In the last eight years, I’ve had the pleasure of working on and visiting many “green homes” across the country. Without exception, clients and homeowners feel good about living a more sustainable lifestyle and the reduction of energy consumption. They’re proud of their green building materials and count kilowatt use and production.
Today, most people who want a “green home” may be motivated by environmental concerns, yet they are also trying to get the most house for the least amount of money. As a result, most green builders now make return on investment (ROI) a central selling point – promising that higher upfront cost will be offset by savings downstream.
ROI may drive the market now, but homeowners are becoming aware of other key benefits of sustainable homes: enhanced comfort and improved indoor air quality (IAQ).
Comfort and IAQ are thus emerging as great new potential selling points for green builders that can both affirm and better rationalize a homebuyer’s decision to “go green.”
Enhanced comfort means:
- The home is draught-free: It has a complete and continuous airtight layer.
- The home has no cold spots: All interior surface temperatures are within 3 degrees of the room temperature, so building design prevents radiant surfaces and convection currents from cooling occupants.
- There’s no overheating: Internal energy gains are accounted for including solar gain with site-specific shading, window tuning and orientation strategies being implemented.
- The home offers fresh air: Energy recovery balanced ventilation system (HRV or ERV) is implemented to create healthy IAQ and reduce energy consumption.
All homeowners, but particularly those with children and households with the elderly, will prize the benefits of good indoor air quality:
- Better breathing: the amount of oxygen with each breath is higher with a lower amount of contaminants. This puts less strain on your body and energizes occupants, also preventing future respiratory issues.
- Better sleep: rates of breath change throughout sleep cycles. Contaminants in the air or a reduction in oxygen can interrupt sleep cycles.
- Reduced allergens: less dust, pollen, mold spores, animal dander, and cockroach debris means less sneezing, coughing, congestion, irritated eyes, and hay fever.
- Balanced humidity: staying roughly in 45 to 55 percent relative humidity reduces mold and bacteria growth while also preventing skin irritation and nosebleeds from dry air.
Builders can increase comfort and IAQ with very modest cost increases, just by incorporating more refined building science. Those who offer these enhancements are making their homes even more desirable to buyers, an avenue to increased sales. To offer this enhanced selling proposition, builders will first take some preliminary steps to avoid common missteps: like incomplete air barriers, unbalanced ventilation, convection loops, thermal bridging, surface temperature, and inaccurate R-value calculations. These are all easily avoided, as follows.
Many homes in Asheville are being built with an airtight barrier exterior of the wall/roof sheathing. This is a good thing! But it’s a part of a whole. The air barrier needs to tie into the foundation to create a complete and continuous airtight layer that effectively reduces infiltration/exfiltration.
I’ve seen many homes in the Asheville area utilizing the ZIP System as their air barrier. A cost savings option would be using at least 3/8th inch thick plywood or OSB with air sealed seams, as the air barrier. Although airtightness isn’t guaranteed in these products, it is commonly found [NRC-IRC: “Air Permeance of Building Materials”]. OSB has lower costs than plywood but is more susceptible to moisture damage, often causing edges to flare, which could damage air sealed seams.
When a building is airtight there also needs to be a balanced air ventilation system, ideally with energy transfer, such as a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).
- HRVs exchange only sensible energy – energy we feel such as temperature.
- ERVs exchange sensible and latent energy – energy that’s stored within a phase change.
I’ve often seen airtight homes that don’t use HRV/ERV, but instead continuously vent out the bathroom. These then rely on make-up air from the leaky windows and doors as well as the foundation if it doesn’t have an air barrier.
These leaky locations are where it’s the wettest and most likelihood for mold growth. This creates poor IAQ that could hurt building occupants, especially children and the elderly, as well as risk structural failure from wood rot.
A HRV or ERV is necessary for an airtight building to have superior comfort and IAQ.
Convection loops and thermal bridging
In addition to the airtight layer being complete and continuous, it also needs to be in complete contact with the thermal layer. This can easily be done by using a loose-fill insulation in the stud bays and/or an exterior rigid board insulation touching the air barrier sheathing. To reduce costs these two practices are often instead done with batt insulation and no exterior rigid board.
Exterior rigid board insulation on the sheathing creates a direct contact with the air barrier and prevents convective air loops. The exterior rigid board insulation also creates a thermal break, preventing thermal bridging at the wall studs. This prevents energy transfer and creates a more comfortable interior.
Batt insulation, even when installed properly, rarely has complete connection with the sheathing, i.e., the air barrier. Even a 1/8th inch gap between the air barrier and thermal layer will create interstitial convection cycling that can reduce the R-value by as much as 45% [AIA Abq: “Thermal Bypass and Air Barriers” by Armando Cobo]. This gap also leads to condensation issues.
Leaky windows, doors, and foundation create interior radiant and convective energy transfer due to the interior surface temperature being so different than room and occupant temperature. This pulls heat from the occupant and makes them feel cold and uncomfortable even if the room temperature is at a comfortable setting.
A critical point in this: the R-value of the building assembly (aka whole-wall R-value) isn’t the same as the R-value of the insulation. The true R-value is affected by thermal bridging, air leakage, wind washing, convective loops, radiation enhancements, thermal and hygric mass, improper installation, and even temperature.
When installing low-performance components (windows and doors) on a high performance house, the installation should be “future-proofed” to easily allow the later installation of high performance components. Similarly, airtight roof systems should be future-proofed to easily allow installation of renewable energy production.
Fairly recent innovations in building science can now be applied to create homes that offer high performance plus increased comfort and greatly improved indoor air quality – benefits that will make sustainable homes even more desirable in the marketplace.
The sales benefits will be optimized as the industry becomes more familiar with new climate-specific best practices. Things will also advance as “green materials” become less costly and more available.
The most useful step in the immediate future will be offering local examples of the new, more ideal sustainable home. Only then will homebuyers readily understand the comfort benefits, and thus be willing to pay for it! … which will drive the housing market and industry to change and grow.
The Asheville green building market is just starting to recognize the sales potential of high performance superior comfort homes. Those able to properly apply building science principles to offer superior comfort — while also carefully controlling costs — will be very attractive to homebuyers.
I think we have an immense opportunity for superior comfort homes in Asheville, and invite further discussion on these topics.
This post was submitted by guest blogger Nick Shaw, a certified passive house consultant and builder who recently relocated to Asheville. Nick can be reached by phone at 804-381-9699 or email at email@example.com.