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Building Science
Content: Building Envelope>Blower Door>Thermal Bridges>Indoor Air Quality>Natural & Alternative Walls>Building Science & Other Links
Asheville Green Building needs more Science.
General contractors, architects, home builders and even the green builders need more education in building science. It shows us the healthiest and most energy efficient ways to build new homes and improve existing ones in Asheville, Western North Carolina and surrounding climates.
Best building science practices focus on the building envelope and fresh air introduction. Explore some cost-effective approaches to energy efficiency and healthy, green building.
Antique Framing for your Building Envelope???
This building envelope is typical for Asheville NC new homes Very Leaky and Poorly Insulated.
The building envelope is the single, greatest factor affecting a home's energy use and comfort. Indoor air quality and other operating costs like maintenance and durability are directly dependent on the building envelope. It is the most permanent feature of a home and the hardest to correct and improve.
To have healthy indoor air quality we must have control over the building envelope. The same is true for our energy costs. Heating and cooling represents the biggest energy use by far but it makes little sense to invest in efficient mechanical systems without a way of controlling that valuable conditioned air. Building Envelope upgrades help pay for themselves through smaller heating and cooling systems with less fuel and energy costs used throughout the life of the structure.
"Stick framing" or traditional framing with studs on exterior walls, creates lots of opportunities for air leaks and cold spots. Any where there is an air leak or cold spot there is also an opportunity for mold to develop. This antique framing could be performing MUCH better with a functional air barrier and exterior insulated sheathing.
This was a good way to build homes in the early part of last century. There are much better ways of framing and insulating a homes envelope that have been thouroughly researched by building scientists with decades of results. The trick is building an air-tight, well insulated envelope for similar costs to conventional construction.
May I have the building envelope please...? 
- Air-tight as possible
- Continuous insulation = Reduce thermal bridging as much as possible
The best building envelopes have minimal thermal bridging and are air-tight. Thermal bridges are gaps in the insulation, occupied by a solid material. They tend to be framing and structural components like 2x4s. Each stud or framing member can represent a hole in the envelope that is absent of insulation, potentially leaks air, transports energy the wrong way and offers the opportunity for mold.
There are many ways to achieve airtightness and minimize thermal bridges. To make the best choices, understand how these two fundamentals work.
A good Building Envelope is as Air Tight as possible.
The Blower Door Test: A new home builders most important diagnostic tool.
This is the greatest invention in Building Science. The results from this test gives one the best idea of new homes and existing home performance relating to energy and comfort. This test reveals the most critical aspect of a building envelope. Results have a big impact on a homes Indoor Air Quality (IAQ).
Important metric to compare new home builders and green homes: Energy, IAQ, Comfort, and Durablility.
A blower door is simply a fan with a pressure gauge. It is attached inside of a doorway. Results show how "leaky" or drafty the house construction is.
| ACH50 | Test Type and Result | |
| < 1.5 | High Performance | |
| 1.5 - 2.25 | Tight | |
| 2.25 - 7 | Average to Leaky | |
| > 7 | Very Leaky |
This tool and test has driven development of green and high performance homes more than anything else. It's a direct meaurement of how much energy a home is likely to consume and the ability of having control over the indoor air quality. Results are increasingly recognized as THE most important metric of green homes and especially energy-efficient homes. High efficiency home builders use past results as third-party verification of true performance. Most Asheville builders and contractors are still struggling to achieve tight results.
If you are involved in building a new home, it is your responsibility to pay very close attention to these numbers and reduce them as much as possible. The most cost-effective path to energy efficiency focuses on achieving excellent blower door results.
Our new homes consistently break the test records in Asheville and all of Western North Carolina.
Dont buildings need to breathe?
Tight envelopes offer better Indoor Air Quality IAQ thanks to fresh air introduction with Energy Recovery Ventilators (ERV & HRVs). An ERV is a ventilation device that contains a fan, heat exchanger and sometimes a filter. They bring in Outdoor air and exhaust indoor air in a balanced and energy efficient manner. Polluted as it is, Outdoor air is always cleaner than indoor air no matter how much trouble one goes through to filter and make the interior VOC free.
Internal heat exchanger of an ERV or HRV. Introduces fresh outdoor air while exhausting polluted indoor air. A simple, affordable method for excellent IAQ.
Buildings need to breathe, but we want to control where, when, and how. The Tighter the envelope, the more control of introduced fresh air and conditioned air. Conditioned air is expensive and accounts for the biggest portion of most people's environmental and social foot prints.
Introduced, outdoor air is healthy as long as one is controlling where, when and how it enters the home or building. ERVs and tight building envelopes offer the most affordable and effective control of healthy indoor air.
Tight Building Envelopes offer more mold resistance
When homes leak humid air through a crack or small hole, that humidity will condense on cold surfaces which is often the studs or structural framing members inside the wall (Thermal Bridges). Condensing humidity inside a wall makes water and the result is often mold. In our climate, this can happen from the interior and exterior in both the Winter and Summer. The only way to de-humidify interior air is by moving it, an important consideration for space conditioning in our climate.
Ignore R-Value until Thermal Bridging has been addressed
Well Insulated = Continuously Insulated = Eliminate Thermal Bridging
Alternative envelope systems that have the most research in our climate include Structural Insulated Panels (SIPS), Pre-Cast Concrete Panels(Superior Walls), and Insulated Concrete Forms (ICFs). These are appealing due to thier affordability, availability, code acceptance and performance. Generally, ICFs and Pre-Cast Concrete panels are used below ground as foundations and SIPs are used above ground for exterior walls and roof.
Nature's Thermal Imaging. Here we use one of our own Asheville custom homes to display Thermal Bridging. The stripes or "snow ghosting" reveal this roof's thermal bridges. They are directly above the 2x6s connecting the SIPS roof panels.
These 2X6 thermal bridges are hard at work: melting snow, wasting money, and polluting the environment. Some SIPS do a better job at eliminating thermal bridges but all SIPS have much less thermal briging than typical stick framing.
Eliminating Thermal Bridges can be more important than the amount of insulation or "R-value."
With an air-tight shell in place, verified by the blower door test, eliminating thermal bridging becomes the most important goal in creating a good building envelope. Thermal bridges represent missing insulation, actively transport energy the wrong way and they can accumulate moisture.
Its important to address thermal bridging before looking at R-value which is the amount of insulation. The danger of R value is that its usually only a measurement of the cavity insulation in between the framing members (studs, rafters, floor joists, etc). High amounts of R value do little good if the heat can enter or escape through other places in the building envelope.
| Typical Stick Framing | 23% |
| Smart Framing | 18% |
| Typical SIP | 8% |
| Eco-Panel SIP | 3% |
All SIPS dramatically reduce bridges in insulation, but not all SIPS are created equal. The percentage of thermal bridging in an envelope can differ greatly among the panel products available. In Western North Carolina, we are lucky to have a local panel manufacturer that claims the highest R-value and has the lowest amount of thermal bridging in thier products.
The most cost effective approach to energy efficiency and green building is creating a building envelope that is
- Air Tight
- Continuously Insulated
- Higher R value or insulation value
Putting it all together
With any airtight and continuously insulated wall or roof system chosen, it becomes even more important for contractors and home builders to ensure that components such as the floor framing, headers, connections, and transitions are well detailed to reduce thermal bridging and well sealed on-site to ensure maximum performance. Below is an expanding quick reference guide for building methods that are being used to obtain high performance at reasonable costs.
Consider traditional construction methods with newer details.
Stick framing or pre-fab (systems built) can compete with alternative envelope systems in a few ways. The most common is to use advanced, smart framing with well sealed, insulated sheathing on the exterior combined with an Airtight Drywall System. This method relies on framing and drywall laborers to seal the extensive amount of cracks that result from the many layers of this stick frame construction. Meticulous attention to detail is required for safe flashing and a reasonable level of airtightness.
Most asheville nc home builders including asheville green builders use little advanced framing, no airtight drywall approach but worst of all skip insulated sheathing altogether.
Cost is much less of an issue than knowledge, regarding home builders and consumers.
Double Stud Walls
Stick framing can also perform very well with double stud walls combined with raised heel trusses. Double stud walls are smart framed with offset, parallel walls resulting in 12 inch thick walls. Advantages include room for more affordable, environmentally friendly forms of insulation. Drawbacks include loss of floor space, less light entering rooms and more use of solid lumber and trim. Many double stud wall designs fall short on performance by not addressing the thermal bridging through other areas of the envelope most notably the foundation and floor framing. Insualted sheathing is a great solution for this. Larsen truss walls are another good way to address it but be careful of excessive labor costs.
ZIp system sheathing is recommended for both forms of stick framing as it creates a very effective air barrier. OSB is increasingly being used as an effective interior vapor and air barrier with a more permeable sheathing on the exterior with these types of assemblies. An additional housewrap is usually recommended for a Weather Resistive Barrier (WRB) and a rain screen detail depending on the siding. There are also new spray coating products that act as a WRB and Air Barrier worth considering as well as exterior spray foam.
Consider SIPS.
Scientific studies from Oak Ridge National Labs among others, prove that Structural Insulated Panels SIPS are one of the highest performing envelopes available.
The best part for builders and your budget is that SIPS are very easy to work with.
SIPS can be panelized or modularized in climate controlled conditions and assembled on site with the same set of skills used in conventional construction. This same set of skills is SIPS advantage to almost any other system. Faster time to "dry in" and less on-site labor help to counter the higher, material cost. Depending on the design and builder, SIPS can be just as affordable up-front and in some cases more affordable than conventional construction methods. Energy Costs are dramatically reduced.
Our favorite reasons to use SIPS :
- Airtight. One of the most consistent and easy to seal building envelope systems available.
- Continuously Insulated. Thermal bridging is dramatically lower than other most other options.
- Strong. More capable of withstanding disasters than typical construction.
- Pre-fab panelization. Using SIP panels offers many advantages of pre-fab in a more flexible and customizable system.
- Most amount of R- value per inch. Keeping wall thickness smaller increases square footage which could also reduce cost per square foot and allow more light (and solar heating) in windows.
- Less on-site waste and labor.
www.sips.org/ Structural Insulated Panel Association website.
Drawbacks include concerns over up-front costs, indoor air quality and to a smaller extent insulative qualities. For simple plans, we find costs are extremely competitive with conventional framing and SIPs are almost always a better value when long term energy performance is considered. Indoor Air Quality is almost always healthier with SIPs as long as they are built air-tight and the home uses an ERV. The insulative qualities vary greatly between panels of different foam types and thicknesses.
There have been some cases of SIP failures but almost all of them are the result of very poor installation practices. Some question the long term durability of SIPs chemistry but unless a home uses diagonal board sheathing, all homes rely on similar adhesives in OSB, plywood and many other engineerd products already common in homes. SIPS need to be protected and maintained just like regular homes and can usually be changed or fixed in the future just as easily.
Perhaps the greatest drawback is that most SIP foams are made from un-renewable resources (as are almost all quality foams). In terms of volume, foams are >95% air and they are resistant to moisture damage. Many feel that insulating housing stock is a good use of this precious resource. It drastically reduces decades worth of dirty energy use into the future.
The building envelope is the most permanent feature of a home. Dirty, monthly energy is used everyday, potentially for the rest of the envelope's life.
Timber Frame
Timber Frames are considered by many to be one of the greenest ways to build. Aesthetics are obvious. From a Building Science perspective, they are excellent, as long as the timber is inside the building envelope. This protects the timber/structural system and reduces the Thermal Bridging. Almost all Timber frame structures use a type of SIP as the exterior envelope. If the building is truly supported by the timber frame (some are only decorative) then a less expensive, non structural SIP can be used to create the envelope.
Natural Alternative Walls
As an Asheville green builder always on the lookout for a better way of doing things we would entertain any alternatives brought to us by a client.
We prefer the methods above. Below are options that have been brought to us before. We are willing to further explore with you these or any other green building methods.
Log Homes
Log homes enjoy much popularity in Western North Carolina. They can be beautiful, long lasting and are arguably, very renewable. From a building science perspective however, they are terrible performers. They are usually very drafty from the beginning and get worse over time as the solid lumber shrinks, cracks and moves. This movement can cause many problems besides air infiltration. Even a perfectly, air tight and stable log wall has an insulative value of R-1 per inch. To meet the poorest performance allowed by law in our area a log wall should be 13" thick, which is very rare. Dont worry log home builders, most stick frame new homes in asheville and WNC dont achieve a legal, true R13 either. Code inspectors generally do not enforce air infiltration or R value requirements of the envelope. An exterior log wall is pretty much a giant thermal bridge and usually a very drafty one at that.
Insulated Log Homes
For those who must have the look of a log home, there are a couple options. For best performance with a sacrifice in some aesthetic details, a high performance wall can be finished on both sides with log siding.
There are also products available that completely mimic log construction. Remember the rules of air tight and thermal bridging. To the right is a product that could perform well in our climate if installed correctly. The white layer represents polystyrene foam, a possible good air barrier and good insulation. Notice that it is a continuous layer, crucial for good performance.
Strawbale
Used increasingly in drier climates, its renewability aspects are tough to beat. Good insulation levels must be balanced with painfully thick walls resulting in less natural light and solar gain. The rounded plaster aesthetics are loved by many. Its the building science and lack of long term case studies in our rainy and humid climate that give the most cause for concern in Western North Carolina. Very high labor costs can be expected without a serious investment of sweat equity.
AAC
Autoclaved Aerated Concrete is a very popular building material in other parts of the world where the need for cooling is dominant. It can have unique aesthetics and its long life is perhaps its greatest virtue. Despite claims made about the extra energy performance gained by its thermal mass properties, its average R value per inch is just over 1. This puts it in the same category as wood or a log home and would be considered a thermal bridge in a well insulated structure.
Compared to all other methods of reducing energy use/environmental footprints, improving comfort and indoor air quality, high performance envelopes are the most affordable and effective.
Building Science basics shows us the importance of building air-tight and minimizing gaps and bridging in the insulation caused by studs, framing and structural members.
Energy, Health, Comfort, Longevity, Environmental and Operating costs are all directly dependent on the Building Envelope.
Heating and Cooling in Asheville and Western North Carolina
It is only after addressing the fundamentals above that it makes sense to look into heating and cooling systems which are the biggest energy users in our homes and buildings. Visit this Asheville Builders blog page to get an overview of the heating and cooling systems most often used in our climate .
Heat Pumps
Heat Pumps for North Carolina Homes looks at how they work and the different options available.
Geothermal, Ground Source Heat Pumps
Ground Source, Geothermal Heat Pumps in Asheville NC looks at how this technology works and important things to consider when choosing a system.
Radiant Floor Heating
Asheville Radiant Floor Heating is an overview of this heating system that continues to grow in popularity throughout Western North Carolina.
Here are some good places to learn more:
www.toolbase.org A good starting point for beginners, put out by the National Association of Homebuilders. Like most things asscoiated with this organization, information is intended to protect home builder's interests and profitability. A good search tool allows one to get an overview of a wide range of topics and products. Good information on green construction products and techniques.
www.ornl.gov/sci/btc/ Oak Ridge National Lab. The best building science organization for building in the Asheville climate. A little technical for most homeowners and house builders, this site has some great information on it. Construction companies from around the world send products here for testing and their experiments continue to influence best building practices throughout the country. Search tools are lacking in quality.
www.BuildingScience.com This site is run by the Buidling Science Corporation, a team of our continents best building scientists. Although it can get technical, they have a broad array of studies and best construction practice recommendations that are easily searched for. Their work is typically oriented for cold climate home building and is intended for the homebuilder community at large, meaning that they simplify some of their advice for uneducated home building professionals.
www.greenbuildingadvisor.com/ This is a great site for smart home buyers and profesional home builders. Much of the information is through blog like discussions which can lend themselves to opinion, sometimes questionable and always debatable. Martin Holladay is one resident expert who always has interesting topics and great advice.
www.passivehouse.us/passiveHouse/PHIUSHome.html This is the United States website version of the German created Passive house standard. See more information on this certification at the bottom of our Philosophy page. The science behind the performance of this building standard is largely outlined above in our Building science page. The site gets into much more detail.
www.ecoiq.com An online magazine dedicated to a more sustainable future.
www.ashevillenc.com A website dedicated to Asheville NC.
www.carolinaliving.com is a site serving the Carolinas.
Our goal is to better educate Asheville Builders and all those involved in building new homes, custom homes and green homes. Visit our Asheville Green Building Design page for cost effective energy efficient design stategies or our Asheville Home Builders page for our building services.
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