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What is R-Value?
The R-value of different insulation materials is a rating of an insulation product's effectiveness as determined by its thermal resistance or insulation value. The higher the R-value, the more insulated the home will be. R-value is measured by Imperial units and is the most common rating of insulation used in the United States and even some 'metric' countries like Canada. You can find a helpful chart showing the estimated R-value per inch of a wide range of common insulation materials here.
What is U-Factor?
The U-factor, also known as the U-value, is a rating you'll more often see on windows or doors, and it's essentially the opposite of an R-value. It is a measure of how fast heat will transfer through the product. For U-factor, lower is better. Check out this Rise energy-efficient windows buying guide for a more in-depth analysis of the U-factor for windows.
What is RSI?
RSI is the same concept as R-value but in metric units. To convert an RSI value to an R-value, multiply by 5.678. Again, the higher, the better. You can expect to find RSI units for products made in the United Kingdom, Australia, or New Zealand. Knowing how to convert this measurement can help make sure you are purchasing insulation products that will maximize your home's energy efficiency and thermal performance.
Effective R-Value vs. Nominal R-Value
There is an essential difference between effective R-value and nominal R-values to make things even a bit more confusing.
What is Nominal R-Value
The nominal R-value is the insulation value (R-value) of the insulation layer only. For example, a 6-inch layer of mineral wool insulation batt has a nominal R-value of R-24.
What is Effective R-Value?
The effective R-value is the insulation value of that part of the building when it's fully assembled.
This measurement considers the R-value of all the different materials that go into the total wall construction, including wood, concrete, drywall, siding, etc.
For walls with framing extending from the exterior to the interior, the effective R-value is lower than the nominal R-value because heat is lost through the uninsulated frame, a process called thermal bridging.
Here are three examples from different climate zones to illustrate how much insulation is used in super-efficient homes.
For every part of the home, Claire has insulated to levels more than double the minimum required by code. By doing this, she has a dwelling that practically heats itself and only needed a minimal and low-cost back-up heater.
Once again, all the home's surfaces are insulated to more than double the minimum R-value required by code. The floor slab is exceptionally well insulated, keeping it the same temperature as the rest of the house, making it pleasant to walk on. The attic was designed with lots of space for insulation, so it was easy and affordable to pile it up to R-100. This entire home is heated with a single electric heater, the size of a large toaster, installed in the air supply duct. Avoiding paying for a regular furnace made up for a lot of the cost of the extra insulation.
To build today's top energy-efficient homes, you need to use substantially more insulation than the minimum code, especially in cold winter climate zones. The payback for that investment is three-fold – superb home comfort, astonishingly low energy bills, and energy security.
A beautiful feature of these homes is that, even in an emergency involving an extended power outage in winter, they cannot freeze on the inside. Because they are so well insulated, even the sunlight that comes in daily through the windows is enough to keep them safely warm. That means your home's interior and water systems are protected from freezing under all circumstances, whether you are home or away.
Tobias Roberts
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.
