Until now we focused on cold and heat entering through walls and windows. But our home is not an empty box. We cook, shower, watch TV, work on computers. All these activities produce enormous amounts of energy.
Let's see how Internal and Solar Gains act as free radiators in winter, but become the biggest enemy of your air conditioner in summer.
Anything consuming energy inside your home ends up releasing heat. This is called an "Internal Gain". The engineer must calculate these precisely - both for heating (winter) and cooling (summer).
The human body (at 36.6°C) emits roughly 100 Watts at rest and 150 W during housework. Invite 10 friends and you've just added 1,000 Watts to your living room - as much as a small space heater! Besides temperature, people also produce moisture (latent gain) through breathing and sweating.
The oven (1,000+ W), cooktop, fridge, TV, even the phone charger - all the electricity they consume ultimately converts to heat. A desktop PC releases 300-400 W - enough to heat a small room on its own.
Boiling water and hot showers don't just raise the temperature - they fill the house with water vapour (moisture) , adding enormous latent energy load. This is particularly challenging in summer, where it intensifies the AC's dehumidification work.
A typical Greek home with 4 occupants, cooking, TV, lighting and a computer produces internal gains of 800-1,500 W at any given time. That's an entire electric heater running constantly - invisible but very real.
The heat entering through windows from Solar Radiation is the largest free energy source for a home. Sunlight passes through the glass, strikes floors and furniture, heats them, and the warmth gets trapped inside (the Greenhouse Effect).
In winter the sun is low on the horizon and penetrates deep into the house through south-facing windows. A 2 m² south window can import 400-600 W of free solar energy on sunny January days.
West-facing windows receive the scorching afternoon sun almost head-on. A 3 m² balcony door facing west can import 1,500+ W of solar heat in July - equivalent to 1.5 electric heaters! Without external shading, this energy dominates the AC's workload.
The G-Value (Solar Factor) of each window determines how much solar radiation passes through. An old single pane (g ≈ 0.85) lets almost everything in. A modern solar-control glass (g ≈ 0.30) blocks 70% of solar energy, dramatically reducing cooling loads.
In Greece, the summer sun is so high that south windows actually receive fewer solar gains than west-facing ones! That's why bioclimatic design recommends large south windows (winter gain) and small west windows (summer protection).
Internal and solar gains behave completely differently depending on the season. In winter they're a blessing - in summer a curse.
In winter, the engineer subtracts gains from heat losses. If the living room loses 2,000 W to cold, but south windows provide 600 W, the TV 150 W and 4 people 400 W, then heating needs to produce only 850 W instead of 2,000. In Passive Houses, these gains alone suffice!
In summer, the equation reverses violently. All gains must be expelled by the AC. Cooking in August + 10 guests + west-facing glazing = AC at 100% just to "offset" the internal heat, before it even starts cooling.
In Passive Houses (Passivhaus), insulation is so extreme that free internal and solar gains provide heating for almost the entire winter. No conventional radiators needed! The challenge shifts to overheating - how to avoid excess warmth.
Depending on use (residential vs office), internal gains change dramatically. An office with 20 computers and lighting may need cooling even in winter! That's why EN 12831 always asks "what will happen inside the building".
The solution is not to stop breathing or cooking! The answer lies in smart bioclimatic design - maximising gains in winter and minimising them in summer.
Awnings, external roller shutters or louvres (brise-soleil) block the sun before it hits the glass. Interior curtains are not enough - the heat is already inside. External shading cuts summer solar gains by 80%+.
Choose glass with the right G-Value per orientation: south windows can have higher g for winter solar gain, while west-facing need low g (solar-control) to minimise cooling loads.
Replace old incandescent bulbs with LED. They don't just save electricity - they produce negligible heat, significantly reducing the AC's summer load. A 100 W incandescent emits 90 W of heat. An equivalent LED emits just 3-5 W.
The ideal design combines large south windows (maximum winter solar gain), small west ones (summer minimisation), external shading and interior thermal mass (solid floor storing heat). This philosophy reduces energy needs by 40-60%.
🔥 Internal and Solar Gains are not a "detail" - they are the hidden protagonist of the energy balance. Good design exploits them in winter and controls them in summer.
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