Natural Insulation
Product choices and considerations for energy savings
Thermafiber’s Mineral Wool Insulation contains over 80% recycled content and helps projects qualify for LEED Green Building Credits.
by Dr. Guido Wimmers .Insulation is only one component of many in the building envelope - but arguably the most critical one in determining energy savings and interior thermal comfort. Every aspect of the envelope design must be considered including the air tightness and the positioning of windows within the thickness of the walls.
From the point of view of thermal resistance, the requirements embedded in building codes are, by the very nature of the process of code-making, bare minimum standards that fall far short of what is achievable [and desirable] at any given time.
Insulation Materials
Over the lifespan of a building the application of insulation will always have a positive environmental impact by reducing operating energy. However, the ecological footprint of the material itself is complicated to define because there are a lot of different factors to be considered.
Fibreglass remains the industry standard in North America, although it is by no means the default choice for all applications. Most fibreglass insulation now contains some recycled content, and some manufacturers have replaced the traditional phenol formaldehyde binder with other more benign alternatives - or use no binder at all. Unlike loose fill, fibreglass batts are considered to have little or no negative impact on indoor environmental quality.
Spray applied foam insulation continues to be used in some higher performance residential buildings as it is easier to achieve continuity of insulation with spray applied products than with fibreglass batts. It should be noted, however, that sprayed foam insulation still requires a vapour barrier on the inside face to ensure that warm humid air does not cause damage to the wood wall studs over time. Despite improvements in chemical
formulas, spray foams are not environmentally benign, and face increasing competition from natural-source alternatives.
Among the natural insulation materials that are commercially available, wood fibre and cellulose fibre [usually recycled newsprint], are now very popular in Europe. Also spray applied, they are quite dense, but do still require an air barrier. Like spray applied foam, their performance is less contractor-dependent. Cotton insulation made out of recycled blue jeans and completely new materials like sheep’s wool, hemp or flax are also emerging as eco-friendly alternatives.
In industrial and commercial construction mineral wool and rigid foam insulation remain popular, mineral wool for its fire resistance, rigid foam as a convenient way of reducing thermal bridging in steel stud construction. Extraction and processing of mineral wool products [a by-product of steel processing] may still be an environmental concern, and rigid foam products are non-recyclable - even if they can be reclaimed and re-used. Again the chemical composition of rigid foam insulation has been improved, but is not environmentally benign.
Selecting the right insulation material
Among the questions to be asked are how comfortable, how environmentally friendly, how healthy and how ‘future proof’ do you want your building to be? Insulation can also be more than an energy barrier, providing among other things, fire resistance, humidity control or noise reduction.
All materials have their own strengths and weaknesses to varying degrees. A lot of the fibre-based materials such as cellulose fibre or wood fibre are sensitive to water exposure, but this is not necessarily a disadvantage. These materials can modify humidity levels, are well suited to so-called breathable construction, and can compensate for minor deficiencies in workmanship. They must however be protected from external moisture sources by careful envelope design.
Classification of insulation materials is quite difficult because there are several general ways of classification and differentiation. They can be differentiated by raw material into inorganic or organic, by whether they are renewable or non-renewable, or by their consistency such as foam, rigid, wool or loose.
By saving energy, insulation contributes to a reduction in the environmental impact of buildings, not to mention the economic considerations. Insulation can also have a bearing on indoor environmental quality and, according to type, insulation can also offer a multitude of other benefits such as found in natural insulations.
Although R-values for equivalent thicknesses of insulation vary somewhat between the different materials [polystyrene, mineral wool and fibreglass are typically better insulators than cellulose or other organic materials], often the choice is also influenced by the other properties - such as thermal storage, fire resistance or noise reduction characteristics.
Specific Heat Capacity
This term is used to compare the heat storage capacity per unit weight of different materials. It is not exactly the same as thermal mass, this term usually being used to quantify the heat storage capacity of a building element or an entire structure.
Heat capacity is not linearly related to weight. If you compare one kg of wood fibre, with one kg of rock wool or fibreglass, wood fiber has nearly 2.5 times the thermal mass. Values for organic foams like polyurethane, styrofoam and icynene lie between these two extremes.
Also, the density of insulation materials is very different. Foams have a density of between 15 and 40 kg/m3, but wood fibre typically has a density of 160 kg/m3 so that the effective thermal mass of equal thicknesses of insulation can vary by a factor of 10.
Fire Resistance
The combustibility of insulation materials is also an important consideration, although deaths in fire situations are most commonly caused by the inhalation of smoke generated by the room contents. Another potential problem is the chimney effect which is caused by the shrinking of insulation in wall cavities, when they are overheated. Products like rock wool or even cellulose and wood fibre perform better in fire situations than polyurethane or polystyrene based foams or fibreglass.
Noise Reduction
Noise reduction can be a very valuable and important benefit of thermal insulation. There are two characteristics that materials need to have a positive influence on noise reduction: mass and flexibility. Polystyrene or polyurethane, for example, have neither and, therefore, nearly no influence on noise. Rock wool, fibreglass and cellulose fibres are soft and have a significant mass so they can make a contribution to noise reduction. The densest insulating material is wood wool which is a very efficient sound deadener.
Insulation in high performance buildings - A European perspective
Canadians have happily gone high tech in the design of their outdoor clothing, with breathable fabrics, super insulated linings and careful detailing of seams and closures to keep out wind, water and cold. It is surprising then to a visitor that this is not also true in regard to the design of building envelopes. When one considers that we spend about 20 hours per day in buildings, it would seem logical to pay at least as much attention to them as we do to our clothing.
Standards of insulation in European buildings, particularly houses, are two to three times higher than those in Canada; and insulation forms part of a holistic and high-tech approach to building envelope design.
The thermal resistance of the insulation, and the design of windows, are the main factors in controlling the interior surface temperature of the exterior walls, and so eliminating the uncomfortable effects of radiation [from warm surfaces in summer, and to cold surfaces in winter] on the occupants of the building.
This is not just a seasonal concern, as at any time the differences in internal surface temperature, which result in asymmetrical radiation, are also important to the sensation of comfort. Human beings are sensitive instruments capable of discerning, and being discomforted by, differences in temperature of adjacent surfaces of as little as 3º C . This has a bearing on the choice of heating system since, for example, forced air and electric baseboard systems both create hot spots relative to poorly insulated adjacent surfaces that can result in occupant discomfort.
Even though thermal mass is widely considered to be beneficial in reducing the energy demand in buildings by moderating diurnal temperature variations, the savings that result may be as little as 10% of operating energy. Typically the benefit of superior levels of insulation is considerably greater.
To make design decisions only on the basis of the cost of energy, the cost of insulation and the desired payback period, is to ignore the long term benefits of better design. Canadian codes and standards tend to focus on saving on the cost of operating energy, rather than saving on the overall quantity of operating energy.
By insulating to a higher standard to reduce the overall energy demand and the temperature of the heat distribution system of a building, one can also increase the beneficial impact of non-traditional energy sources. We always have to focus on the energy itself and not on the cheapest source of supply because the price of energy is not necessarily an indicator of an environmentally-friendly energy production.
Taking a performance-based, as opposed to a prescriptive approach to design serves to emphasize the importance of insulation. It is only after the various building envelope options have been modelled and optimized that one can establish the amount of input energy required for the building and how to distribute it. The projected consumption in KWhr/m2/annum will help to determine what energy sources would be most appropriate for the given design.
As a point of reference, the most rigorous European standard - the Passive House Standard [see sidebar to Canadian Green Housing Initiatives article in this issue] - regulates input energy to a maximum 15KWh/m2/annum - about one tenth of that in a typical new 200 m2 Canadian house, a difference equivalent to 300 litres of oil, 300m3 of natural gas or 3000KWh of electricity.
The thickness of insulation required to reduce energy consumption to these levels results in an exceptionally steep temperature gradient, across the wall section, and requires rigorous control of thermal bridging, moisture and air movement to avoid condensation and mould growth.
When we refer to breathable construction we are not talking about promoting air flow. What we want is a vapour open construction which allows water in the form of humid vapour to defuse from high saturation into lower saturation without hitting the dew point.
The trick is to design a sequence of insulation layers that will forgive little mistakes in construction. Therefore, in Europe, the general rule of thumb is to choose a layer which acts as a vapour check near the inside face, and increase the vapour transmission step by step [to about a factor of 5] by introducing other insulation materials progressively in layers toward the outside.
Energy conservation and the desire for higher levels of comfort may bring Canadian practice in line with European practice. We have the insulation products. The caution is that, with these higher standards, comes the need for a greater understanding
of building physics, and strict control of workmanship on site.
passive house design services, aiming to combine the best of North American and European sustainable design practices. .
