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What is R Value?


Homeowners and business owners who are looking to reduce their heating costs through effective insulation need to be able to answer the question, ‘What is R Value?’. For a precise definition, R Value is the measure of a material’s thermal efficiency and defines how quickly heat transfers through it.

Mankind has long understood the need to insulate walls and cavities to reduce heat loss from homes, and science has developed far more effective means of modern insulation than materials used in the past such as straw, horse hair and newspapers.

How Do Heat Transfers Take Place?

However, to properly answer the question, what is R value, it is important to understand how heat transfers take place due to either conduction, convection or radiation and that most homes lose their heat mainly through conduction. The heat in a home will continue to be lost until the temperature inside the home matches the outside temperature.

Home or office insulation is designed to slow down and reduce that heat loss, to minimise the amount of additional heating required to maintain the warmth within a home or other building. Conversely, the opposite is equally true in summer where walls and roof voids which are well insulated with materials having a high R value will block the heat of the sun and keep a home or business at a cooler and more comfortable temperature, without the need for air-conditioning.

Reducing the need for man-made intervention, to add heat to a building in winter, or to remove heat from a building in summer, helps minimise global warming and reduces a buildings carbon footprint. In addition, the investment made by a building’s owner to ensure it is thermally efficient can be repaid many times over in savings made on utility bills.

So what actually is R Value?

In simple terms, it can be explained as a measure of thermal resistance to heat flow due to conduction. The higher the R value, the more efficient an insulating material is, and the slower the heat passes through it via the process of conduction.

An insulating material with an R value of 20 offers twenty times the resistance to heat than the resistance offered if there were no insulation present. An R value of 10 offers ten times the heat resistance of having no insulation present but only half the resistance to heat loss of an R20 rated insulating material.

What is the relevance of R Value when Installing Insulation?

Insulating material for walls, such as fibreglass sheets and blankets are efficient due to the millions of tiny air pockets within the material that slow down the passage of heat. When installing insulation, it is important to avoid compressing the chosen insulating material as this compression removes the air pockets and reduces the efficiency of the material.

However, where space permits to add additional layers of insulation in roofs and walls, those working out what is R value can simply add together the R value of the different layers, provided the layers haven’t been compressed. Therefore, using two layers of insulation each with an R value of 13 gives an answer of 26.

Working out the R value of a wall would mean adding together the R value of the brick, plus the one of the sheathing, the one of the insulation, the R value of the plasterboard and the one of anything that is added to the plasterboard such as heavy-duty wallpaper.

R value is a great method of comparing different products resistance to heat loss, provided the product is correctly installed. It should be remembered though, that it does not measure heat loss due to convection, radiation or infiltration by draughts or cold winds.

Comparing the R Values of the most common Insulation Materials

The most common forms of insulating materials are:

  • Fibreglass - Usually in a blanket or roll form and well suited to installation between joists in roofs and studs in wall spaces.
  • Mineral Wool - Similar R value per inch to Fibreglass and heavier, with better sound insulation and is a better fire retardant - often made of recycled materials and an environmentally sound choice.
  • Cellulose Loose Fill - A good option for retrospective addition of insulation to avoid opening up walls.
  • Rigid Foam Boards - A good choice where space is limited - offering a higher R value per inch of thickness than fibreglass or mineral wool.
  • Spray Foam - Offering the highest R values but generally requires a professional installer, though can be used by handymen in a spray can form for small gaps and spaces. 

What is the Requirement?

Working out what value is required for walls, floors and loft spaces does vary depending on location. Large countries such as the USA having recommended R values for seven different zones varying from zone 7 covering the Arctic in the North, down to zone 1 in Hawaii and Southern Florida.

Even in the UK, there is a significant difference between the levels of insulation required for homes in the Orkneys, compared to those in the Channel Islands. Most local authorities and local professional builders will be able to advise on recommended R values within their area.

Hopefully, this article has thrown some light on the answer to the question what is R value and highlighted that it is an efficient way to compare the heat resisting properties of a range of different insulating materials, thus helping buyers make a better-informed choice.