Understanding R-value and U-value

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Whether you are in the market for a home, building a new home or considering a renovation, you understand the importance of having a secure building envelope. A robust building envelope will mean your energy bills are lower, your home is more comfortable and you reduce your impact on the environment. The trouble is, there are so many products on the market to choose from and deciphering R-value and U-values can be difficult.

What is R-value?

R-value is a rating of how resistant a product is to heat flow i.e. its thermal resistance. The higher the R-value, the better the product is at preventing heat from flowing out of a home in winter and keeping cool, conditioned air in the home over the summer. Products with a high R-value, when installed correctly, will create a better building envelope.

The R-value applies only to specific materials and not to the systems as a whole. To get the R-value of whole systems (like a wall system) add up the R-value of each component.

To optimise performance, you must ensure that every aspect of your building envelope is performing well and that quality craftsmanship means that there are no gaps or holes in the envelope.

What is U-value

U-value measures the rate at which a product transfers heat. That means that the lower the U-value, the better the product is at insulating and the tighter your building envelope will be.

U-value is usually attributed to doors and windows so opt for a low number to mitigate heat energy loss through these openings. U-value demonstrates the product’s thermal performance and is effective in describing the performance of the whole window or door system, even though there are many different kinds of materials which make up these components.

Mathematically, U-value is the reciprocal of R-value i.e.  U = 1/R and R = 1/U. Let’s take a look at an example: a product with an R-Value of 5 has a U-value of 0.2 (1 divided by 5).

The important thing to remember here is that choosing products with a high R-value and a low U-value will guarantee that your building envelope is sound.

What is an ideal R-value and U-value?

The answer to this question varies depending on the building climate zone you live in. You must balance a good building envelope with your budget. There is a point where the product may not warrant the cost when you look at what it is cutting from your heating and cooling costs.

Even if you have the best building materials, they will only be able to perform if they are installed properly and according to the manufacturer’s specifications. If you have walls made with products which have a high R-value, but there are gaps between panels or between your wall systems and the roof or floor, then all your conditioned indoor air is still going to escape.

All of your building products must work in unison to create the kind of building envelope that keeps you comfortable and keeps your energy bills low.

Topics

Building Envelope, Energy Efficiency

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