If you are planning to install or replace a cedar shakes roof, you should definitely choose a thermal insulation layer accordingly. The decision you make will affect, among other things, the cost of heating and cooling your home, the quality of your sleep, the longevity of your roof and the value of your property on the secondary market. Although cedar roofs have, as a rule, quite a high insulating potential, by themselves they do not keep the attic warm very well. For this reason, we’ve put together a guide for you to discuss the best materials for insulating cedar roofs and explain what you should choose for your home.
What is Thermal Insulation in Roofing?
Roof thermal insulation is nothing more than a layer of material that reduces the thermal conductivity of a building’s roof. In addition to reducing heat leakage through its conductivity, well-constructed thermal insulation also reduces the phenomenon of thermal convection, which in turn translates into improved
attic ventilation.
By design, the insulating layer in a cedar roof is meant to ensure that the temperature of the air in the attic does not immediately drop when the air outside gets colder and does not immediately rise when the weather gets warmer. This effect is achieved by sealing the entire roof surface and weakening the thermal conductivity of the materials.
Of course, even a roof without any thermal insulation will have a certain ability to keep heat inside. However, the more you use mineral wool, cellulose or other material, the stronger this effect will be. Indeed, it is estimated that a well-sealed roof reduces heat loss by up to 30%. For the average home in the United States, therefore, the savings can reach as much as $750 a year, and for larger buildings, it will easily exceed $1,000. So we are talking about real amounts of money that are relevant to most households in the country.
Why Do Cedar Roofs Need a Proper Thermal Insulation Layer?
No commonly used roofing material provides insulation performance that would allow the building to operate economically without an additional layer of mineral wool or foam. In the case of cedar roofs, the installation of a proper insulation layer is even more important – the formation of leaks and gaps between cedar shakes is a natural phenomenon that results in the problem of heat leakage to the outside of the building worsening year after year. Unlike modified bitumen, PVC, TPO and even asphalt shingles, wooden shakes simply do not form an airtight layer around the sheathing.
Consider, too, that the need to ensure good thermal insulation is even greater the more significant the changes in air temperature are, both throughout the year and on a daily basis. Since cedar roofs are highly susceptible to moss and algae growth and rot, many homeowners cut down the branches of the trees above their homes, thereby depriving themselves of shade. This has the effect of reducing moisture and improving the condition of cedar shakes, but at the same time makes it necessary to provide adequate thermal insulation for the building.
It should also not be forgotten that cedar roofs are very often installed mainly on old houses, which were designed 40, 50 or 70 years ago. As construction technologies were much more imperfect at that time, the phenomenon of thermal convection is much more noticeable in these buildings, so more heat escapes through the roof. This effect is especially noticeable in storied buildings.
What is Thermal Conductivity and Heat Capacity?
Before we get to discussing materials well suited for cedar roofs, we need to explain to you two terms that we will use later in the article. They are:
- Thermal conductivity coefficient
and
- Heat capacity
These are two of the most important concepts in thermal insulation design, and as a cedar roofing contractor operating in Highland Park, we use them every day.
Thermal Conductivity Coefficient – How Quickly Does Your Roof Conduct the Heat?
In the simplest terms, thermal conductivity factor determines how good the insulation performance of a material is – the lower this parameter is, the better for your household budget. Thermal conductivity factor is expressed in terms of lambda (λ) in units of Watts/Meter*Kelvin, while in the imperial unit system, it is more common to use the designation k and the formula (BTU / h * ft * °F). However, it doesn’t really matter – the idea is simply to keep the value of this number as low as possible.
The lower the λ or k, the better. In engineering, the thermal resistance coefficient is also used, but in the case of insulation materials, its values are rarely given. If someone needs to know its value, he will most likely be able to calculate it quickly.
| SI (λ) | Imperial | Examples: | |
| Great | ≤ 0,020 | ≤ 0,14 | Aerogel, a top-grade PIR foam |
| Good | 0,021 – 0,030 | 0,15 – 0,21 | PUR foam, XPS, premium EPS |
| Decent | 0,031 – 0,045 | 0,22 – 0,31 | Mineral wool, standard EPS, cork |
| Poor | 0,046 – 0,060 | 0,32 – 0,42 | Perlite, cheaper varieties of mineral wool or styrofoam |
| Very poor | > 0,060 | > 0,42 | Concrete, brick, solid wood (non-insulating materials) |
The most important relationship is that the lower the lambda value, the better the insulating properties of the material. In simple terms, if an insulating material has a low thermal conductivity coefficient, this means a lower lambda value, which in practice means that heat escapes from or enters the building at a lower rate.
Heat Capacity – The Amount of Energy the Insulation Material can Absorb
The heat capacity of a material, on the other hand, shows how much heat can be absorbed by that material before it heats up. The higher the heat capacity of the material, the slower it will give up heat, which is very important, especially in summer – it prevents the attic from heating up.
Best Thermal Insulation Materials for a Cedar Roof
So what materials should you choose to make your cedar roof as efficient and safe as possible? There are several options to choose from.
Glass or Rock Wool
Mineral wool is a material very often used in construction due to its relatively low thermal conductivity coefficient combined with a very attractive purchase price. It is a relatively good and, above all, very well-known insulating material that also has good vapor permeability, which is a key parameter for insulating cedar roofs, which are, after all, prone to rot.
Free vapor transmission and an acceptable lambda coefficient (usually around 0.035) furthermore go hand in hand with very good fire resistance – as an inorganic material, both mineral and glass wool are not prone to burning, which is extremely important for a roof that is acutely vulnerable to fire in the middle of summer. In a sense, the mineral or glass wool insulation layer provides a natural barrier for the roof to protect it from fire in the event of a fire.
Although mineral wool does not provide the highest possible level of thermal insulation, it does a very good job of sound attenuation, mainly due to its considerable volume combined with softness. This is a big advantage, especially if you live in a larger city like Glenview.
Mineral wool has several disadvantages that you should be aware of.
First, it is prone to losing its properties when it soaks up water – you need to watch out for this, such as keeping an eye on whether the roof has leaks. It has a lower heat capacity than the other two materials, and can settle over time, causing thermal bridges.As a result, the lifespan of mineral wool, especially in wetter climates, is quite limited and, in our opinion, is a material for 15-25 years of normal use.
Blown-in Cellulose
Cellulose is a material that has not enjoyed much popularity in recent years – in fact, it could be said that it was treated as a budget solution for people who did not care about obtaining the highest possible roof insulation performance.
Standard cellulose roofing insulation has a lambda coefficient of about 0.040, so it is 15% less efficient than mineral wool, although it is worth noting that with the passage of time, the efficiency of this form of insulation deteriorates even further.
Since cellulose is made from recycled paper, which is shredded under controlled conditions and soaked in compounds that make it fireproof, one might expect it to be a material prone to soaking, which is true. As a cedar roofing contractor, we regularly find ourselves in attics where mold develops, precisely because of the type of thermal insulation used. Since cedar shakes are also a naturally derived material, you can guess that a roof insulated this way will tend to degrade and rot much faster.
However, cellulose is great if the shape of your roof is highly irregular – tiny pieces of paper perfectly get into every crevice and hole, so that, at least initially, not too many thermal bridges are formed. However, you need to be aware that over time, they will almost certainly form, as the insulation will inevitably decrease in volume. Along with the average good insulation performance, there also follows a good sound absorption, which is a real advantage of cellulose. It is also worth remembering that it is the most environmentally friendly insulation material used in roofing!
Unfortunately, cellulose’s fire resistance is much worse. Although it does not burst into flames when heated, it begins to glow, which is extremely dangerous if your roof gets very dry during the summer season.
It is also a material with a very high susceptibility to water – in contact with it, it begins to settle, thus reducing its mass and thus worsening its effectiveness.
PUR Insulation Foam
PUR foam is a modern insulation material that is formed from the chemical reaction of polyols and isocyanates. As the name suggests, PUR is a spray-applied foam that adheres tightly to the substrate and does not create gaps that cause thermal bridges. It has a very low thermal conductivity coefficient, by far the best of all the materials listed here. It is being used more and more in modern construction because of its reliability and high insulation performance.
PUR foams can be divided into closed-cell and open-cell. This parameter determines how airtight the foam will be. Among other things, this translates into vapor permeability, which is quite important for cedar-clad roofs. Open-cell foams will allow more water vapor to pass through, for that they will have a higher thermal conductivity coefficient, so you will need to apply more of it than closed-cell foam to get a similar result. On the other hand, closed-cell foams are not vapor permeable at all – this may seem like a big problem, but with proper ventilation it is not an obstacle. For this you should definitely contact a professional.
Wood Fiber Boards
Another material that will work well with a cedar roof is wood fiber. It is made from finely ground conifer, so it too is of plant origin. In doing so, it retains all the necessary functions that an insulating material should have in a cedar shakes roof. Like the previous two materials, it is vapor-permeable and sound-absorbing. They are usually chosen for their eco-friendliness and better heat capacity than cellulose and mineral wool. They are also easier to install.
However, they have their limitations. Due to their lower mechanical resistance and ecological nature, they are more expensive to purchase and more difficult to access. Their fragility requires special care in transportation and installation. With the latter, you also need to be careful not to install them in bad weather – if the panels get wet during installation, they can deform and lose their thermal insulation properties. With all this, it has a higher thermal conductivity coefficient, so to achieve a similar value to cellulose or mineral wool, you need to install a thicker layer, which translates into the cost and weight of the entire roof. Wood fiber boards are not very popular and rarely used, but they are one option for thermal insulation.
Combination of Several Materials
A practice you may sometimes hear about is the installation of several materials simultaneously, in order to maximize efficiency and complement each other’s disadvantages of different materials. Such a solution is used quite rarely, especially in new projects, due to the difficulty of calculating the actual energy performance of a house, leading to problems with building certification. The most common scenarios in which such methods are used are situations where the roof and floor are insulated separately (e.g., with PUR foam and mineral wool) or gaps are filled with cellulose. Other examples of the use of such practices include:
- cellulose with a layer of glass wool underneath – a very popular, simple combination where glass wool provides the base and cellulose fills in hard-to-reach areas;
- wood fiber boards above the rafters and mineral wool in between – an efficient combination that includes all the most important functions for insulation;
- outer layer of wood fiber boards and cellulose blown inside the baffle – a highly breathable roof that retains the most important properties.
Summary
In the case of a cedar roof, several characteristic properties of the materials are very important, which are offered to the greatest extent by 3 materials: mineral wool, cellulose and closed-cell insulation foams. In our opinion, however, these days it is not worth deciding to buy materials with poor performance, so if you can afford it, closed-cell insulation foams, such as PIR, remain by far the best choice. With this technology, your roof will have everything it needs to be energy efficient, durable and safe for years to come.
