Keepin’ it cool in the pool – heat gain / heat loss

As a business that builds glass structures for homeowners, we know a lot about keeping a building warm or cool.   It’s a big concern for almost all of our customers, especially when considering the impact on construction costs and long term heating / cooling costs.

Heat loss, measured in kW or BTUs,  reflects the total transfer of heat through the building from the inside out.  Since pool enclosures are made from aluminum and glass or polycarbonate, the information in this article will deal only with these elements. Although heat loss and heat gain can happen through any part of a building.

The opposite of heat loss is heat gain, also referred to as solar gain.  Heat gain occurs when when warmth comes into the space via radiant heat as the sun shines through the glass.  It’s a sign of a low U value rating. Heat gain also occurs as cool air escapes through seals.

Knowing what heat loss and heat gain is, how to reduce it, and what it means when you are in terms of the glass choices for your pool enclosure, will help to answer many questions about the internal building environment.

Many times, the benefit of a pool enclosure is that you can get the light and warmth from the sun to help offset the costs of electricity and heating.  It’s true that in Northern climates the heat gain can help other heating systems perform better, increasing overall comfort in a room.  But for the pool enclosure to actually be comfortable year round, the glass or polycarbonate used must provide enough heat gain to offset any heat loss.


As mentioned above, heat loss is measured in kWs or BTUs and is a function of heat transfer rates.
Heat transfer rates are measured in U values. The U value is the overall heat transfer coefficient and indicates how well glazing will heat. A low U value signifies glass that transfers less heat whereas a high value indicates that the glass is not going to perform well thermally.

The opposite or reciprocal of the U value is the R value.  R values are commonly used with rating types of insulation but can also be applied to glazing.

The solar heat coefficient is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward.  The solar heat gain coefficient is expressed in a number between 0 and 1.  The lower a window’s solar heat gain coefficient, the less solar heat it transmits.

So if you have glass with a low U value and high R value you can be sure it has less heat loss and will then help keep the room warmer. If you have a lower solar heat gain coefficient, the glass will gain less heat, thus it helps keep the room cooler.


The amount of heat loss is important for several reasons:

  • You’ll want to be sure that your pool enclosure is adequately heated or cooled as necessary for best year round swimming
  • Understanding heat loss is a key factor in keeping the costs of heating and cooling at optimal levels
  • When you know the potential heat gain or loss you will be able to plan for additional heating or cooling with an HVAC system, if necessary.

Different climates have different needs. So if you are building a pool enclosure in the Northern part of the United States you’ll want glass with a high heat gain to help warm up the space.  But if you are in Texas, you’ll want to choose glass with a low heat gain.


There are several ways to reduce energy consumption and heat loss (or gain).  These include:

  • Improving the insulation through the use of dual pane glazing or sunshades
  • Reducing air leakage by using larger glazed panels (less edges mean less heat loss / gain)


Carefully planning for heat loss and heat gain will save you money.  It can reduce heating / cooling costs while giving you the benefit of an additional daylight source.  We recommend low emissivity (Low-E) glazing with a low U value to help keep heat gain or heat loss within acceptable levels.

If you have questions about which type of glass would be best for your pool enclosure, contact us!


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