can store). This article covers the first two. Thermal mass has its own article.

External shading

The wizard offers three choices: external venetian blinds (often called Raffstore or brise-soleil), roller shutters (Rollläden), or no external shading. Venetian blinds tilt their slats to block the direct sun and still let daylight in. Roller shutters close completely: they block almost all of the sun, and the room goes dark while they are down. Run the same house with each to see what full closure buys you on the hottest days. Internal blinds and curtains are not modelled at all. They stop sunlight only after it has passed the glass, so most of the heat is already inside.

In the model both are a flat screen mounted on the outside, a few centimetres from each window. When deployed, the screen stops the sun’s energy before it reaches the glass: most of it for venetian blinds, almost all of it for roller shutters. How much a real product lets through depends on the product and, for blinds, the angle of the slats. The model uses one representative value for each type. The wizard starts with external shading switched on, and you can change it to none.

When the blinds deploy

A window’s venetian blind is down when two things are true at the same time. The sun falling on that window, direct plus diffuse, is above 300 W/m² (watts per square metre of glass; full sun on a clear day is roughly 1,000 W/m²), and the outdoor air is warmer than 10 °C. There is no calendar. The blinds can move all year, and the two conditions decide.

On a bright day in January the air is colder than 10 °C, so the blinds stay up and the sun heats the house. On a bright day in May they go down. The 10 °C lock is a choice made for this model, not a manufacturer value, and it matters: with blinds that close on the sun alone, all year, the heating demand of the reference house rose by more than a third, because every bright winter hour lost its free solar heat. That is also why a small house with a lot of glass can still cool on a bright cold day in March. The blinds stay up on purpose, and closing them would cost more heating than it saves in cooling.

Each window decides for itself: east windows close in the morning, south windows around midday and west windows in the afternoon. The control looks at the sun on the glass and the outdoor air at each time step, not at indoor temperature, wind, rain or whether someone is home.

Roller shutters follow the same sun threshold, but they wait for a warm day: they close only when the outdoor air is above 20 °C. Below that, the room keeps its daylight and the shutters stay up. Like the 10 °C lock, the 20 °C is a choice made for this model, not a manufacturer value. We chose it by running a small, heavily glazed house with floor cooling with the lock at 18, 20, 22 and 24 °C. At 20 °C the house stayed almost as cool as at 18 °C, with about a quarter fewer hours in the dark. From 22 °C the hours above 26 °C indoors doubled. Heating did not change at any of these settings, because the shutters never close in the heating season.

The sun threshold and the outdoor temperature locks are set by the model, not offered as inputs. Real sun sensors work the same way: they close on brightness, and some add an outdoor temperature threshold or a winter setting that keeps the blinds open.

What leaves the house in summer

Heat leaves through the envelope (the shell of walls, roof, floor and windows), through ventilation and, if you have one, through a cooling system.

Natural ventilation means no fan: air comes in through the gaps in the building and when you open a window. The model uses a background rate of 0.3 air changes per hour (the air in the house replaced 0.3 times an hour), which in everyday terms is about half an hour a day with the windows wide open. It runs only while the house is warmer than the outdoors, the indoor temperature is between 18 and 32 °C and the outdoor temperature is between 0 and 32 °C. The rate does not rise when the house gets hot, with one exception.

A house with no cooling gets night airing. From 23:00 to 06:00, when a room is above 23 °C and the outdoor air is above 12 °C and cooler than the room, the windows count as open and the air exchange is topped up to 2 air changes per hour. That is the figure the German standard DIN 4108-2 uses for night ventilation through windows. A house with floor cooling or split air conditioning does not get it, because its cooling already holds the setpoint. There is no airing by day, and no stronger cross-ventilation between floors.

With mechanical ventilation and heat recovery selected, the natural term drops to 0.1 air changes per hour and the ventilation unit takes over. That unit is a pair of fans and a heat exchanger that passes the heat of the outgoing stale air to the incoming fresh air. It runs all year at a constant flow and recovery effectiveness, with one exception: a summer bypass, the flap a real unit opens so that cool outdoor air reaches the house without passing through the exchanger. The model has one, per heated floor. The exchanger is bypassed when three things hold at once: the room is warm, the outdoor air is above 12 °C, and the outdoor air is cooler than the room. “Warm” switches on above your daytime heating temperature plus 2 K and switches off again below that temperature plus 1 K. With daytime heating at 21 °C, the bypass opens above 23 °C and closes below 22 °C. While it is open the fans keep running at the same flow, but the exchanger stops exchanging, so cool night air comes in unwarmed. The numbers sit inside the ranges that unit makers publish for their controllers. Real units differ in their settings, and the wizard does not offer these as inputs.

Active cooling is a separate equipment choice: floor cooling, split air conditioning, both, or none. Floor cooling reuses the underfloor circuit, so it needs underfloor heating. A house with radiators can have split air conditioning or no cooling. A dew-point guard keeps floor cooling from cooling the floor below the dew point, the temperature at which moisture in the air condenses on a cold surface.

Reading the result

The results show the air temperature of each thermal zone (a floor, treated as one body of air) through the year, with peak, mean and minimum values, and the cooling energy where a cooling system exists. To see what shading does, run the same house twice with the same weather year, once with shading and once without. Use the hourly chart to see how long the temperature stays above the level you care about. Without a cooling system, cooling energy is zero whatever the temperature, so read the temperature and not the energy figure.

The weather year matters. A real hot summer shows overheating that a typical year (an average assembled from many years of records) smooths out, so run both.

Limits of the model

• Temperatures are averages for a whole floor. They do not tell you whether one bedroom under the roof is comfortable.
• Window opening follows a fixed rule: 0.3 air changes per hour, plus night airing at 2 per hour in a house with no cooling. It does not follow what your household does. If you keep the windows shut at night, your house will be warmer than the run shows. The airing also reaches the ground floor, where some people keep windows closed for security.
• Shading is all or nothing, with one fixed share of the sun let through for each type, and the same device on every window. You cannot put shutters on the south and west faces only. Slat angle, daylight and glare are not modelled, and there is no wind lock (the retraction of real blinds in strong wind) or manual override.
• Fixed overhangs, balconies, trees and neighbouring buildings do not shade the house. Only the blinds do.
• Humidity and moisture are not part of the comfort result.