What is an exposed perimeter?

…And how do I work it out?

When trying to identify the thickness of insulation required to insulate a floor, many of us are faced with the onerous task of calculating the perimeter / area (P/A) ratio.

While establishing the area of a floor is fairly straightforward, identifying the exposed perimeter can cause quite a lot of confusion:

• “What is it?”
• “Does this wall count?”
• “What if it’s next to my garage?”

Hopefully by the end of this short guide, understanding and calculating the P/A ratio, or more specifically the exposed perimeter for your floor, will become somewhat of a walk in the park.

The exposed perimeter is essentially the total length of all exposed walls surrounding the floor you are proposing to insulate. This includes externally exposed walls and walls adjacent to an internal unheated space. Unlike the total floor perimeter, the exposed perimeter focuses solely on the edges where heat loss would occur.

For example, if you were to imagine the diagram below resembled your home, and you were looking to insulate the living room, the sum of edges A, B, C and D would make up the total perimeter of the living room. In comparison the sum of edges A, B and C, which are all either externally exposed to the outside elements or to the unheated garage, would make up the exposed perimeter measurement.

This figure when divided by the given floor area will provide you with a P/A ratio, which contributes in identifying the required thickness of insulation to achieve your target thermal performance.

So, now that you have your exposed perimeter and are able to calculate your P/A ratio, have a go on our free, online U-value Calculator to discover the thickness of insulation you need.

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