It is vital to understand the pros and cons of insulation materials in order to make the correct choice for a specific situation. No matter how good the product, all can be undone with poor installation and/or wrong specification.
When specifying insulation, Duncan Goldsmith, managing director of Rigifoam, warns against basing decisions on a salesman’s pitch or bias data sheets. “There are numerous options, but thermal insulation should insulate against all types of heat transfer, namely conduction, convection and radiation,” he states.
What to look out for
The better the thermal conductivity, less material is necessary to achieve the required thermal resistance (R-value). As part of the calculation, it is important to know that thermal conductivity (lambda value) is measured in W/mK, and is also expressed as the k-value (also k, λ or κ). The key is to consider the real value and not just the claimed laboratory value.
An insulation material’s R-value is calculated by thickness/thermal conductivity. So for 30mm Lambdaboard® at 0,024W/mK, the R-value would work out to be 0,03/0,024 = 1,25m2/Wk.
Don’t be mislead
• Radiant barriers
Goldsmith explains that a radiant barrier (low emissivity substrate) does not have an R-value, but if installed with an airgap, it will insulate against radiant heat transfer while convection and conductive heat will still pass through. However, if in contact with any heat-conducting material such as steel, all insulation properties are lost. “Think of a roast chicken, the foil is direct contact with the chicken hence it cooks from convection and conductive heat,” he illustrates.
If installed correctly, it will reduce the heat gain of building in a hot climate, but is not able to insulate against the cold in cooler months. Radiant barriers are best used in conjunction with other insulation products.
• Bulk insulation
The R-value of bulk/fibrous/cellulous insulation installed over purlin under roof sheeting, with the compression factor, is up to 31% overstated compared to controlled condition testing, according to the latest ASHRAE 90.1 table.*
“What is often not mentioned, is that air movement influences the performance of open-cell products, just as wind blows through a jersey,” explains Goldsmith. “These products also fill with moisture over time, from people breathing as well as the humidity from bathrooms and kitchens. The water formed after condensation reduces insulation properties.”
If installed correctly as per international standards, both radiant barriers and bulk insulation do insulate, but they need to be protected from elements that reduce their lifespan.
Rigid closed-cell insulation’s thermal conductivity is not only as low as 0,024W/mk, but more importantly, the polyisocyanurate material doesn’t lose shape over time and isn’t influenced by wind, moisture or dust. It is also resistant to solvents and heat from down-lighters.
According to Goldsmith, Lambdaboard® has the lowest thermal conductivity value of all commercially available insulation media, with the highest R-value per 25mm. This R-value isn’t compromised during installation since the board doesn’t compress and its dimensional stability changes less than 0,1% in a temperature range of minus 40°C to 120°C.
Lambdaboard® insulates against convection, conduction and radiation in both hot and cold environments. Being closed cell, it does not absorb water, although in humid conditions a vapour barrier is required as with other types of insulation. The thermosetting polyisocyanurate material also doesn’t soften, melt and drip like thermoplastic insulants, and when exposed to fire, it forms a char layer to prevent flames from spreading.
Goldsmith points out that polyisocyanurate accounts for more than 80% of all retail, commercial and industrial roof insulation in the USA, with a demand of over 400 million square metres per year. It is also the most popular insulated backing board for shingles roof covering.
Tel: 011 421 0313
Website: www.rigifoam.com or www.lambdaboard.co.za
• Over purlin.
• Hot and cold deck.
• Under floor.