In 1979 the US Department of Energy published a study on passive thermal management of residential buildings that contained an unthinkable fact: homes with properly installed blackout curtains recorded indoor temperatures on average 6-8 degrees lower compared to those without shielding, but only under the condition that the windows would remain closed during the hours of maximum sunshine. Homes in which the windows were open with the curtains drawn had higher indoor temperatures than those without screens. It’s not an intuitive result, and that’s precisely why almost no one applies it.
Common sense says that circulating air cools. This is true, but only when the outside air is colder than the inside air. In the central hours of a summer day with outside temperatures from 34-36 degreesopening windows does not introduce coolness: it introduces warm air that adds to the air that is already there. If there is a blackout curtain behind this open window, the problem is multiplied.
The cavity that turns into an oven
When a blackout curtain is drawn with the window open during peak hours, an enclosed space is created between the fabric and the glass. Warm outside air enters, gets trapped in this cavity and cannot circulate. The blackout fabric, which absorbs solar radiation and retains heat, begins to heat up until it reaches surface temperatures 40-50 degrees in the most extreme cases. At that point it behaves just like a radiator: it releases stored heat into the indoor environment by radiation, increasing the perceived temperature even when the air in the room appears still.
This phenomenon has a name in building thermal physics textbooks: radiant effect from a hot surface. It is not exclusive to blackout curtains, it is produced with any surface that absorbs heat and comes into contact with indoor air. The difference is that a curtain has a very large surface, often corresponding to the entire window wall, and radiates evenly throughout the room.
Quality blackout fabrics like those in the range Natté di Mermetwith certified solar coefficient up to 3%of tissues Blackout Screen di Sunbrellathey are designed to reflect solar radiation outwards and not absorb it. But this property only works when the fabric is directly exposed to the radiation with the window closed. With the window open, the mechanism is reversed: the fabric receives heat from the warm air circulating in the cavity and releases it inwards.
Time management that changes everything
The basic principle of summer passive ventilation is called free cooling and has been applied for decades in the bioclimatic design of buildings. It works like this: you accumulate coolness during the night and morning hours by opening everything up, and seal the building envelope during hot hours to keep that coolness as long as possible.
In practice, the window with a blackout curtain must follow a precise sequence. From early morning until around 8-9open windows without curtains: the cool night air still lingers and the sun does not yet warm the surfaces. Among them 9am and 7pmdepending on exposure, windows closed and curtains fully drawn: double insulating layer, no hot air penetration. Give it to her 19-20 and then, when the outside temperature drops below the inside, everything opens again for a change at night.
The exposure of the room changes the time window. A room exposed to East receives the morning sun and must be controlled early, but can be opened as early as the afternoon. A room exposed to West it receives afternoon sun and must be controlled from 1pm onwards, but can be opened in the morning without problems. The rooms a sud require continuous control for the entire central period of the day, from 10 A.M. until at least 6 p.m.
Not all blackout curtains work the same
Buying technical curtains for home use is much more complicated than the generic products sold in furniture stores suggest. The difference between a basic blackout fabric and a quality technician is measured by two values: the solar factorwhich indicates the percentage of total blocked solar energy, and the heat transfer coefficientwhich measures how much thermal energy passes through the fabric.
Fabrics with white acrylic coating on the outside, like the series Obscura di Kvadratreflect a greater amount of solar radiation than dark or colored fabrics. The color of the side facing the window is more important than the one facing the room: a fabric that is light on the outside and dark on the inside has a better thermal performance than one that is dark on both sides. It’s a detail retailers rarely communicate that makes a measurable difference 2-3 points to the surface temperature of the fabric.
Roller blinds placed as close to the glass as possible, with very little cavity space, work better than those placed away from the window. The less space there is for warm air to accumulate, the less the fabric heats up. Side track systems, which seal the edges of the tent to the wall, almost completely eliminate the infiltration of warm air from the sides and significantly increase thermal efficiency.
Blackout curtains and air conditioning: the relationship that no one calculates
A study ofAENEAS of 2021 on the summer management of Italian residential buildings estimates that proper solar shading of glass surfaces reduces the summer heat load of an apartment in 20-30%with an equivalent reduction in air conditioner consumption. It’s not a negligible saving: on an average summer bill, it can translate to 40-60 euros per month less.
The air conditioner and the blackout curtain work in synergy only if the scene is managed properly. An air conditioner that cools a room with closed windows and drawn curtains maintains the temperature with short cycles and low consumption. The same air conditioner in a room with open windows and closed curtains runs continuously because the thermal load is too high to compensate: it cools the air that is immediately heated by the radiant curtain and by the infiltration of warm outside air.
External blinds, shutters and shielding systems placed on the outside of the glass remain the most effective solution of all because they block solar radiation before it reaches the glass. But for those who live in an apartment without the ability to install external control, an internal blackout curtain managed in the correct time sequence comes pretty close to the results. The difference is not in the product: it is in the method.
A blackout curtain that is bought well and used badly is just an expensive curtain. The tent itself, with the windows closed at the right time, is a passive air conditioning system that consumes nothing and makes no noise.
