In general terms, there are two types of roofs, hot and cold. While it sounds counterintuitive, cold roofs are primarily built in the northern areas of the country. You might wonder why you would want a cold roof in Minnesota, for example. That’s why it’s important to understand the underlying design of these two roof types.
A cold roof is designed specifically for cold weather climates and more specifically, areas where you have to deal with frost. A cold roof is designed with insulation above the plasterboard but well below the roof floor, allowing a degree of ventilation to pass, through, up, and out.
To prevent frost damage, you have to keep the roof cold, but how do you keep the roof cold without those sub-freezing temperatures flowing down and into the house? Heat rises after all and cold air tends to settle below the hot air.
So, naturally, you would assume that if the roof is cold, then some of that cold would make its way down into your very own living room. Thanks to some ingenious engineering, it doesn’t quite work that way, even though cold air will indeed still make it into the rooms below.
How Does a Cold Roof Work?
Imagine a system of layers that starts from the sheetrock just above your head, in the living room. If you have a cold roof, and you’re looking up at the ceiling, just above your sheetrock is a series of joists that run across the entire length of your home, except where they connect to the larger beams.
The spaces between the joists are stuffed with insulation. It’s not necessarily specific insulation, just that there is insulation between the joists. Above the insulation and joists, is a space where there is absolutely nothing. The minimum width of that space is around two inches.
Above that space of emptiness is the bottom of your roof deck, which is usually a form of plywood that sits below the tiles and everything that comes with the tiles. It may not seem like much, especially not for an area in which you would think that there should be much more to protect from the cold elements.
However, there are several advantages to this type of roofing system that is more beneficial than you know.
- Strategically placed vents
- Cold roofs are lower roofs and lower roofs mean that there is potential for higher ceilings
- Prevents snow from melting on the roof, which could lead to refreezing in the form of solid, heavy ice
Unfortunately, everything comes with a caveat, no matter how good it sounds. It’s simply the nature of the beast and there are a few concerns when it comes to a cold roof.
- A cold roof is considered to be more difficult to build
- “More difficult to build” means more expensive to build
- Thermal bridging is common with this design
- Higher utility bills in the summer
So, if you’re involved in new home construction, in terms of the purchaser having your home built, and you are in a northern climate, you have some hard decisions to make as far as how you want your roof construction to be.
How Does Ventilation Work in a Cold Roof?
When a cold roof is designed and constructed there must be a ventilation system somewhere that allows for the passing of cold air from one end of the roof to the other. However, this ventilation occurs underneath the roof deck and above the joists and insulation.
This cold air is what facilitates the coldness of the roof which, in turn, allows snow to settle without melting. The vents will generally sit in areas where the roof meets the outer walls of the residence.
You will know that you have a cold roof if you go up in the attic and study the roof itself or locate these vents at the top of the wall and bottom of the roof.
These vents draw the cold air in—cold air as in the outside air—and this cold air will flow below the rood deck, transferring the ambient air to the roof deck, even if the roof is black and sitting in direct sunlight.
There are several things that this design contributes during the winter months. It keeps the snow from melting and refreezing, which contributes to the overall weight increase and potential damage as a result.
It also keeps this refrozen snow from building up the weight on the eaves, which is not only dangerous in terms of what the weight damage can do to your roof but that a re-melt could result in an avalanche from your own roof.
After all, eaves are designed to protect the sides of your home and they aren’t much good to anyone if there is s ton of ice build-up on the edges and it ends up snapping your eave or falling on top of you when you are standing underneath of it.
Is Thermal Bridging a Significant Problem with a Cold Roof?
There’s little doubt that warm roofs are more protective against thermal bridging. It’s one of the few negative factors that you have to consider in a cold roof design.
The only reason that a cold roof allows thermal bridging to occur is for the same reason that it protects against adverse winter conditions. The same roof design that copes with snowfall by not allowing it to melt, also allows thermal bridging to occur.
The warm air is easily able to flow around the gap created between the insulation and the roof deck. If the joists aren’t appropriately insulated, there is even more room for thermal bridging to occur because it simply creates more space for heat to travel against an underlying, thermal barrier.
Final Thoughts
A cold roof is the way to go when you live in fiercely cold areas that receive a good degree of annual snowfall. Mountainous regions are included, especially in areas like the Rocky Mountains. A cold roof is designed to keep the roof cold, keeping the snowfall from melting and refreezing on your eaves.
Without a cold roof, you will have to employ a lot more preventative maintenance in the winter months, to stave off potential damage on your eaves.
We hope this blog post helped you understand a little more about cold roofs. They can definitely benefit you in areas where snowfall is common. If you have any questions or would like to learn more, please contact us today.
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