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21 September 2020

R-Values and Product Selection

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In my Detailed Blog of September 2017 (‘There is More to Insulation Than Just the R-value’), I commented on some of the factors which influence the performance of thermal insulation including the two-way flow of heat; the speed of losses or gains of thermal energy being dependent upon temperature differences; and the effect of thermal bridges, all in relation to comfort for a building’s occupants. My comments were made to bring to the reader’s attention the very dynamic and complex nature of thermal energy flow, and the consequential complications this causes when the multiple and often conflicting factors involved in construction design are needing to be juggled.

While all this thermal theory is very well, it is when actual building design/construction begins that the selection of specific thermal insulation materials and proprietary products must be made and finalised. At this stage, other non-thermal factors come into play to dominate the design decisions around the permanently integrated thermal performance required within the various building elements. This involves a variety of often conflicting aspects such as the fundamental of dimension limitations; internal vapour control; material durability; long-term weather and waterproofing; mechanical and environmental protection; fixing/installation methods and skill requirements; temporary protection during adjacent construction; future maintenance; costings and availability; aesthetics etc.

Fundamental to all this is the overriding need to achieve and maintain the calculated construction R-values required for the particular building elements being designed. A typical example is the common use of flexible and semi-rigid pads in timber-framed housing. These can be made from glass-fibre, polyester, sheep’s wool and other flexible fibrous base materials, even straw-bales, each with their own special characteristics.

Although they are available in a standard range of R-values, they are not necessarily interchangeable if a particular construction R-value is to be achieved or maintained for a specific thermal wall. This is because for any particular R-value the different materials often need to be of different thicknesses. The range of pads suitable for a 90 x 45mm stud wall may be limited, due to thickness, whereas if there is a structural requirement for 140 x 45mm studs then there can be more choice. Should a client insist on a specific product which is thicker for a given R-value, then 140mm studs may need to be used when 90mm framing would have been sufficient for another product or material, resulting in 50% more timber being used and a consequential cost penalty. Be warned, squashing a product to fit the construction reduces the R-value from that on the product label, as building inspectors well know.

A similar balancing is needed with rigid sheet thermal insulation where the same situation occurs with the different grades of polystyrene and other plastics having different R-values for particular thicknesses. An advantage of rigid insulation is that it doesn’t tend to slump over time as semi-rigid pads might.

In the past few years, much has been made of the need to insulate the external corners of timber-framed walls while there are more significant areas of localised R-value reduction which are ignored. The first is the deep solid timber lintel over a wide window where, for some walls, immediately above is either a roof truss or the outer joist of the upper floor, both of which make the lintel redundant. The second results in the downgrading of the ceiling insulation especially when high construction R-values are being asked for. On a flat ceiling under a pitched roof, there is the low triangular zone adjacent to the eave where the full depth of insulation cannot be fitted, and therefore the full R-value is not achieved. Some may say that the percentage area is small so it doesn’t matter, but heat is lazy and it doesn’t bother with the R4.0 at the ceiling centre, it just pours out via the minimally insulated band at the exterior wall junction. A simple change of product at this zone could easily resolve this problem.

There are other thermal insulation products which by calculation can be shown to have good R-values but in practical use have the potential to cause significant problems unrelated to thermal performance. Often these are used in existing walls where pads and rigid sheets cannot be economically fitted. Because the condition of the building wrap behind the cladding cannot be inspected, or at least not extensively, if there is any un-remediated damage then the insulation will contact the back of the cladding and thereby have the potential to create a path for moisture to enter the structural wall cavity where it should not be.

The design, installation and product selection of insulation for the thermal envelope of any building is not a simple exercise.

Through EcoRate Ltd – Architect I provide objective independent thermal performance analysis and advice on sustainability matters, to architects, designers, builders, manufacturers, and others in the construction industry, including those proposing to build a new home. I am also a Homestar Assessor.
For more information feel free to contact Keith at EcoRate Ltd on 021 890 251, [email protected], or our website

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