The path to near net-zero is underfoot
February 2, 2014 | By Steve Rohrbaugh
Homes use a significant amount of energy to operate. Depending upon the location, about 63 per cent of that energy may be consumed by space heating according to Natural Resources Canada. Given the continual escalation of greenhouse gas emissions, status quo is no longer acceptable.
In many jurisdictions there are initiatives exploring high-efficiency homes and even net-zero energy residential construction, which may develop into new requirements over the next decade. A net-zero home produces as much energy as it uses over the course of a year. But most homeowners are not yet prepared to invest in on-site renewable energy systems to reach net-zero. Perhaps, approaching near net-zero should still be the goal for every residential new construction and renovation project. Now is the time to begin getting comfortable with the higher performing technologies that will shape the future of residential buildings.
The path to near net-zero begins with reducing energy consumption. The crucial first step is reducing the need for energy with a well-insulated, tight building envelope, paving the way for the latest HVAC technologies to have maximum effect. Radiant heating brings a lot to the equation and can be done even better than in the past. For example, combining a high-performance radiant system with a geothermal heat pump and advanced mixing controls puts you on track to cut your heating energy usage by more than half. Once you have selected the optimum wall and roof construction along with low U-value windows and doors, a well-designed radiant heating system should be your next priority in your journey to near net-zero.
CONSIDERING RADIANT DESIGN OPTIONS
Understanding current radiant heating installation methods and their performance attributes is fundamental to achieving energy savings. There are three practical and proven methods for residential installations. One is for installers to secure PEX pipes with aluminum heat transfer plates to the underside of the subfloor in joist cavities. Another is to pour a thin slab of concrete that covers the PEX pipes, which are fastened to the top of the subfloor. A third is to fasten heat transfer panels to the top of the subfloor, then snap the PEX pipes into these panels. The choice between these installation methods is influenced by the scope of each project.
With many renovations, the homeowner may want to keep the existing wood, tile or marble floor. Heat transfer plates make radiant feasible under these conditions. To drive heat into the room, the underlying joist cavity is filled with insulation typically four to five times greater in R-value than the resistance of the flooring above. The minimum amount of insulation is specified in the CSA B214 Installation Code for Hydronic Heating Systems. A downside to heat transfer plate constructions is that optimal space heating comfort is not achieved. First, it is impossible to configure perimeter and occupied pipe spacing layouts. Second, it is difficult to zone rooms because the joists prevent the installation of the most efficient pipe layouts. These constraints limit system efficiency.
THIN SLAB OVERPOUR
An above-the-floor system such as a wet overpour is widely used in new construction. Overpour installations allow for the quick and easy placement of complex piping layouts that optimize the occupants’ thermal comfort. The thin slab also has a sound deadening effect. Installers need to make sure they have considered the extra material and labour costs to reinforce the subfloor and double plate the walls. Other trades should not be allowed to work inside the home while the thin slab is being poured and the floor finishers should not be scheduled for weeks to months later until the slab is cured.
Heat transfer panels are an above-floor alternative that fits tighter construction schedules and requires less coordination. Panels and pipes install quickly using typical framing tools and complex piping layouts are easy to configure with basic carpentry skills. Finished flooring can be installed the same day after pressure testing the PEX pipes.
Some renovation projects – such as when there is a full tear out of existing floor and wall coverings – are nearly equivalent to new construction. Either above-floor panels or below-floor plates can be installed. In these retrofit projects, it is generally not practical to reinforce the subfloor. An overpour should not be undertaken without the prior approval of an architect.
Radiant heating is not only under your feet. Heat transfer panels can also be installed in walls and ceilings, behind the gypsum board, as heating surfaces to increase the heat output into the room.
EVALUATING RADIANT PERFORMANCE
The different radiant floor installation methods have significant differences in performance. Pipes underneath the subfloor, either with or without plates, are the least efficient radiant systems at transferring heat into the above room. Plate installations require higher water temperatures to heat the room and there are maximum temperature limits that must be adhered to. Floor coverings, underlayments, adhesives and grouts can be damaged by excessive temperatures leading to discoloration, noise, delamination, warping, cracking and deterioration.
More important than protecting the flooring is to ensure the occupants foot comfort and safety by limiting the surface floor temperatures in accordance with CSA B214. Working within these design temperature limits, the designer often will select supplemental heat to achieve the remaining heat requirements. These separate systems increase the installation costs and are less efficient, resulting in higher energy usage.
Overpour and high-performance heat transfer panel installations are more efficient at spreading the heat evenly underneath the flooring, allowing the room to be heated with lower water temperatures. High-performance heat transfer panels, with their low thermal mass and high heat conductivity, result in a quicker response time than overpour radiant systems. Quicker response, particularly during the spring and fall “shoulder” months, adds efficiency and increases comfort by reducing the tendency to overheat the room, which can occur with slab systems. Also this fast pick-up time can be combined with advanced controls for nighttime or unoccupied setback, resulting in less demand on the high-performance radiant panel system which translates to less energy consumed.
PAVING THE WAY FOR GEOTHERMAL
High-performance panels may provide up to 50 per cent higher heat output in comparison to thin-slab overpour or traditional panel and plate systems, and more than double the heat output of just pipes installed in the joist cavity. This higher performance results in a lower required heating water temperature at design conditions. As radiant is optimized, there’s a sweet spot in performance levels where radiant and geothermal are perfectly compatible.
Performance matters with space heating system designs. Energy savings is typically 25 to 50 per cent with geoexchange compared to other HVAC systems, according to the Canadian GeoExchange Coalition. Geothermal exchange systems do not burn combustible fuel to make heat and they provide three to four units of renewable energy for every one unit used to power the system. High-performance radiant heating panels are best suited to maximize this geothermal advantage by heating the room with low water temperatures that fit in the operating range of geothermal heat pumps without requiring supplemental heating systems.
DESIGNING FOR NEAR NET-ZERO
A radiant specialist should get involved early in the project when there is a better chance to influence the design selections. It takes careful planning and involvement from the whole team – architects, radiant designers, general contractors and radiant installers – t
o achieve maximum energy savings. Together, this team can take steps to get nearer to net-zero by reducing the homeowner’s energy requirements with:
• Well-insulated walls, ceilings, windows and doors
• Efficient and comfortable space heating with high-performance radiant heat transfer panels
• High coefficient of performance energy sources such as geothermal exchange
Why wait until net-zero is a mandate? With additional planning and a focus on maximizing performance, near net-zero construction is within reach today. Let’s get a few steps down the path by gaining experience with higher performing technologies now and preparing ourselves for a convincing dialogue with homeowners and decision makers.Steve Rohrbaugh is an applications engineer in the building technologies, civil engineering and infrastructure divisions of REHAU North America.