Solar gain and the Irish climate

Solar gain and the Irish climate

Considering the ever-improving window energy efficiency standards across Europe, it is important to understand the various contributing parameters and their impact. What it ultimately comes down to is finding the right balance between U-value and solar gain, while understanding their significance in determining the actual thermal performance of glazing.

Before discussing the principles and importance of each parameter, it is worth briefly noting what they mean in simple terms. U-value quantifies the amount of energy – specifically heat energy – lost through the window, while g-value (solar gain) represents a potential amount of energy that can be captured by sunlight passing through a window in specific circumstances.

Consider this crucial point for a moment: while the heat loss represented by U-value is certain, the solar gain represented by g-value is only potential and dependent on conditions.

A window with a U-value of 0.9 W/m²K will lose such amount of heat regardless of its environment. It does not matter which part of the house the window is in, at what height or which direction it is facing. A solar gain of 60% represents energy that we may potentially capture if there is enough sunlight passing through the window for a specific period of time. In reality, even south-facing windows will not achieve the theoretical energy gains in a country like Ireland during winter, where such gains would actually be beneficial.

Heat loss occurs during every hour of every day, in a year that is 8,760 hours. Therefore, it is certainly worth our best efforts to minimise it. Solar gain, however, can only be considered to contribute to a building’s energy balance when the sun shines through the window. With an average of around 1,200 hours of sunlight per year[1], Ireland can take little advantage of the sun’s natural supply of heat energy. During the 6-month heating season (4,380h), considering average daylight hours, we can at most hope for around 380h of direct sunlight.

If we assume that the days are half as sunny as during the other 6 months, we arrive at 190h. This is, theoretically, the amount of time that solar gain may contribute to a home’s internal temperature – against 4,380h that U-value has a definite effect on the indoor climate.

U-value is constant, while solar gain is inconsistent. What we mean by this, in simple language, is that it is always good to have well insulated windows, and indeed other parts of the building – this will keep you warm in the winter and cool in the summer. On the other hand, solar gain in winter would be valuable (albeit an unrealistic expectation during an Irish winter), while solar gain in the summer, where it is far more likely, can actually cause the building to overheat.

Some professionals believe in the benefit of sacrificing U-value performance in return for better solar gain, and often specify double glazed windows on the south facing parts of the building. Doing this will result in compromising a certain heat retention measure in pursuit of higher potential of heat generation from an unreliable source. Below is a sample calculation of the trade off involved in downgrading from a high quality triple-glazed unit to a standard double-glazed one.

Oknoplast 4/18/4/16/4 triple-glazed unit

g-value = 53%

U-value = 0.5 W/m²K

Typical double-glazed unit

g-value = 64% (11% gain)

U-value = 1.1 W/m²K (120% loss)

Practically speaking, conditions to make solar gain relevant in Ireland are rarely met. In conclusion, solar gain is a sound engineering concept, but its influence is greatly diminished by the nature of the Irish climate. Meanwhile, U-value is a parameter whose significance remains unaffected.