The ‘language of insulation’ is not what you would call romantic. It’s unlikely to inspire a book of sonnets any time soon; budding Blakes, Chaucers and Shakespeares tend to have weightier things on their minds! by Paul Forrester in the April Issue of Insulate magazine
No: insulation is a technical, logical language, born of the laws of physics. Much of it is black and white, but there remains room for nuance. Used well, it is a powerful tool for communicating ideas about building design and performance. The words themselves may not be beautiful, but they can help create great spaces that inspire great work from the people who use them.
A fundamental measure of a thermal insulation material is its thermal conductivity (lambda value, units: W/mK), the amount of heat energy it conducts. Generally, thermal conductivity doesn’t change with thickness, so thermal resistance (R-value, units: m2K/W) is a more accurate measure of a material’s ability, at a specific thickness, to resist heat transfer.
The resistances of the materials in a wall or roof are combined with internal and external surface resistances, then the reciprocal taken, to give the thermal transmittance (U-value, units: W/m2K) of the element. Additional calculation procedures apply to floors, basements and steel framed structures.
Conduction through materials is one mechanism for heat transfer; another, via the movement of air, is convection. The third principle mechanism occurs due to bodies emitting and absorbing electromagnetic radiation at different rates, depending on temperature and emissivity.
Thermal insulation is most effective when installed as a continuous layer, forming a thermal envelope. Interruptions to the insulation layer are thermal bridges (or cold bridges), and should be avoided as far as possible. They can be the result of structural requirements, poor design, or poor installation.
Thermal bridges can be repeating, occurring at consistent intervals (like timber rafters, a structural frame, or mechanical fixings), or linear, where the geometry of a construction element changes or at the junction between elements (the floor/wall junction, the wall/roof junction, or around door and window openings).
Gaps in insulation layers can result in warm and cold air mixing, and circulating around the insulation layer. This is one type of thermal bypass, a collective term for different types of air movement (also including air infiltration and wind washing) that result in significantly increased heat loss.
Taking a fabric first approach to construction centres on reducing heating demand and energy use through efficient building fabric (i.e. insulation and well-detailed thermal bridges) and passive heating techniques. Maximising solar gains through good design maintains comfort in winter, with solar shading to limit the risk of summer overheating.
A fabric first approach also relies on airtightness, to stop warm air leaking from the building, and controlled ventilation, to make sure occupants have fresh air to breathe. Design for low energy construction typically features a continuous airtightness line within the thermal envelope; the level of airtightness dictates the ventilation measures that should be installed.
Air holds moisture vapour. The warmer the air, the more moisture vapour it can hold. When air comes into contact with colder surfaces it drops in temperature. The temperature at which the air is saturated by its current moisture load (100% RH) is the dew point; when the air drops below the dew point it can no longer hold the excess moisture vapour and deposits it as condensation.
Surface condensation occurs on surfaces that are colder than the surrounding construction, such as windows or badly detailed thermal bridges, while interstitial condensation typically occurs unseen within the layers of a construction.
In terms of general moisture management, materials should be selected appropriately. Closed cell products absorb limited quantities of moisture and do not allow the passage of moisture vapour, unlike vapour permeable materials. Older buildings, especially with solid walls, often rely on moisture transport through the building fabric, so any retrofit measures must be sympathetic.
Acoustic Insulation and More
As well as thermal insulation, fibrous materials also provide sound insulation. They can reduce airborne sound or act as a resilient layer to deaden vibrations. At their heaviest, they can add mass to a building and reduce impact sound, including deadening rain noise on a roof.
Other performance characteristics influence a material’s suitability for certain applications. Compressive strength is the ability to bear imposed loads; some products also declare compressive creep to demonstrate their behaviour under load for an extended period.
Specific heat capacity is used to calculate the contribution to an element’s thermal mass. Thermal insulation products resist the transfer of heat rather than storing it, so effective use of thermal mass – also part of a fabric first approach – depends on the types of structures into which the insulation is installed.
The introduction of Construction Product Regulations, including requiring products to be CE marked to harmonised European standards, has raised quality and improved the consistency of insulation products.
An overview like this can never exhaustively cover ‘the language of insulation’ but we hope it has proved a useful summary of insulation properties and some of the issues associated with its use. Did we miss anything from the topics covered? Are there other insulation-related topics we could do a follow-up on? Get in touch and let us know!