Insulation Basics: Designing for Temperature Extremes in Any Season

When it comes to insulation, more is more. The more insulation you’ve got, the more efficient your house will be — not just in winter but all year. This year’s summer in the Northern Hemisphere was a bit tricky to characterize. Thanks to a return appearance of the polar vortex, lower latitudes in North America reached chilly temperatures in the 50s (Fahrenheit), while Canada was left to endure scorching heat over 100 degrees. In Europe it was similar, with Baltic and Scandinavian countries receiving a blast of heat in July while the wineries in Italy complained of not enough sun to ripen the grapes.

In times of uncertainty, it’s best to plan for all occasions. With extreme temperature swings becoming the new normal, the safest design response is a house that can handle whatever the weather has to offer. The best way to do that is by insulating as much as possible. Because when it comes to protecting your house, there’s no such thing as too much insulation. That being said, there is such a thing as too much money! And insulation can be costly. So how can you balance costs and enough insulation to meet both winter and summer needs? Here’s what to know.

The owners of many homes insulated for cold climates have had chances in recent years to see how well their insulation layers perform in extreme heat as well. Many homeowners in the north may have noticed this summer that their home lacks enough shading and heats up fairly quickly during the day. Then it holds that heat through the summer night, when it may have already cooled off outside.

This is why choosing insulation that also performs well in summer may be a good idea.


Not all insulation is created equal. Thermal capacity is the amount of energy needed to change a material’s temperature. Insulation materials with a low thermal capacity allow heat to transfer through them faster than materials with a high thermal capacity. (This is a different measurement from the often-cited R-value or U-value of an insulation material.) Thermal capacity is a characteristic of insulation that is not usually discussed in the home building sector with the client, and your architect or contractor might not mention it to you.


The less insulation you have, the more it matters what kind of material it is. This is similar to the idea that the less money you have in your project budget, the more you want it to really count. If you’re going to have only a thin layer of insulation, for reasons we’ll set aside for the moment, then it could be valuable for you to consider materials with a high heat capacity. In this way not only will you be protecting against winter cold, but you’ll also be using a material that will help you a bit more in the summer.



On the other hand, if you’ve got a thick enough layer (more than 10 inches, or 25 centimeters) of almost any kind of insulation, it matters less what kind it is. Winter or summer, it will take a while for heat to move through it.

A Passive House, for example, can be insulated with anything from polystyrene to sheep’s wool and it will still perform relatively well in winter and summer, simply because the standard requires an insulation layer so thick that the heat takes a long time to get through it, no matter what material it is made from. This would probably be similar in a LEED-certified home with a high energy and atmosphere rating, or a home specified to any similarly high level of energy performance.


The average house on the market, however, does not meet the passive or LEED energy-efficiency objectives with superthick layers of insulation, but instead has much thinner layers acting as a barrier for the transfer of heat and relies on traditional heating and cooling to meet comfort needs. In these situations (which are the majority), it can be useful to know the type of material used for the insulation layer, because it can have a fairly noticeable impact on comfort in summer.


Thermolok InCide Cellulose InsulationSo what kinds of insulation materials perform well in summer? Those with high thermal capacity. In other words, natural materials. At least for the time being, there is no synthetic material on the market that has the high thermal capacity of natural materials like wood fiber, cellulose and cork. This is because historically, insulation products have been developed for cold climates, where until recently homeowners have not had to worry much about summer heat waves.

The high cost of natural materials, however, makes them trickier to specify for the average home, and a traditional cost-benefit analysis may not accurately reflect the payoff.


One of the best natural insulators is cork, the natural bark of the cork oak (Quercus suber), native to the Mediterranean. There are around 2.5 million hectares (6.2 million acres) of cork forests worldwide, half of which are in Spain and Portugal (pictured here is a forest on the Italian island of Sardinia). Once a tree has reached 25 years, it can be harvested approximately once a decade for the rest of its life, around 200 years.

As always, there are advantages and disadvantages to every material. Cork has a good balance of thermal performance for both cold and hot temperatures, but it does cost more than synthetic materials.

In cold temperatures cork does not provide quite as much insulating power as some of its synthetic counterparts, like polystyrene. However, the high heat capacity of cork makes it a stronger option than EPS (extruded polystyrene) for locations that also get hot summers.


Strategies to Use in Combination With Insulation

The subject of thermal performance, and insulation in particular, is pretty complicated, and there are many variables that are easy to overlook.

But if you decide to invest in a higher-cost natural insulation for the benefit of higher thermal capacity, then it would be a mistake to overlook some other basic characteristics of building envelope design that can help your house stay comfortable in summer.


A ventilation layer may be beneficial, for example, either as part of the facade or in the roof, to allow the external air to flow through and carry away the heat. Again, the value of this detail will be felt even more in homes with low to medium levels of insulation. Employing a ventilation layer, as well as reflective components (often called “radiant barriers”), can help counter a lack of sufficient insulation and can be quite effective when used in combination.

It’s also important to remember the importance of the roof. Heat radiation from the sun affects this horizontally sloping surface most, especially the closer you are to the equator. The hotter the climate, the more priority should be placed on insulating the roof.


Shade, shade and more shade! Lastly, it’s essential to remember that none of this matters at all unless you shade from the summer sun. You might as well throw money out the window — or use $100 bills as insulation — if you don’t have proper shading devices that block heat from reaching your glazing, the weakest point in your building envelope. In the Northern Hemisphere, the west, south and east facades (usually in that order) should take priority when you’re adding shading.

See 6 ways to cool off without air conditioning


The ultimate goal is to maintain a comfortable temperature at a reasonable cost, allowing your house to gain some immunity against sudden swings in the weather’s mood that would otherwise present you with unexpected energy bills.

More: Insulation Basics: Heat, R-Value and the Building Envelope

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