Friday, March 4, 2011

Energy Audit of a Shelter Home - Part II

By Gaius Hennin, P.E.
Last month on this blog, we shared with ShelterBuild readers details of a free class Shelter recently hosted on the subject of energy audits and energy efficient homes. Aaron Despres from Energy Solutions For Maine and Up-Country Building Inspectors, Inc. explained to the class three levels of energy audits you might receive from a commercial vendor and what you might expect from each.

Tipton Home
In this post, we'll share the results of an energy audit Aaron performed on the Tipton home, which we've mentioned previously on this blog. (You can also read about the Tipton's home and their goal of building a near net-zero home on their blog, Simple Living, where they describe the process of designing and building their home as well as the energy performance of the house.)

Shelter Design Build was responsible for the shell of the Tipton's home including the timber frame and structural insulated panel enclosure (from R-Control), Marvin doors and windows, and McElroy hidden fastener metal roof.

Aaron’s audit started with an exterior inspection of the home: checking the orientation of the home, type of siding, type of foundation (and lack or presence of insulation on foundation wall), roofing type and lastly checking for any indications of obvious envelope leakage. There is no such thing as a house built too tight, only inadequate ventilation.

Next, Aaron moved inside. He first checked the water temperature at the tap to check for excessively hot water, which would indicate a scalding danger and wasted energy due to a needlessly high thermostat setting. Water temperatures above 120 degrees expedites corrosion and mineral build-up in a storage-type heater. Also, for every ten degrees above 120 degrees that you set the thermostat, expect to add as much as 5% to your water heating bill.

While inside, Aaron performed a visual inspection and measured the house to approximate the volume of the house for air leakage calculations. A very expensive, high resolution infrared camera was used during the audit to find areas of air leakage, shown as color differences on the camera’s screen. The camera can also be used to spot moist areas in the walls and roof which indicate leaks originating from the outside due to rain and snow or from the inside due to excessive indoor relative humidity (read: inadequate ventilation).

The stove should be out and the door and damper
should be shut tight during depressurization.
The next step in the process is to create a slight negative pressure inside the building to exacerbate any leaks. I should note here that prior to depressurization, Aaron (a seasoned Auditor) ensured that all combustion appliances were off (to avoid drawing in dangerous combustion gases) and the woodstove was completely out with the stove door and damper shut tight (ashes want to stay IN the stove). The standard depressurization is -50 pascals with respect to outside air pressure (roughly equal to the negative pressure developed by a 25-mile-per-hour wind), which is only -.0073 pounds per square inch. The negative pressure is accomplished by blowing air out of the house with a specialized device called a blower door, which mounts into an exterior door opening. The pressure difference is measured with a manometer. The fan has a variable speed motor which allows the operator to get the pressure in the house down to -50 pascals, at which time the volume of air going out is measured. This leakage rate at a standardized negative pressure gives you a number to compare to other homes and to industry accepted levels. The Shelter Design Build shell leaked 525 cubic feet per minute when depressurized by the fan (a basketball is roughly equal to one cubic foot); at a total volume of around 12,000 cubic feet, the total volume of air in the house will be exchanged 2.5 times each hour at this artificial depressurization (known as 2.5 ACH50, since this is the number of Air Changes per Hour at -50 pascals). The average new home ACH50 ranges from 5-10; older homes range from 11-15. The International Residential Code’s energy conservation code allows an ACH50 of 7; Energy Star requires an ACH50 of 5 or less; the first production production-built zero-energy home in the southeast has an ACH50 of 2.3. Upon hearing this we felt like new parents pushing their infant in a stroller down a crowded street to a chorus of, "what a cute baby!"

With the house still depressurized, Aaron started walking around the house aiming the infrared camera at likely leak points. Leaks that had not shown up earlier were now obvious as outside air flowed in under influence of the outside positive pressure. Leaking areas included the penetration of the insulated chimney pipe through the roof, the French doors, the intersection of wall and roof structural insulated panels, electrical outlets in exterior walls, the fresh air inlet for the wood stove and a section of stick-built wall where the cellulose insulation had settled several feet (this was not dense pack cellulose). The double-hung windows, notoriously leaky, performed exceptionally well prompting Aaron to comment, “These are the best double hung windows I have tested.” (Nice work Marvin.)

We were disappointed at the amount of air leaking in around the chimney as it has a Dektite rubber flashing boot, which is attached to the metal roof and to the stainless chimney pipe with silicone caulk. (See our recent blog post on this subject.)
Metal Roof Penetration
Example Slideshow
We can only assume that air was entering the standing seam rib of the metal pan at the eave and ridge and spilling into the space under the Dektite then into the house around the chimney pipe. The leak through the French door was not surprising; they are notoriously leaky where they meet in the middle. The leak at the wall-roof panel intersection was disappointing though not surprising as this is a tough area to seal 100% with expanding foam. The leak was minor but points to a lapse in quality control. The leaks through the electrical outlets are very common as they represent a tunnel that runs through the walls, up to the roof and down to the basement. Electricians (like plumbers) are charged with running holes through framing, through floors and often through the roof to ply their respective trades but are rarely armed with a knowledge of building science and even more rarely armed with a can of expanding foam to seal the penetrations. This is a fairly easy fix involving turning off electricity to the appropriate outlets and switches, removing said plates and spraying a small amount of expanding foam through the hole at the back of the box where the wire enters to seal the electrical chase in the SIP. The fix for a stick-built house is more complex since even insulated stud bays typically allow air to move through them. (Yet another reason to build with SIPs, as if you need one.) In a stick-built house the entire box needs to be replaced with a sealed box that has a rubber gasket to seal around the wire and a foam gasket to seal to the interior wall finish. Don’t bother with the foam gaskets available at the big box stores which seal the cover to the box; the air will still blow through the plug receptacles. The stick-built utility room was insulated with loose cellulose which settled leaving the top of the wall uninsulated. Installing dense-pack cellulose (cellulose blown in at a density of about 4 pounds per cubic foot) in the affected areas would prevent further settling. The settlement of cellulose in stick built walls is a common problem, easily avoided by building with SIPs!

After a thorough examination of floor, walls, and roof to find leaks, Aaron shut off the blower door and moved on to an inspection of the gas cookstove for propane leaks. He then checked the broiler in the gas oven to ensure that it was not emitting excess carbon monoxide while running. As is common, when the broiler was warming up a significant amount of CO was present, but within a few minutes the measured CO readings were back down to acceptable levels. Next Aaron performed a depressurization check to see if any part the current ventilation system (bathroom fan and cookstove hood) created a negative pressure which could cause combustion appliances to backdraft. He checked this by turning on the cookstove hood and measuring the pressure at the wood cookstove and comparing the measured pressure to acceptable standards. It turns out that the cookstove hood created a -32 pascal pressure in the kitchen, yet -5 pascals is the acceptable standard. This is a red flag indicating that in the winter, when all doors and windows are closed, when the hood fan is running and there is a fire in the wood stove there is potential for carbon monoxide gases to leak back into the living space. To prevent this, the hood fan would need to be swapped for a smaller fan or provided with a fresh air inlet to prevent unacceptable negative pressure. According to ASHRAE Standard 62.2-2007, the fresh air inlet would need to be at least 10 feet from the hood exhaust.

Reflectix (photo from product website)
Last, we headed down into the crawl space to have a look. The foundation wall is about 4’ tall, poured on ledge, and the homeowner had placed crushed stone on the ledge for leveling purposes and then placed a vapor barrier over the top. Basement insulation consisted of 2” of extruded polystyrene foam (R-10) on the outside of the wall and Reflectix insulation stapled to the underside of the floor joists. Reflectix is a layer of bubble pack faced on both sides with a layer of metalized aluminum. This material was chosen to prevent radiant heat from the in-floor radiant heat system from leaking down into the basement. As Aaron pointed out though, as soon as even a thin layer of dust forms on the top side of the reflective product, it is no longer effective. He prefers to see a product with more significant conductive r-value, such as rigid foam (tedious to install between joists) or spray foam insulation (easy to install between joists though pricey.) Air tight homes, such as this one, suffer from excessive indoor relative humidity in the winter time when the average family of four produces roughly four gallons of water per day simply cooking, bathing and breathing. Crawlspace basements on ledge can be a virtually infinite source of moisture as the stack effect (or ‘chimney effect,’ so named because it creates air flow similar to that in a chimney—as lighter warm air escapes through the upper part of the house, cooler heavier air is drawn in down low) draws in cool air through the basement that naturally rises up through the house, bringing vapor moisture from the basement with it. One way to mitigate the flow of crawl space vapor moisture into the living space is to install a vapor barrier over the floor of the crawlspace. When doing so, be careful to overlap seams at least 6” and run the barrier up the foundation walls at least 6”. Seal the barrier to itself and the wall with expanding foam, appropriate construction mastic, or acoustical sealant. Aaron also suggested insulating the header, even though the SIP covers it, due to the cold sill plate and potential for leakage on top of it and below the SIP. Cutting and fitting rigid foam in between each joist bay would be tedious here, spray foam quite painless.

The house does not have a traditional heating plant (boiler or furnace), so Aaron was unable to perform an inspection of that appliance, though it is a typical part of the audit. We decided that the next time we offer this class, we will ask Aaron to audit a shell more representative of America’s building stock: a bit leakier, with a boiler, and more typical insulation. A house built or enclosed with SIPs is naturally very energy efficient (as witnessed by the air infiltration rate and whole wall r-values) and doesn’t give many opportunities for illustrating improvements.


  1. Hi
    Thanks for the informative article. It appears from your article that the home that was tested and you built the shell for using timber frame and SIPS had an ACH50 of 525cfm, but that a good number to shoot for is 5 to 7 cfm. What am I missing?

  2. Thank you David for pointing out a labeling mistake in our blog. We incorrectly stated “The International Residential Code’s energy conservation code allows an ACH50 of 7 cfm; Energy Star requires an ACH50 of 5 cfm or less; the first production production-built zero-energy home in the southeast has an ACH50 of 2.3 cfm.” When we meant to say “The International Residential Code’s energy conservation code allows an ACH50 of 7; Energy Star requires an ACH50 of 5 or less; the first production production-built zero-energy home in the southeast has an ACH50 of 2.3.” The numbers 7, 5 and 2.3 respectively refer to the number of total air changes per hour at the 50 pascal negative pressure created by the blower door. Our SIP house had an ACH50 of 2.5; 525 cfm refers to the homes’ CFM50, or the cubic feet of leakage that occurred at the 50 Pascal negative pressure. We regret any confusion and have made the corrections in the copy of the blog article.

    Blueberry Beeton and Gaius Hennin