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Helpful? 3

Questions and Answers About Air Barriers

Every home needs an air barrier that limits infiltration and exfiltration

Posted on Jan 14 2011 by Martin Holladay, GBA Advisor

Builders of a certain age — say, those older than about 55 or 60 — started their careers at a time when no one talked about air leakage or air barriers. Back in the early 1970s, even engineers were ignorant about air leakage in buildings, because the basic research hadn’t been done yet.

Times have changed, and most residential building codes now require builders to include details designed to reduce air leakage. Today’s young carpenters are working on job sites where air barriers matter.

Q. What materials make good air barriers?

A. A wide variety of materials make good air barriers, including poured concrete, glass, drywall, rigid foam insulation, plywood, OSB, and peel-and-stick rubber membrane. Although air can’t leak through these materials, it can definitely leak at the edges or seams of these materials. When these materials are used to form an air barrier for your home, additional materials such as tape, gaskets, or caulk may be required to be sure seams and edges don’t leak.

To make a good air barrier, a material not only needs to stop air flow; it also needs to be relatively rigid and durable. If you want to determine whether a material is an air barrier, hold a piece of the material up to your mouth and blow. If you can blow air through it, it’s not an air barrier.


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MORE INFORMATION

Air Sealing: A Guide for Contractors to Share with Homeowners

Making Air Barriers That Work


Engineers distinguish between air barrier materials (drywall, for example), air barrier assemblies (for example, plywood with taped seams attached to wall framing), and air barrier systems (all of the materials and assemblies that make up a building's air barrier).

Q. My builder installed Tyvek under my siding, so I already have an air barrier — right?

A. Not necessarily. Although Tyvek and other brands of plastic housewrap are sometimes marketed as air barrier materials, the primary function of housewrap is to act as a water-resistant barrier (WRB). In other words, the Tyvek is there to protect the wall sheathing from any wind-driven rain that gets past the siding.

Some builders have experimented with using Tyvek as part of an air barrier system. If the seams of the Tyvek are taped, and if the gaps between the Tyvek and window openings are carefully sealed, and if the transitions between the Tyvek and other materials at the bottom of the wall and the top of the wall are detailed in an airtight manner, then Tyvek can work as part of an air-barrier assembly. But siding contractors often rip holes in the housewrap with their ladders; they also penetrate the housewrap with hundreds of nails and staples. Builders interested in achieving a tight air barrier have found that other air sealing methods are more effective than an approach that depends on housewrap to be a wall’s primary air barrier material.

Q. My home has polyethylene under the drywall. Is the polyethylene an air barrier?

A. Probably not. During the 1980s, interior polyethylene was widely promoted as an interior vapor barrier. Its use in new homes is now relatively rare, except in very cold locations (for example, Minnesota, Canada, and Alaska).

Most polyethylene installations leak a lot of air — especially at the seams between adjacent sheets of poly, at penetrations, and around electrical boxes. That’s not usually a problem, since polyethylene is an effective vapor barrier even when it is not installed in an airtight manner.

Some cold-climate builders have successfully used polyethylene as part of an air barrier system. To act as an effective air barrier, however, polyethylene needs to be installed with careful attention to a long list of fussy details, including the use of acoustical sealant (non-hardening caulk) at all seams and the use of airtight electrical boxes. This type of polyethylene installation is relatively rare.

Q. Where are the most common air barrier defects located?

A. Most air leaks happen at the seams or cracks between different materials: for example, where the mudsill framing meets the foundation, where floors meet walls, and where walls meet ceilings.

Although gaps around windows and doors — the first areas of concern for many homeowners — occasionally contribute to air leakage problems, the most significant air leaks are usually in hidden areas. Because such hidden leaks — called “thermal bypasses” by weatherization contractors — usually don’t cause obvious drafts, homeowners are often unaware of their existence.

Here’s a list of some of areas that are often poorly sealed, and therefore responsible for significant air leakage:

  • Basement rim joist areas;
  • Holes cut for plumbing traps under tubs and showers;
  • Cracks between finish flooring and baseboards;
  • Utility chases that hide pipes or ducts;
  • Plumbing vent pipe penetrations;
  • Kitchen soffits above wall cabinets;
  • Fireplace surrounds;
  • Recessed can light penetrations;
  • Cracks between ceiling-mounted duct boots and ceiling drywall;
  • Poorly weatherstripped attic access hatches; and
  • Cracks between partition top plates and drywall.

Q. What’s the best way to test whether my house has a good air barrier?

A. Tracking down air leaks can be tricky, especially for builders or homeowners who are unfamiliar with the devious paths that air can take. For example, builders are often surprised to learn that significant air leakage paths can occur through interior partitions located far from exterior walls.

The best way to test the integrity of a home’s air barrier is to perform a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. To learn more about blower-door testing, see “Blower Door Basics.”

Q. During the winter, water sometimes drips from the can lights in my cathedral ceiling. A contractor told me that the problem is that my ceiling doesn’t have an air barrier. Can you explain what he means?

A. If you have punctuated your insulated cathedral ceiling with recessed can lights, it’s very difficult to keep the ceiling airtight. (Unfortunately, even so-called “airtight” can lights are actually fairly leaky.)

In a ceiling like yours, warm humid air enters the insulated rafter bays through cracks around the can-light trim. The air is drawn into the rafter bays by the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.; the air usually exits the rafter bays through cracks near the ridge.

Since fiberglass batts are air-permeable, they do little to slow air movement. Regardless of whether the rafter bays are vented or unvented, there are usually plenty of cracks that allow humid indoor air to find its way to the underside of cold roof sheathing, where the moisture condenses. On cold nights, a layer of frost can build up on the underside of the roof sheathing. When the weather warms up, the frost melts, leading to dripping can lights.

The solution is to create a tight air barrier at the ceiling plane. The best way to achieve this goal is to remove the can lights, patch the drywall, and substitute surface-mounted light fixtures like track lighting.

Q. How do I create an air barrier for my basement?

A. An air barrier system is a three-dimensional balloon surrounding the conditioned area of a home. Assuming that your basement is part of your conditioned area — and I think it makes sense to include it — then there are several areas of concern.

First of all, it’s important to note that the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. depressurizes basements during the winter. In fact, stack-effect depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. is strong enough to pull air through soil under the basement slab. (Believe it or not, most soils are porous enough to allow a connection between the air in the soil — even 7 feet below grade — and exterior air above grade.)

A basement slab is an effective air barrier. However, the perimeter crack where the basement slab meets the footing or foundation wall is a source of air leakage and should therefore be caulked.

Basement sumps can allow significant volumes of air to enter a house. The solution is to install a sump with an airtight lid; these are available from Jackel, Inc. in Mishawaka, Indiana (574-256-5635).

Most builders know the importance of installing sill-seal between the top of the foundation wall and the mudsill. If the sill-seal is ineffective, this joint may need to be caulked or sealed from the inside with spray foam.

If the rim joist in your basement isn’t insulated, it should be. Closed-cell spray polyurethane foam is the best insulation for this location; fortunately, spray foam also helps seal air leaks in the rim-joist area.

To limit the stack effect, you may want to consider weatherstripping the door at the top of your basement stairs.

Q. What’s the “neutral pressure plane”?

A. Several different driving forces affect air leakage rates. Some of these driving forces — including wind, exhaust appliances like bathroom fans, and unbalanced or leaky ductwork used for forced-air heating or cooling systems — are intermittent. However, one driving force — the stack effect — acts continuously on a house, as long as the exterior air temperature is significantly below the interior air temperature.

Because of the stack effect, it’s fairly easy to predict infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. patterns during the winter. Warm air usually exits the house through cracks near the top-floor ceiling. The stream of departing air pulls air into the house through cracks in its lowest level — for example, through the crack between the basement wall and the mudsill.

In other words, the air at the top of the house is pressurized with respect to the outdoors, while the air in the basement is depressurized.

In the center of your house, maybe somewhere in the vicinity of your living-room windows, the indoor air is neither pressurized nor depressurized. It’s at about the same pressure as the air outdoors. This is called your home’s “neutral pressure plane.” Even if your living-room window has all kinds of cracks, not much air will leak through — as long as the wind isn’t blowing. That’s because there is no driving force. The outdoor air and the indoor air are at the same pressure.

What happens to your home’s neutral pressure plane when you turn on an exhaust fan in an upstairs bathroom? It moves upward a few feet, that’s what. More air is now leaving your house, so more air needs to enter the house to replace it. As more air enters — perhaps through those cracks around your living-room window — the neutral pressure plane moves a few feet higher, up near the ceiling.

Understanding these pressure dynamics helps guide the efforts of weatherization contractors when they perform air-sealing work. The most important cracks to seal are those under negative pressure — that is, cracks in a basement or crawl space — and those under positive pressure — that is, cracks in the attic floor. Air leaks in the center of the house — in the vicinity of the neutral pressure plane — are less important.

Q. I don’t know whether my air barrier is on the outside or the inside of my wall. Can I tell where my home’s air barrier is located by looking at the house plans?

A. Probably not. Unfortunately, most house plans don’t indicate the location of the air barrier. As a result, a home's air barrier details — if such details even exist — were probably made up by the builders on the job site.

A wall’s air barrier can be located at the exterior sheathing (by taping the sheathing seams), at the interior drywall (by following the Airtight Drywall Approach), or in the middle of the wall (by using spray polyurethane foam). If the details are done correctly, any of these three methods can result in a very tight air barrier.

Q. My designer brags that his house plans include air-barrier information, and that the air barrier can be traced “without lifting your pen from the paper.” What’s that mean?

A. It means you have chosen a good designer. Although it’s rarely done, a good section drawing of a house design should indicate the location of the air barrier. Because this air barrier must be continuous, without any interruptions where the walls meet the ceiling or at other transitions, it should be possible to trace the air barrier on the section drawing — from the basement slab, up the walls, over the ceiling, and down the other side — without lifting your pen from the paper.

Q. I live in a hot climate where heating bills are low and air conditioning bills are high. Do homes in a hot climate need a decent air barrier?

A. Yes. If your home lacks a decent air barrier, lots of cool indoor air can escape through cracks in your walls and ceiling. That departing cool air is replaced by hot outdoor air that sneaks in other cracks, forcing your air conditioner to work overtime.

If your summers are humid, these air leaks carry a double penalty. When outdoor air enters your home, your air conditioner struggles not only to cool the air, but also to wring the moisture out of the air. In states with humid summers, a significant portion of your air-conditioner run time is actually devoted to dehumidification. The tighter your home, the easier it is is to keep your indoor air cool and dry.

Q. I want to make my house as airtight as possible. Is there any way to do that without spray foam?

A. Absolutely. Many builders have achieved very low levels of air leakage without any spray foam at all. Moreover, many homes that have been insulated with spray foam still have high levels of air leakage.

How is this possible? The reason is simple: most walls and ceilings don’t leak in the middle of the wall or ceiling. They leak at edges, penetrations, and transitions. So even when a wall is insulated with spray foam, you still need to worry about the details.

The lowest levels of air leakage are achieved by builders who have studied airtight construction techniques, who think through potential air leakage paths during each phase of construction, who are conscientious and methodical, and who have already built a few homes that were tested by a blower door.

Most air sealing details don’t require fancy materials. In many cases, a few rolls of gasketing material and some tubes of high-quality caulk are all that’s necessary. Attention to detail usually matters more than high-tech tools or equipment.

When it comes to air sealing, the proof is in the pudding — in other words, the blower door results.

Last week’s blog: “Net-Zero-Energy Versus Passivhaus.”


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  2. Fine Homebuilding

1.
Fri, 01/14/2011 - 08:01

air barrier
by Terrell Wong

Helpful? 0

Love the article, but the picture of the outlet on the exterior wall kills me! Best thing I ever decided was to remove all wiring, venting or any other service except those very few that are absolutely necessary in the exterior walls. Brass (yes they are expensive) floor outlets work great and were easy to install. I put the minimum required by code in and then peppered the interior walls with regular outlets as required. 2nd best thing I did was to make drywall continuous on the exterior wall. Interior walls butt into the drywall. Got my house http://rosedalehouse.ca down to 1.6ACH. Not bad for an historic, therefore interior only retrofit project . I believe the historically "correct" double hung windows are my downfall. Look good, but leak like hell!

Thanks for all your posts - keep them coming


2.
Fri, 01/14/2011 - 10:02

Two steps forward one step back?
by Lucas Durand - 7A

Helpful? 0

Martin, does the rate of air leakage through a particular "crack" increase with the "tightness" of a home?
In other words, will crack sealing increase the rate of leakage through the cracks you don't seal (everything else being equal)?


3.
Fri, 01/14/2011 - 10:23

Edited Sat, 01/15/2011 - 05:06.

Response to Lucas
by Martin Holladay, GBA Advisor

Helpful? 0

Lucas,
The answer to your question is, "it depends." I think it's safe to say:

1. Sealing leaks will lower the air leakage rate of your house and will lower your energy bills, even if you don't track down every last crack.

2. As far as I know -- and any physicist who cares to contradict me is welcome to chime in -- there are some circumstances in which sealing one crack might increase airflow through an unsealed crack. For example, if a house has a very leaky ceiling, the basement will be under negative pressure. If a homeowner seals one crack in the basement, the airflow through unsealed cracks in the basement might increase somewhat. But the overall air leakage rate for the entire house should decrease.


4.
Fri, 01/14/2011 - 11:08

Edited Fri, 01/14/2011 - 11:08.

Code enforced air tightness requirements?
by j chesnut

Helpful? 0

Martin,
Much appreciated article.

Putting aside the question what the air tightness requirement(s) would be, am I safe to assume since we can all agree air tightness is a component of responsible building that we would all agree that it should be a code enforced performance requirement for new construction?

In the process of trying to determine what the requirement would be, do you think it is more of a factor for protecting the assembly from moisture damage - or - a factor for limiting conditioned air escaping through the envelope?

I was at an Building Enclosure Council meeting the other day. ASHRAE is currently testing a sample of current wall commercial buildings from different US climates for their air tightness performance. As I understand it this is the first commissioned study by ASHRAE to study the air tightness performance of commercial buildings. A good sign.


5.
Fri, 01/14/2011 - 11:16

Edited Fri, 01/14/2011 - 11:23.

Code requirements for air tightness
by Martin Holladay, GBA Advisor

Helpful? 0

J,
The code requirements for air sealing in the 2006 IRC were already fairly stringent but were almost never enforced. Here's what I wrote on the topic in an earlier blog:

"In the 2006 International Residential Code (IRC), air-sealing requirements can be found in section N1102.4. The code requires that 'The building thermal envelope shall be durably sealed to limit infiltration. …The following shall be caulked, gasketed, weatherstripped, or otherwise sealed with an air barrier material, suitable film, or solid material:
1. All joints, seams and penetrations.
2. Site-built windows, doors and skylights.
3. Openings between window and door assemblies and their respective jambs and framing.
4. Utility penetrations.
5. Dropped ceilings or chases adjacent to the thermal envelope.
6. Knee walls.
7. Walls and ceilings separating the garage from conditioned spaces.
8. Behind tubs and showers on exterior walls.
9. Common walls between dwelling units.
10. Other sources of infiltration.'

"The real zinger in this list is #10: 'Other sources of infiltration.' That just about covers it, doesn’t it?

"The IRC requires all sources of infiltration to be 'caulked, gasketed, weatherstripped, or otherwise sealed with an air barrier material, suitable film, or solid material.' The code does not include any exceptions. In other words, there is no such thing as a legal crack or air leak.

"Unfortunately, the requirements of Section N1102.4 are rarely enforced, and as a result, many new American homes — especially fiberglass-insulated homes — perform poorly."

So, that sums up the story for the 2006 IRC. The latest version of the code, the 2009 IRC, has upped the ante. It has retained the air sealing requirements found in the 2006 IRC, while adding additional requirements.

In Section N1102.4.2, the 2009 IRC gives builders two options: either have a third-party inspector verify completion of an air-barrier checklist (shown in Table N1102.4.2), or perform a blower-door test to confirm that the home's air leakage is less than 7 ach50 (a low bar, but better than no bar at all).


6.
Fri, 01/14/2011 - 11:33

thanks
by j chesnut

Helpful? 0

Thanks for outlining the current state of the residential code on air tightness.

More specifically (here's my language 'challenge' again) I'm trying to gauge whether there is enough consensus within the green building community to create a lobbying effort to require blower door confirmations of requirement(s) more stringent than 7 ach50 and do away with the visual inspection option.

Not that you alone can gauge the whether the consensus is there, but I would be curious to hear thoughts from others if they agree that this is possible route forward to raising the bar on typical residential construction; or are there obstacles to improving the codes or gaining a consensus within the green building community that I am not acknowledging?


7.
Fri, 01/14/2011 - 11:34

Air leakage
by Doug McEvers

Helpful? 0

Lucas and Martin,
Back in the old days when a blower door test was performed on a new house at least 2 results were noted. ACH50 and ELA (equivalent leakage area) in square inches. Reduce the ELA and lower the ACH50, I believe Martin has it right.


8.
Fri, 01/14/2011 - 11:42

Air Leakage
by Doug McEvers

Helpful? 0

TEC describes the leakage area measurement far better than I did.
http://www.energyconservatory.com/support/support4.htm


9.
Fri, 01/14/2011 - 12:00

Edited Fri, 01/14/2011 - 12:01.

High bar, low bar
by Martin Holladay, GBA Advisor

Helpful? 0

The 2009 IRC requires that new home builders achieve 7 ach50.

Nine years ago, in 2002, a study of new Wisconsin homes showed a median air leakage of 3.9 ach50.

If that's what new home builders were averaging in 2002, before there were any blower-door requirements in the code, why are code officials setting the bar so low? (Or, to speak more accurately: why are they setting the leakage rate so high?)

Here's part of the answer: not everyone builds to Wisconsin standards. The code also has to work in Louisiana, Alabama, Oklahoma, and Texas, where building techniques are different from those in Wisconsin.


10.
Fri, 01/14/2011 - 14:48

Edited Fri, 01/14/2011 - 14:53.

Response to Martin
by Riversong

Helpful? -1

"Builders of a certain age — say, those older than about 55 or 60 — started their careers at a time when no one talked about air leakage or air barriers."

Not all of us. And anyone who's ever lived in a drafty old house understood the importance of keeping the wind out. Below is a picture of an air/vapor barrier I installed in a double-wall fiberglass-insulated house in TN in 1982, using butyl caulk at every edge and seam. I also used the vapor barrier as an air barrier in my first Larsen Truss house, built in 1987 by placing it outside the sheathed structural frame like a housewrap with taped seams before attaching the trusses.

"Air leaks in the center of the house — in the vicinity of the neutral pressure plane — are less important."

Until the wind blows. The 50 pascal blower door test simulates a 20 mph wind, but a 40 mph wind exerts 200 pascals of positive pressure on the windward side and almost as much negative pressure on the leeward side and the roof. Winds may not be as consistent as the stack effect, but can have much more dramatic impacts when they do blow.

The answer to Lucas' question, "does the rate of air leakage through a particular "crack" increase with the "tightness" of a home?" is not "it depends" but YES. Just as the more R-value we install in a house the more important become the voids or thermal bridges, the same is true of air leaks.

It's true that any effort at tightening a house will reduce it's overall leakage rate and save energy, but the same stack effect and wind effect pressures will drive more air faster through the remaining cracks - making those areas more vulnerable to moisture damage. It's for this reason that a 97% complete vapor barrier will be 97% effective (vapor diffusion is an area-weighted phenomenon), while a 97% complete air barrier can be 97% ineffective (because air leakage is a localized phenomenon).

W&D Int.jpg 108.jpg 113.jpg


11.
Fri, 01/14/2011 - 14:59

Response to Terrell Wong
by Riversong

Helpful? -1

In a well-designed house, there is no need to avoid or skimp on electrical outlets in exterior walls, and few owners are going to be happy with floor receptacles. Proper planning and proper air sealing is no harder to do than any other air barrier detail. And running exterior wall drywall continuously behind (and above) partitions, requires a two step drywall and rough wiring process that few builders or subcontractors would tolerate, and still requires penetrations for wiring which have to be sealed. Proper ADA techniques makes sealing at partitions just as effective and keeps the construction sequencing on track.


12.
Fri, 01/14/2011 - 14:59

Edited Fri, 01/14/2011 - 15:01.

Response to Robert
by Martin Holladay, GBA Advisor

Helpful? 0

Robert,
Like you, I was paying attention to airtightness when building houses in the early 1980s; when I framed my own house in 1980, I used Typar as well as exterior EPS over the sheathing. When you were building in 1982, and when I was building in 1980, we were both benefiting from the pioneering research conducted by the Princeton House Doctors, who got their first federal grant in 1974, and from similar research in Canada and Sweden.

Gautam Dutt, the discoverer of the thermal bypass, had his famous "aha!" moment in a townhouse in Twin Rivers, New Jersey, in 1977. He was the first to figure out why existing energy modeling programs weren't accounting properly for significant heat loss in existing buildings.

Your point about how wind affects leaks at the so-called "neutral pressure plane" is exactly the same as mine, which is why I wrote that at "your home’s neutral pressure plane," "even if your living-room window has all kinds of cracks, not much air will leak through — as long as the wind isn’t blowing."

The answer to Lucas's question is "it depends," because leaks near the neutral pressure plane only leak when the wind is blowing or fans are operating on them -- so the leakage depends on the wind and fan operation. These cracks will not be as affected by changes in the basement cracks in the same way as unsealed cracks in the basement.


13.
Fri, 01/14/2011 - 15:04

Code Requirements
by Riversong

Helpful? 1

Codes are catching up with air sealing standards. And let's not forget that code standards are MINIMUM standards - in other words, just meeting them earns you a D-. We should all be exceeding them.

While IRC may require only 7 ACH50, Energy Star has required 5 ACH50 and their new 2011 version 3.0 has a climate zone specific requirement:

Infiltration rates shall be less than or equal to the following values:
6 ACH50 in CZs 1,2
5 ACH50 in CZs 3,4
4 ACH50 in CZs 5,6,7
3 ACH50 in CZ 8
Envelope leakage shall be determined by a Rater using a RESNET-approved testing protocol.

And the Energy & Environmental Builder's Alliance (EEBA) requires less than 0.25 cfm of leakage per square foot of building surface area (including floor) @ 50 Pa.


14.
Fri, 01/14/2011 - 15:17

2009 IRC Visual Inspection Checklist
by Riversong

Helpful? 1

Here is a link to the new thermal bypass checklist.


15.
Fri, 01/14/2011 - 15:33

Edited Fri, 01/14/2011 - 15:35.

Eureka Moment?
by Riversong

Helpful? 0

I find it humorous to see such statements as "Gautam Dutt, the discoverer of the thermal bypass, had his famous "aha!" moment in a townhouse in Twin Rivers, New Jersey, in 1977."

It has the same ring to me as "Columbus discovered America".

Perhaps Dutt can take credit for giving it that name or for quantifying it, but I think most old timers knew that warm air leaks into an attic when it's cold outside.

It's funny how long it takes for scientists to "discover" or "prove" what ordinary people always knew.


16.
Fri, 01/14/2011 - 16:56

Edited Fri, 01/14/2011 - 16:57.

On discovering the thermal bypass
by Martin Holladay, GBA Advisor

Helpful? 0

Robert,
You are quite correct that builders have long known that a hole in the ceiling will leak air.

However, it took until the late 1970s for researchers to quantify residential air leakage.

Because air leakage had not be quantified before then, researchers and builders underestimated it.

Because air leakage was underestimated, remedies and good building practices were not implemented, and chases into attics were not sealed.


17.
Fri, 01/14/2011 - 17:37

Response to Robert Riversong
by Michael Blasnik

Helpful? 1

The answer to Lucas' question, "does the rate of air leakage through a particular "crack" increase with the "tightness" of a home?" is not "it depends" but YES. Just as the more R-value we install in a house the more important become the voids or thermal bridges, the same is true of air leaks.

It's true that any effort at tightening a house will reduce it's overall leakage rate and save energy, but the same stack effect and wind effect pressures will drive more air faster through the remaining cracks

Your claims are incorrect. The air leakage through a hole is a function of the pressure difference across the hole and the size/shape of the hole. If you seal half the holes in a house, then the "same stack effect pressure" will produce only half as much infiltration.

It's true that if you seal leaks only at the top of the building, then the remaining leaks at the top will leak a little faster because you have moved the neutral plane away from them. But if the leakage distribution doesn't change (i.e., if you seal all the leaks about equally) then the leakage rate drops in direct proportion to the reduction in hole size and the reduction in blower door reading.


18.
Fri, 01/14/2011 - 18:06

sheathing effectiveness?
by 5C8rvfuWev

Helpful? 0

If sheathing is taped and penetrations sealed so it forms a proper air barrier .... then the siding guy shoots nails to attach the siding through the sheathing (and into the studs) so thousands of nails have penetrated the sheathing ... is the sheathing still considered an effective air barrier?

If so ... what if he somehow missed the studs with a half dozen or more nails ... is the sheathing still an effective air barrier?

And if so, why?

Thanks for another terrific summary, Martin.

Joe


19.
Fri, 01/14/2011 - 18:22

For michael Blasnik
by Garth Sproule Zone 7B

Helpful? 0

You stated that "The air leakage through a hole is a function of the pressure difference across the hole and the size/shape of the hole."
I agree, but, as you plug these leaks, the pressure differential will increase and cause any remaining holes to leak more. No??


20.
Fri, 01/14/2011 - 21:09

Air Leaks
by Doug McEvers

Helpful? 0

If you look at the Energy Conservatory link and read 3#, they explain the Canadian EqLA measured at 10 pascals and the ELA method of LBL at 4 pascals, there is no mention of a "multiplier" when air leakage is either raised or reduced. It is a straight calculation.

http://www.energyconservatory.com/support/support4.htm


21.
Sat, 01/15/2011 - 04:53

Do nail holes hurt an air barrier at the sheathing?
by Martin Holladay, GBA Advisor

Helpful? 0

Joe W.,
For a further discussion of the question you raise about nails in sheathing, see the comments posted at my earlier blog, Airtight Wall and Roof Sheathing -- especially Garth Sproule's comment of 6/12/10 and my response of 6/13/10.

Here's the summary: as long as the siding nails stay put, the air barrier is tight. But if you are using the roof sheathing for the air barrier, there is a potential problem: when roofers come along in 20 years to strip the asphalt shingles from the roof for a re-roofing job, they'll be pulling up the old roofing nails, leaving holes in the roof sheathing. That's definitely a problem.


22.
Sat, 01/15/2011 - 10:05

If we are allowed to Guess(about crack pressure)
by John Brooks

Helpful? 0

I agree with "depends"...
it depends on where the cracks are
what is causing the pressure
and how the cracks are "balanced"


23.
Sat, 01/15/2011 - 11:00

Fireplace walls
by John Reimers

Helpful? 0

The check list that Robert posted above includes an air barrier on fireplace walls. I'm assuming that this refers to wooden fireplace chases. Does it apply to an internal fireplace and chase? Is ADA drywall adequate or is something more required or recommended? To go above and beyond are there other practical procedures?

The fireplace has a sealed door. What is the logic behind this requirement, especially on an internal chase.

Thanks, John


24.
Sat, 01/15/2011 - 11:46

Edited Sat, 01/15/2011 - 12:41.

Response to Garth - holes and pressures
by Michael Blasnik

Helpful? 0

Garth said:

as you plug these leaks, the pressure differential will increase and cause any remaining holes to leak more. No??

The answer is generally no -- the pressure will not increase - but there are situations in which it can.

In terms of natural infiltration pressures -- wind and stack effect -- the pressure differences induced across the holes in the house will depend on the distribution of the leakage locations in walls vs. ceilings vs. floors/foundations. If you reduce the size of all holes in a home in half, the stack and wind pressures will stay the same and the natural infiltration rate will be cut in half. But if you shift the distribution of the holes in the building by mostly sealing leaks at the top or at the bottom for example, then some leaks will experience larger pressures and others will experience smaller pressures. The net effects can be roughly estimated by infiltration models such as the LBL model (which lets you specify the fraction of leaks in walls vs. top and bottom and the difference in leakage areas between top and bottom) or analyzed in even greater detail using a model such as CONTAM. In practice, reductions in air leakage as measured by a blower door are pretty much proportional to changes in infiltration rates.

When it comes to air leakage created by mechanical systems such as exhaust fans/ventilation or unbalanced duct leakage, then sealing leaks in the building will tend to increase the pressure difference across the remaining leaks (actually, just those leaks leaking inward) and lead to increased air leakage rates. The relationship is a little complicated and changes once the fan flow rate is sufficient to make all holes leak inward.


25.
Sat, 01/15/2011 - 12:25

On air barriers behind zero-clearance metal fireplaces
by Martin Holladay, GBA Advisor

Helpful? 0

John Reimers,
If a fireplace is located against an exterior wall, then the framed wall behind the fireplace is part of the home's thermal boundary, and therefore requires an air barrier as well as insulation.

If a fireplace is located in the center of a home, an air barrier is only required under the fireplace (this might consist of a concrete slab, for example) and at the point at which the flue chase penetrates the ceiling air barrier. Follow the chimney manufacturer's recommendations for such penetrations; if the chimney manufacturer's details are not airtight, it may be necessary to complete the air sealing with pieces of galvanized sheet metal sealed with high-temperature silicone caulk.


26.
Sat, 01/15/2011 - 12:26

To Michael Blasnik
by Martin Holladay, GBA Advisor

Helpful? 0

Thanks for your very helpful responses, Michael -- much appreciated.


27.
Sat, 01/15/2011 - 13:13

Edited Sat, 01/15/2011 - 13:30.

Second image, building details, KISS is missing.
by aj builder, Upstate NY Zone 6a

Helpful? 0

Martin, Looking at the image (image 2) showing all the details to seal and insulate a home. I would as of today anyway never build that home. Complicated labor intensive, foam intensive, tape intensive, caulk intensive, etc etc etc. To me it looks like that home if built where I live would have temperatures in the wall conducive to mold growth more than anything built to date. Even if not, it lacks the quality where I would say, wow... this follows the KISS rule beautifully. Yaa know, like E=MC^2

Is the wall moisture/mold proof? Riversong wants thick cellulose walls to be permeable to the exterior which the example is not. 1" of foam is not enough foam to meet the 1/3 rule. Showing wettable insulation against the rim joist and the concrete is not accepted, unless what you are showing is not fiberglass or cellulose.

I think the image should be pulled. More wrong to be learned than right.

And just to get Robert all up in a knot, the image to me proves more than ever that simply doing one of three types of wall is better than combining the three.

1- Exterior only insulation works. Bruce Brownell hasn't been sued out of business that I know of. Taped foil faced foam in two layers continuous around 6 sides of a home gives a high whole R value and a low ACH. Unrefutable and simple comparatively. I mean, look. your image shows no connection of your air barrier between your second floor inner foam to the first floor air barrier or whatever it is supposed to meet up with. If it does, then I need another detail posted to guide me.

2- Larson Truss/Riversong Truss with high perm exterior, double wall, cellulose only, no exterior foam to reduce perm or cause inner wall temps to near mold conditions in framing. Less complicated than the image shown here. Good wall for those that desire to use the least foam practical in a build.

3- Spray foam and Spray foam flash. Another less complicated way to build. Sealing all up for a low ACH is very easy, (I have done such quickly and inexpensively) R value is not great but there are ways to overspray framing to get some advantage of whole wall R value and the R value maintains better than fiberglass mostly. The proof is in the utility bills not in what we all think about it on websites. In my case we have seen a major reduction in energy needs for two Icynene projects.

KISS is a great rule. Keep it simple.

Where would I like to be frame wise? For my location I like the PH idea of 90% less Energy needs.

How to get there? Let's get there as simple as possible!

Robert, it's mighty cold here as I type. Fire us up. Your derogatory comments entertain as much as your knowledgeable posts inform. Living and loving all of you these days neighbor.


28.
Sat, 01/15/2011 - 14:07

Edited Sat, 01/15/2011 - 14:11.

Thanks for all the contributions.
by Lucas Durand - 7A

Helpful? 0

In the interest of exploring the "it depends" answer I'll restate my original question, then expand a bit on what I was thinking about when I asked it:

Martin, does the rate of air leakage through a particular "crack" increase with the "tightness" of a home?
In other words, will crack sealing increase the rate of leakage through the cracks you don't seal (everything else being equal)?

I originally asked the question because I was wondering if/how Bernoulli's principle (more specifically "venturi effect")could be applied to understanding air infiltration.
The basis for "venturi effect" is that:
"According to the laws governing fluid dynamics, a fluid's velocity must increase as it passes through a constriction to satisfy the conservation of mass, while its pressure must decrease to satisfy the conservation of energy." - Excerpt from Wikipedia.

I was imagining cracks that leak air inwards, at the mud sill say. I imagined that if the pressure gradient across the envelope remained constant, and the total cross sectional area of those cracks was decreased, the result might be somewhat like the effect of tightening the constriction within the throat of a venturi - which is to say that the rate of air infiltration would increase through the un-sealed cracks (even though the total volume of air exchange had decreased) - two steps forward one step back...

Now, after reading all the informative replys my original question generated, It is clear that pressure dynamics within a house are greatly complicated by a number of "real world" factors.

Is it even relevant to consider simple "steady state" pressure scenarios with crack sealing or does the act of crack sealing alter the "steady state"?

Regardless of changes in the pressure gradient across a particular crack, is Bernoulli's principle applicable or not?
Does the inward leaking air leak faster as the "inward cracks" become fewer and does the outward leaking air leak faster as the "outward cracks" become fewer?

Possibly the answer to my question is here: "...while its pressure must decrease to satisfy the conservation of energy".
If the rate of infiltration increases, does the cooresponding decrease in pressure mean that the two effects cancel each other out in terms of energy flux?

Thank you all for the consideration of my questions - I have an inquiring mind ;-)


29.
Sat, 01/15/2011 - 14:16

Edited Sat, 01/15/2011 - 14:26.

Lucas, both
by aj builder, Upstate NY Zone 6a

Helpful? -1

Any nozzle that I have used will increase velocity and decrease quantity at the same time. Via conservation of energy some side effects to balance out equations are change in temperature of such things as the nozzle getting warmer, the fluid getting warmer. As to homes, the level of pressures being discussed and the difference in pressures being so little, that in engineering we would just not work with certain aspects of all as being variable to a point of being worth the effort to figure. I have been away from class too long to do all the math for you but in the real world of nozzles, changing to smaller holes only leads to less going thru the hole at a higher velocity.

Speaking of engineering on a site with builders... I have recently picked up a digital caliper made by General Tools. I am building cabinets with it and it is helping me with going between mm and inches as I am using all Blum hardware which specs dimensions in mm. What a fine instrument for the dollars. If you need to be at 5/32 or 20mm, no problemo. Done. No finish carpenter/cabinet maker should be without one.

General makes lots of neat tools. My caliper came from HD.

http://www.generaltools.com/Products/Fraction-plus--Digital-Fractional-Caliper-(6)__147.aspx


30.
Sat, 01/15/2011 - 15:27

Flow through leak nonlinear with pressure drop
by Dick Russell

Helpful? 0

One may reason that sealing some of the holes in a house's envelope reduces net flow through the house and thus lets it support a greater pressure differential at places away from the neutral plane. This would tend to increase the flow through unsealed orifices. However, pressure drop across an orfice varies with the square of velocity (IIRC). As a first approximation, I would expect inward flow through a hole to go up with the square root of the fraction increase in pressure drop.

Anyway, the increase in pressure drop across the envelope won't go up linearly with fraction of holes that are plugged. As a large number of leaks in a house are plugged one by one, I would expect the supportable pressure gradient to approach some limit asymptotically.

Imagine a limiting case, with just two small and equally sized holes in the upper floor ceiling leaking air out and another two down low leaking air in. If you plug one hole up and one hole down, you cut the number of holes in half, but the pressure gradient change is essentially nil, so the air leakage through each of the remaining holes stays the same and the total leakage essentially is cut in half.


31.
Sat, 01/15/2011 - 17:08

Calking Mudsill
by Ross Neuman

Helpful? 0

Martin comments that "Most builders know the importance of installing sill-seal between the top of the foundation wall and the mudsill. If the sill-seal is ineffective, this joint may need to be caulked or sealed from the inside with spray foam". Here in BC, Canada the sill plate is typically sealed with a closed cell foam gasket between the foundation wall and the sill plate (which is usually not pressure treated). This gasket is typically wider than the sill plate and serves as both an air seal and a moisture seal between the concrete and the plate. I would also like to calk the sill plate for added protection against air leaks but am uncertain how best to do this. If I trim back the sill gasket to give a clean edge where the sill plate meets the concrete to use spray foam I am concerned that I will have compromised the moisture seal at this point especially if the foam ever becomes damaged. If I do not trim the sill seal I am unlikely to get a continuous foam seal. Comments?


32.
Sun, 01/16/2011 - 06:46

Wide sill seal gasket
by Martin Holladay, GBA Advisor

Helpful? 0

Ross Neuman,
Do you have any evidence that air is leaking at this location? If not, don't worry.

If a blower-door test (or direct observation) shows this crack to be an air leakage location, I would trim back the gasket with a sharp utility-knife blade (back to the mudsill), and seal the crack from the interior with high quality caulk or spray foam.


33.
Sun, 01/16/2011 - 11:07

reply to Lucas Durand
by Michael Blasnik

Helpful? 1

No, the Venturi effect is about flow through pipes, not natural infiltration in a house. They are very different situations. The pressure differences acting across leaks in homes are determined by the magnitude of the driving forces (wind speed, temperature difference, unbalanced ventilation) and the requirement that the amount of air leaking in equals the amount leaking out. The distribution of locations of the holes will affect the pressure in the home needed to maintain this mass balance.

It's a little too complicated to explain in a blog comment but if you're interested you might want read up on it -- you can find info in many places including references like ASHRAE fundamentals.


34.
Sun, 01/16/2011 - 12:23

Edited Sun, 01/16/2011 - 12:29.

Reply to Lucas and Ross
by Michael Chandler, GBA Advisor

Helpful? 0

Lucas you bring up an important point.

In some of our tightest homes (less than 2 ACH-50) we have had problems with wooden exterior doors that can warp slightly due to humidity differentials during the winter months which tend to open up small gaps at the bottom of the door near the sill. These homes have either conditioned crawls or radiant slabs so a small draft at the doorway results in a noticeably colder patch of floor and customer phone calls. They are usually pretty easy to fix and I don't see us switching to fiberglass doors or multi-point hardware but the point remains that in a tight house any small flaw is significantly more noticeable to the occupant.

Ross, the ANSI-700 National Green Building Standard is very specific about recommending that sill seal should be backed up with caulk to create a durable air barrier between the mudsill and the top of masonry. When possible we prefer to put this on the exterior to repel insects or when using a double wall system with separate bottom plates rather than the full width plywood bottom plates.


35.
Sun, 01/16/2011 - 14:05

sealing mud sills
by Tibor Breuer

Helpful? 2

What I ,as a builder, have been doing for a dozen years of so for air and bug penetration is to put down a bead of constuction adhesive on the top of the stemwall.Then I put a 1 foot wide piece of tyvek(cut a foot off the roll with the chop saw)on top of the bead and push it over the j bolts and then put on the sill seal the mud sill and crank it all down.After that when I lift the walls(on top of another bead of caulk) with the plywood sheating hanging down past the rim joist and mud sill, I staple the tyvek that's hanging outside at the bottom up onto the plywood before I staple down the tyvek on the wall.Makes for a neat package --no air and no bugs getting behind the ply at he bottom.


36.
Sun, 01/16/2011 - 15:52

House Pressure Differentials
by Riversong

Helpful? 0

Is it even relevant to consider simple "steady state" pressure scenarios with crack sealing or does the act of crack sealing alter the "steady state"?

If a butterfly's wing motion in Kansas can cause a tsunami in Asia, then yes - everything you do to change the air flow dynamics in a house will impact the internal pressure balance. And that pressure balance changes minute-by-minute with occupant behavior (doors, fans, flushing toilets) as well as environmental variables such as wind speed and direction, air temperature and relative humidity.

It's theoretically true - but trivial - that, if you seal the exact same proportion of high leaks as low leaks, then the internal pressure regime won't change. But, in the real world, nothing works that simply.

In theory, if a house has exactly the same cross-sectional area and shape of leaks bottom and top, then the neutral pressure plane will be centered in the height and the negative pressure at the bottom will be exactly the same as the negative pressure at the top.

If the only leak were an open door at the bottom, then the neutral pressure plane would be at the ground floor and the whole house would be under positive pressure. Conversely, if the only leak were an open window at the upstairs ceiling, then the entire house would be under negative pressure.

If you sealed most of the leaks at the ceiling but failed to bother with the bottom leaks (or didn't seal them adequately), then the neutral pressure plane would shift downward, increasing the positive pressure at the top and increasing the flow rate through the remaining holes. There would be less overall air exchange, but those remaining leakage areas would become highly vulnerable to moisture damage.

Those who have lived in leaky old woodstove heated houses understand this principle. Sometimes you have to crack a downstairs window to start a fire without a lot of backdrafting. If the fire gets started and you close the window, the negative pressure at the remaining "leak" (the woodstove) would increase and create backdrafting. In other words, sealing one leak (the window) increases the flow rate at the remaining leaks (the woodstove door and connector pipe).

So, in the real world, every time the leak dynamics are altered, the pressure balance is changed and flow effects may become locally problematic (drafts or moisture problems).


37.
Sun, 01/16/2011 - 16:16

Edited Sun, 01/16/2011 - 17:43.

Bernouli Ain't Home
by Riversong

Helpful? 0

Bernouli's principle operates only in Italy or in venturis and on airplane wings. Diffuse air flow is directly proportional to the pressure differential, but air flow through an o-r-i-f-i-c-e (the SPAM filter thinks this is a dirty word) is proportional to the square root of the pressure differential.

The range of actual holes and cracks in a house fall somewhere on the spectrum between the two.


38.
Mon, 01/17/2011 - 13:20

Edited Mon, 01/17/2011 - 13:47.

more on pressures
by Michael Blasnik

Helpful? 0

Robert wrote:

If the only leak were an open door at the bottom, then the neutral pressure plane would be at the ground floor and the whole house would be under positive pressure. Conversely, if the only leak were an open window at the upstairs ceiling, then the entire house would be under negative pressure

Actually, if the house only has one hole -- like an open door at the bottom -- then the neutral pressure plan will be located in the middle of that opening. The air leakage in must equal the air leakage out and so air would flow both directions through the hole. If the only hole were horizontal, then the specific location of where air starts to leak out (or in) would be essentially random but still the hole would leak in and out equally -- otherwise the house would explode/implode.

Also, for a little more detail about flow regimes, the types of holes and cracks in homes leads to an average empirically-determined flow exponent of about 0.65 and the vast majority of homes test between 0.60 and 0.70 (square root flow = 0.5 and laminar flow = 1.0). After air sealing, the exponent tends to increase slightly, implying that the holes being sealed were more like an o-r-f-i-c-e than a smooth long passage that would be associated with laminar flow.


39.
Mon, 01/17/2011 - 21:15

Fiberboard
by Jason MacArthur

Helpful? 0

I am looking to create an air barrier outside of board sheathing. I would like this barrier to be durable and tight, but also vapor permeable. I have been considering using fiberboard sheathing and taping the joints.

I have read some air permeability ratings for fiberboard that are a little troubling- essentially they indicate that too much air moves through fiberboard for it to be considered a true air barrier. If my memory serves me well, I don't think one could "hold a piece of the material up to your mouth and blow" through the stuff, and my intuitive sense is that it would work.

Does anyone have any experience with these products as air-barriers, or know that in fact they are too porous to suffice?


40.
Tue, 01/18/2011 - 00:31

Fiberboard
by Riversong

Helpful? 0

Of course it depends on the brand, but in one test of air permeance ½" fiberboard passed 1.6 l/s-m2 @ 75 Pa (air barrier material can allow no more than 0.02), the same as an uncoated brick wall.

In another, 7/16" plain or asphalt-impregnated fiberboard passed 0.82 l/s-m2 @ 75 Pa, about twice as much as #15 asphalt felt.


41.
Tue, 01/18/2011 - 00:54

House or Balloon?
by Riversong

Helpful? 0

Michael said: the hole would leak in and out equally -- otherwise the house would explode/implode

I don't think that the typical 5 Pa (0.1 psf, 6.5 mph wind equivalent) stack effect pressure in a 2-storey house would implode or explode the envelope. Nor would the 100 pounds of buoyancy likely lift it off its foundation.


42.
Tue, 01/18/2011 - 00:55

Caulk, foam, accoustical sealant, tapes?
by Robert Dickinson

Helpful? 0

Are there any good rules of thumb for when it is best to seal cracks, joints b/w materials, framing members, etc. with caulk vs. trigger foam vs. accoustical sealant vs. sealant tapes, etc?

I've seen more variations of what to use to seal these various intersections, but never much logic behind why one would choose one sealant materials versus another.

It would be both interesting and very valuable to hear the reasons why people use different sealing materials in different circumstances. To me, this seems worth of a whole "Musings of an Energy Nerd" column, but I would welcome any blog comments that folks are willing to post.


43.
Tue, 01/18/2011 - 04:37

Caulk, foam, and tape
by Martin Holladay, GBA Advisor

Helpful? 0

Robert Dickinson,
Your suggestion is a good one. Although I haven't addressed caulks in comprehensive article, I have surveyed tapes and gaskets. In case you didn't see it, here is the link: Air-Sealing Tapes and Gaskets.


44.
Tue, 01/18/2011 - 07:46

Response to Robert
by Michael Blasnik

Helpful? 0

I don't think that the typical 5 Pa (0.1 psf, 6.5 mph wind equivalent) stack effect pressure in a 2-storey house would implode or explode the envelope. Nor would the 100 pounds of buoyancy likely lift it off its foundation.

. I didn't say that 5pa of pressure would implode/explode the house, I said that if the flow into the house didn't equal the flow out of the house then it would explode/implode. I'm not sure you realize that a 5pa pressure difference is created by removing about 1 cubic foot of air from a house without replacing it. If you could make 200 cubic feet of air enter a home without any air leaving, that would create a 1000 pa pressure difference and the windows would blow out. My point was that the flow balance is maintained in a home and the pressure is a result of maintaining that balance. .


45.
Tue, 01/18/2011 - 12:27

gba advisor status for Blasnik
by aj builder, Upstate NY Zone 6a

Helpful? 0

Great info Michael, I would like to have you do some blogging here.

Post, post, post


46.
Tue, 01/18/2011 - 20:37

Edited Tue, 01/18/2011 - 20:38.

Can the stack effect itself be reduced?
by j chesnut

Helpful? 0

The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.

Before I read this definition I had assumed that in a home built to meet the PH standard of .6 ACH and that included a balanced ventilation system that the stack effect would be negated or largely diminished. Would this be the case or is the force (or potential force) of the stack effect independent of these factors in terms of the physics?


47.
Tue, 01/18/2011 - 21:05

Response to J Chesnut
by Martin Holladay, GBA Advisor

Helpful? 0

J,
In a Passivhaus building, the stack effect still operates. However, the cracks and holes are smaller, so the air leakage rate is less.

Remember, 0.6 ach is your air leakage rate. If you achieve the standard, you know the rate at 50 Pascals.


48.
Wed, 01/19/2011 - 08:05

stack effect
by j chesnut

Helpful? 0

Martin,
I understand that .6 ACH is the leakage rate. When the leakage rate is measured by the blower door the balanced ventilation is off and sealed.
At a low air leakage rate does the force of the balanced ventilation system negate or overcome the stack effect? In other words does the tandem of low leakage rates and balanced ventilation (air pushed @ ~30 cfm) overcome the stack effect so that you no longer have negative pressures at the bottom of the house and positive pressures at the top?


49.
Wed, 01/19/2011 - 08:30

Edited Wed, 01/19/2011 - 08:31.

Another response to J Chesnut
by Martin Holladay, GBA Advisor

Helpful? 0

J,
You wrote, "When the leakage rate is measured by the blower door the balanced ventilation is off and sealed." I'm not sure what you mean by "the balanced ventilation," but if that refers to a balanced mechanical ventilation system (like an HRV system), then I would have to disagree with you.

In most cases, ventilation system intake or exhaust vents are NOT sealed during blower door testing. (For more on blower door test protocols, see Blower Door Basics.) Of course, a homeowner, builder, or blower-door contractor can decide to have a blower-door test performed any way they want. If you want to seal the intake and exhaust vents of your mechanical ventilation system before conducting a blower-door test, you certainly can. But that's not the usual protocol.

Concerning your question about operating a balanced ventilation system: if the system has been carefully commissioned to be perfectly balanced, then it will neither pressurize nor depressurize the house with respect to the outdoors. The stack effect still applies, of course. If you have done a good job of air sealing, the stack effect won't hurt you very much, though, because the cracks and holes in your envelope will be so small.

One final point: some installers of HRVs prefer to commission their systems to be slightly unbalanced, for a variety of reasons. This will have the effect of slightly pressurizing or depressurizing the home. You can discuss whether either option makes sense with your ventilation contractor.


50.
Wed, 01/19/2011 - 12:52

HRVs and blower door testing
by Michael Blasnik

Helpful? 1

Much as i hate to disagree with Martin, it's actually a pretty common protocol (including part of RESNET's standard test procedure) to seal off continuously operating ventilation systems when performing a blower door test.


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