Net Zero Energy (NZE) houses are designed, modelled and constructed to produce as much energy as they consume on an annual basis. Super-efficient NZE homes use sustainable rooftop solar energy production and small electric-powered heating systems, such as air source heat pumps, to produce as much energy as they consume.
Passive House homes adhere to a voluntary standard for energy efficiency in a building, designed to reduce the ecological footprint. Like Net Zero homes, passive houses are built to optimize thermal gain and minimize thermal losses. Passive houses rely on mechanical ventilation with heat recovery, and the energy required to heat a passive house is typically 90% lower than that of conventional buildings.
Net zero or passive house, these high-performance buildings share several core design objectives: a sealed building envelope, superior insulation, high-efficiency windows and doors, and minimizing thermal bridging. Thermal bridging occurs when a home has low resistance to heat transfer and therefore leaks thermal energy more quickly.
By meeting energy efficiency targets, the homeowners of high-performance homes enjoy reduced operating costs, a more comfortable and quiet living space, high air quality and other health benefits, a more durable building, and the satisfaction of knowing they’re making a significant contribution to reducing carbon emissions.
Insulation is at the heart of energy efficiency, because the more insulation you have, the less energy you’re going to use heating and cooling your home.
Sub-slab insulation and insulating the foundation walls
Most passive house and net zero building designs include rigid insulation below the concrete slab. Not all building codes recognize its value or require it, but concrete is not good at trapping heat or keeping moisture out.
Up to 10% of the heat in conventional homes escapes through the floor, accounting for roughly 10% of the family’s energy bills. Rigid insulation with a minimum of R-20 is the perfect way to protect your home’s concrete slab, keeping water away, preventing cracks and structural damage, while noticeably reducing energy consumption. Rigid foam is installed on top of the gravel and covered with a vapour barrier. The reinforced concrete slab is then poured directly on top.
The best performance and durability are achieved by also insulating both the interior and exterior of the foundation walls of the building. Foundation wall insulation can increase the thermal performance of basements while reducing the chance of interior moisture issues by raising the temperature of the concrete. Insulated Concrete Form (ICF) foundations are a stay-in-place expanded polystyrene concrete forming system designed to deliver superior energy performance.
Optimum Value Engineering (OVE) advanced framing
Advanced framing is a construction technique employed to reduce the amount of lumber used and waste created when engineering and building wood-framed houses. The principles of Optimum Value Engineering were originally established as a way to maximize the profitability of a construction project, but replacing timber with insulation is also a brilliant way to improve the energy efficiency of the building envelope.
Maximizing the insulated wall space helps to eliminate thermal bridging, thereby improving the whole-wall R-value. By utilizing the structural quality of materials like rock wool panels, for example, advanced framing design has been able to increase the on-centre spacing of the floor joists, wall studs and roof rafters. In addition to continuous rigid insulation, SIPs (structural insulated panels), strips of insulation applied to the top of wood studs, and double-studded walls can increase the R-value while minimizing thermal bridging and thermal breaks.
To achieve the high R-values required by the Passive House standard, thick walls are built using ‘super insulation’. Using typical insulating materials like mineral wool, polystyrene or cellulose, the thickness required is roughly 300mm.
Most passive houses rely on very heavy insulation like R-40 to R-60 in 12″ to 24″ thick walls, R-50 to R-100 in 12″ to 24″ thick roofs, and often R-30 to 50 sub-slab insulation, triple- or quad-glazed low-e windows, and exceptional avoidance of thermal bridges. Thermal bridges need to be accurately accounted for in the building’s design calculations.
To meet the Passive House standard the building must not have an air change rate of more than 0.6/hour ( 0.6x the volume of the house), or 5 times tighter than ENERGY STAR®. The ultra-airtight construction and the thick walls dictated by R-value requirements, usually lead designers to choose simpler shapes in their design. It’s also more difficult to deal with thermal bridging challenges in a house with many angles, twists, turns and corners.
Insulation is rated with an R-value, which is a measure of thermal resistance. Here are the most common insulation materials used in BC high-performance homes.
Mineral wool fibre panel insulation
Mineral wool usually refers to either rock wool or slag wool. Rock wool is a man-made material manufactured from natural minerals like basalt or diabase. Slag wool is a man-made material manufactured from blast furnace slag. On average, mineral wool contains an average of 75% post-industrial recycled material.
Mineral wool is a preferred material when constructing high-performance buildings. It delivers superior energy efficiency, fire protection and maximum indoor comfort. Mineral wool is created in commercial furnaces where minerals and other raw materials are heated to roughly 1,600° C (2,910° F) and subjected to a current of steam or air. Oil is added during production to decrease the formation of dust. The molten rock is rotated at high speeds in a spinning wheel, similar to the way that cotton candy is made.
Rock wool has an R-value of between 3.10 and 4.0. It should be protected from exposure to water because it can retain a large amount of it.
Dense-pack blown-in insulation naturally fills all gaps and cracks. Walls and floors can be filled with dense-pack fibreglass or cellulose to achieve the required insulating value. Dense pack fibreglass has an R-value of about 4.2 per inch. A more sustainable, natural, recycled material, blown-in cellulose makes a good alternative to fibreglass. Dense-pack is installed at a density of 3.5 pounds per cubic foot to avoid settling. It must be protected by a moisture barrier.
Spray foam is one of the commonly used insulators in the walls and rafters. Closed cell foam – also called high-density foam – is impermeable to water vapour. This makes it a good choice for unvented attics or crawl spaces. Because of the way it’s applied, closed cell foam can fill any voids, thereby greatly improving the airtightness of the envelope.
Rigid foam board
Rigid foam board insulation is typically used under concrete slabs or as a reasonable alternative to blown-in fibreglass or cellulose in limited spaces, where more R-value is needed. Foam board may be used on the exterior of regular walls, where added R-value is required. It can also be used above the roof sheathing, as part of an unvented vaulted ceiling, as a way to increase the insulation value near the eaves of a low-slope roof. It is also commonly used in locations where plumbing or ducts need to be placed close to the outside wall sheathing.
Continuous insulation and a weather-resistant barrier
No matter its R-value, or thermal performance, if insulation isn’t continuous throughout the building envelope, heat will escape, wasting energy and consequently money. Continuous insulation – aka outsulation – is a term defined by the American Society of Heating, Refrigerating and Air-conditioning Engineers 90.1 (ASHRAE 90.1) documentation. The insulation is uncompressed and continuous across all structural members except for some fasteners and service openings. Wrapping a building’s envelope with a layer of continuous insulation not only improves its effective R-value; it is an essential element of a high-performance building.
Continuous insulation can reduce the wall’s capacity to release moisture from inside the wall assembly. Trapped moisture within a wall system could cause mould growth and wood rot, so a vapour permeable air and moisture-resistant barrier is an essential part of the building envelope.
Exterior doors and windows
A high-performance home should be fitted with ENERGY STAR® certified doors, windows and skylights, appropriate for the climate zone. ENERGY STAR® certified windows typically lower energy bills by roughly 12 percent.
Windows and glazing used throughout in passive house or net zero projects are typically triple- or quad-glazed to retain heat. A common passive house specification is for U=0.15 (0.8 W/m2 K) or less for windows. As high-performance homes have become more popular the price of high-efficiency windows has gone down, performance has gone up – with some achieving R8 – and there’s an ever-increasing variety of options.
Low-emissivity (Low-E) coatings are microscopically thin layers of metal applied to the inner surfaces of the glass, or in some cases, on layers of clear film suspended between the glass panes. The coatings reduce heat transfer. Inert gases, like argon or krypton, often fill the space between the panes and allow less heat transfer than air does. High-performance glazing units often use thermally-improved plastic spacers, or some kind of plastic “thermal break” in the spacer, to reduce heat transfer around the edges.
High thermal mass
Many of us grew up watching our parents turn down the thermostat in the mornings, maintaining a lower constant temperature during the daytime when everyone was out, to save on energy costs. Less heat loss occurred during the day when the house was cooler because it decreased the differential between indoor and outdoor temperatures. Modern high-performance homes take advantage of an airtight envelope, super insulation and often thermal mass, and turning down the thermostat may result in less than a degree of temperature change over an entire day.
Both net zero and passive house building designs can benefit from utilizing high thermal mass in the building envelope to stabilize temperature shifts by absorbing and storing heat energy. Designers will often use high thermal mass materials like concrete, stone and brick rather than steel, wood and carpeting.
Advantages of thermal mass include heating during off-peak hours, heat security in the event of power outages, and extraordinary comfort. During the hottest summer days, thermal mass can be a mixed blessing, if the windows will be opened to take advantage of cooler outdoor air.