EnergyLogic is in the news! Check out this interview with Robby Schwarz, one of the founders and continuing principals here at EnergyLogic. You’ll find the article supporting how EnergyLogic continues to help builders, salespeople, and consumers in our industry better understand the long-term benefits that energy efficiency will bring to their lives.
To access the article, please click on the link below:
Who to Contact:
Principal, Director of Builder Relations
The Q3 Field Fusion event delved into air-sealing and sound transmission challenges in multi-family units through a guided discussion that included perspectives from Code Officials, Insulators, and Raters. Read more here.
There are many complexities that accompany building townhomes and duplexes. For example, townhomes and duplexes built with common fire separation walls (party walls) are twice as leaky as single family houses that are twice their size.
The shaft wall, which we see most often in Colorado, is open directly to the outside through the designed gap between the shaft liner and the framing, thus creating a leaky assembly. An additional complexity arises when the reduction of unit-to-unit sound transmission is taken into account, which requires correctly installed insulation.
EnergyLogic’s August 31st Field Fusion delved into the details of these assemblies through a guided discussion that included perspectives from Code Officials, Insulators, and Raters.
We must first define what part of the shaft wall assembly is fire-rated, as the entire assembly is not. This is an important distinction that allows for more air sealing options once understood.
In chapter 3 of the IRC, Section R302 “Fire Resistant Construction” and Section R302.2 “Townhouses” states, “The common wall shared by two townhouses shall be constructed without plumbing or mechanical equipment, ducts or vents in the cavity of the common wall. The wall shall be rated for fire exposure from both sides and shall extend to and be tight against exterior walls and the underside of the roof sheathing.”
This statement in the IRC is our first indication that the two layers of sheetrock in the shaft liner wall are the fire-rated two-hour wall, designed to slow the spread of fire from unit to unit. Thus, the two layers of 1” drywall cannot be penetrated with ducts.
The framing (which is held off the fire-rated assembly by a clip) often has ducts or plumbing in it and is specifically designed to burn and separate from the two-hour assembly when the clip melts. This allows one unit to burn and fall before fire is able to pass through to the adjacent unit. The UL (Underwriters Laboratory) listing for many of these assemblies’ references section 705 of the International Building Code (IBC) which states in Section 705.2 “Structural Stability”, “Fire walls shall have sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall for the duration of time indicated by the required fire-resistance rating.” This is another indication that UL listings and the code are in agreement that the fire assembly is the two layers of 1” drywall and not the framing adjacent to the drywall.
It is important to point this out because throughout Colorado there is not a common understanding of what constitutes a shaft liner fire-rated party wall assembly. Some jurisdictions still hold that the assembly is the drywall, air gap, clip, framing and interior drywall while others hold that it is as explained above. What is consistent is the understanding that the assembly must be built continuously from the foundation to the roof deck.
EnergyLogic suggests having a discussion with jurisdictions, in an effort to:
- Ensure a common understanding of this assembly
- Determine how the assembly will be air sealed to control airflow to meet the air leakage requirements of the energy code.
One thing to note: jurisdictions throughout the state require that the two layers of sheetrock run continuously from the foundation to the roof deck, but not the entirety of the rest of the assembly. The most conspicuous example is the interior drywall, which is always missing on the ventilated attic side of the party wall.
Challenge: How to Achieve 3 ACH50?
Now that a common understanding of the assembly has been achieved, it is time to determine how the assembly can be air sealed in order to meet the air leakage target of 3 ACH50 for the 2012 and 2015 IECC. Most jurisdictions have not amended the requirement to meet this airtightness level, so pre-planning is crucial in order to be successful.
The clip that holds the framing off the two-hour party wall assembly creates a 1” gap that is connected directly to the outside at the front and back of the unit, as well as to the attic. This is where the UL listing of the assembly comes into play. UL is an American safety consulting and certification company that provides the one or two-hour rating for fire-rated assemblies by testing them in a laboratory environment. The UL listing for these assemblies is often mixed up with code’s definition of the assembly, which creates confusion regarding what materials are allowed to be used to seal them.
UL often refers to fire-blocking materials. Fire blocking materials are usually defined within the UL assembly and can be any one of the following:
- 2” nominal lumber
- Two thicknesses of 1” nominal lumber with broken lap joints
- One thickness of 0.719” wood structural panel with joints backed by 0.719” wood structural panel
- One thickness of 0.75” particleboard with joints backed by 0.75” particleboard
- Gypsum board, including 1” DensGlass Ultra® Shaftliner and 5/8” DensArmor Plus drywall
- Batts or blankets of mineral wool or fiberglass
- Other approved materials installed in such a manner as to be securely retained in place shall be permitted as an acceptable fire block (Section 717.2.1, 2006 IBC).
As some fire blocking materials are air barriers and some are not it important to choose a material that can stop the flow of air. EnergyLogic has seen the most success when builders tackle fire blocking on each individual floor.
Application: The Picture Frame Method
When looking at the party wall assembly, envision a picture frame around the perimeter of the wall. All four sides need to be fire blocked. The material of choice right now is the same 1” gypsum board used in the 2-hour rated assembly. Install the 1” gypsum board in the 1” gap between the interior of the unit and the outside at the front and back of the units, between floors, and to the attic. Again, picture framing the party wall.
Depending on your foundation type, if you are standing on the first floor you will need to air-seal the two layers of gypsum and the bottom plate to the slab in the shaft wall, or address the rim joist connection in the basement or crawl space to the shaft wall. At the rim joist be sure sill seal has been installed between foundation and sill plate as it is your primary capillary break, then seal the sill plate to foundation, seal rim board to the sill plate, and seal the rim board to sub floor. Pay special attention to any knockouts for foundation bolts.
Once the large 1” gap has been fire blocked with an air-impermeable material such as gypsum, seal the smaller gaps between the fire block and the shaft wall and the fire block and the framing. A fire-rated caulk or expanding foam works for this. Following these steps, with careful attention to detail, should enable you to successfully achieve 3 ACH50.
A few words of caution:
- Ensure that the drywall lid is air-tight: duct boots and other penetrations need to be sealed. In addition, as required by ENERGY STAR, the drywall to top-plate should be sealed. (This is a requirement of code that is generally not enforced.)
- Mechanicals can derail all good air-sealing intentions. Undampered ducts run to the exterior for combustion or ventilation air as well as atmospherically vented appliances. These combustion air ducts can ruin one’s ability to build a tight home that gains control and predictability of the airflow in the building.
Don’t Forget: Sound Reduction
Lastly, these assemblies should reduce sound transmission from dwelling unit to dwelling unit. The party wall is assumed to be an adiabatic wall, i.e. there is no heat loss or gain through the wall between two conditioned spaces as the temperature is the same on each side. Therefore, the insulation is primarily installed to lower sound transmission. The principles of sound reduction and heat flow are the same, so proper installation of the insulation in the framed cavity of the party wall is imperative.
NAIMA, the North American Insulation Manufacture Association, states that the installation of insulation in a party wall application should “comply with the manufacturers’ instructions including filling the entire stud cavity and cut to fit around outlets, junction boxes, and other irregularities in the cavity.” In other words, the insulation in a common party wall should be installed to a RESNET, Grade 1.
To learn more please see EnergyLogic’s Tech Bulletin on “Fire-rated Party Walls” which includes an article by Building Science Corporation.
Have a technical question? Contact Robby Schwarz.
Our next event will take place on November 16th. It is focused on Selling High-Performance Homes. Our guest speaker, Todd Gamboa, President of Building Trust LLC., has a wealth of experience and perspectives to share. Please see details here.
If you have suggestions for topic you would like to see discussed in depth, please let us know. We will be releasing our Q1 2017 event topic and date soon.
Principal / Director of Builder Relations
Introducing our new HVAC Designer, Scott Olson
Q&A: Learn more about Scott, his design philosophy, background, and credentials!
What was your first job in residential construction industry?
Construction Superintendent for a National Home Builder in Denver. The Job consisted of front and back‐end scheduling, and warranty work.
How and when did you first become interested in high performance homes and energy efficiency?
While working for a local home builder, I helped create their High Performance testing on all homes. I received my Residential Energy Services Network (RESNET) Certification and tested homes for energy code compliance and made sure homes were ENERGY STAR compliant. I balanced the needs of a production builder with ENERGY STAR best practices.
How did you get into HVAC Design?
I was challenged by my boss, while working for a large production builder, to take on a whole new role as an HVAC Designer. My construction experience as a superintendent was a huge advantage and I strived to make my designs be trade friendly and cost-effective.
What insights did you gain on HVAC designs when you worked directly for a large production builder?
Designed cost effective, construction friendly and functional HVAC systems
What elements of EnergyLogic’s HVAC design philosophy resonated with you and attracted you to the job?
EnergyLogic’s HVAC design services, with a goal of ensuring comfort and efficiency with properly built envelopes integrated with properly-sized and designed HVAC systems.
What are some of the common design mistakes or misconceptions builders should watch out for?
Sizing HVAC with a rule-of-thumb calculation. With today’s tighter building envelopes, rule of thumb often results in oversized systems that cost more upfront and cost more to operate over time. Oversized systems also don’t dehumidify as well as properly sized systems because the run time is shorter, so occupants are actually paying more for discomfort.
What do you like to do in your free time?
In my free time, I enjoy grilling, working on projects around the house, and camping.
Up and down the front range of Colorado we are seeing more and more code jurisdictions adopt either the 2012 or 2015 IECC. From a building science perspective, this is a step forward toward better performing buildings because these two codes require mandatory air leakage targets be met by all segments of residential construction three stories or less. What I mean by this is that the code understands the importance of gaining control and predictability of the air flowing through our buildings. I like to say that air is a freight train and like a train it carries cargo from point A to point B. The cargo it carries is heat/energy, moisture, and pollutants. The issue is that air does not always carry its cargo in a straight line on tracks from inside a house directly outside the house and deposit its load into the ambient air. A properly ducted fan may take air and its cargo to the outdoors, but often air takes its cargo into building assemblies and deposits it there, causing long-term building durability and efficiency issues for our homes. The code now recognizes that tight homes increase durability and efficiency and now understand that visual inspection itself cannot ensure house tightness. The fundamental change in the code from the 2009 IECC to the 2012 or 2015 IECC is this recognition and the move from a choice to administer a blower door test or visually inspect to a mandate that you visually inspect and test to ensure tightness of the homes we build.
EnergyLogic tests homes and has been helping builders use the Simulated Performance pathway through code since the 2006 IECC. Unlike other pathways in the energy code, the flexibility gained in the performance path allows for the most cost-effective means to develop the energy specifications for a house because we can trade off house tightness for R-values and U-values in the thermal envelope. This means that we understand that there is absolutely no problem achieving the code required 3 air changes per house at 50 Pascals (3 ACH50) in a single-family home. In fact, the 2012 and 2015 IECC offer a checklist for how to be successful in the mandatory air barrier and insulation table R402.1.1. We also know with certainty that it is not easy for multi-family buildings to achieve this same air leakage target. Currently, code does not recognize the difficulty of achieving 3 ACH50 in multi-family homes and buildings. The City of Denver has accepted EnergyLogic’s code amendment to allow multi-family homes and buildings to have a leakage rate of 4ACH50. The city of Fort Collins allows a CFM/sqft of shell area measurement to be used to express air leakage in multi-family project. So far they are the only jurisdictions in the state that I am aware of that have amended this section of the code to better reflect the realities of creating airtight multi-family buildings. On a national level, EnergyLogic has submitted a code change proposal for the 2018 IECC that makes a clear distinction between single-family detached homes and multi-family attached homes with achievable air leakage targets for both. We will have to wait until October to see if the proposal is accepted.
So where does that leave our multi-family builders?
Whether you are building duplexes, townhouses, or stacked multi-family buildings, house tightness is solely dependent on attention to detail with regard to air sealing adiabatic common fire rated walls, floors, and ceilings. We recommend beginning by removing all draft-stopping materials (rock wool and fiberglass that are air permeable) in these fire rated assemblies and replacing them with solid fireblocking materials that actually stop the movement of air. By doing this you now have reasonable-sized holes that, depending on the jurisdiction, the assembly, and the skill level of the air sealing contractor, can be sealed. Next, you must treat common walls, floors, and ceilings as you would treat assemblies that separate conditioned space from the outdoors. The codes air barrier and insulation mandatory table/checklist must be applied to these common fire rated assemblies. For example, if there is a tub or shower pan, drop ceiling, or knee wall adjacent to an adiabatic common fire rated wall, floor, or ceiling, an air barrier needs to be installed. Lastly, these details cannot be an afterthought! They need to be addressed from the first design charrette, through a trade partner kickoff meeting, to mid-phase air barrier and insulation rough quality assurance inspection, if there is to be any hope of achieving 3 ACH50 when the home is blower door tested at a final inspection. Blower door testing occurs when the home is complete and when it is impossible to achieve significant air sealing objectives at this point of construction. Small changes in the tightness of the home may be able to be achieved, but air takes the path of least resistance; so if you have not blocked and sealed it out behind the drywall it is unlikely that you will be able to do more that achieve a small increase in the tightness of home at a final stage of construction. In other words, the work has been done at the time of the rough inspection and the evaluation of the work is done at the final inspection.
In conclusion, get involved and take full advantage of EnergyLogic’s third-party inspection and testing services. Get us involved as early as possible in the design of your multi-family project, and let us train all your trade partners at a kickoff meeting, how to successfully work toward meeting the requirements of code. It is not easy, but attention to detail, quality assurance inspection, and greater understanding by the trade base will make 3 ACH50 achievable.
Who to Contact:
Principal, Director of Builder Relations
As houses continue to be built tighter and tighter, the importance of ventilation and providing adequate fresh air to occupants grows. Ventilation requirements of ENERGY STAR® and the 2012 IECC reflect this growing importance. Recognizing that many builders are installing exhaust fans to meet whole-house ventilation requirements, manufacturers are offering some new-innovative fans and controllers to meet this growing demand.
Panasonic Whisper Green® series and Broan Ultra® series are two products we see often. Both fans are designed to operate at a continuous ventilation speed for background ventilation. They can also be ramped up to an exhaust (maximum) speed for spot ventilation. When properly wired to a wall switch, the fan ventilates when the switch is off and exhausts when the wall switch is on. Ventilation speed is set by a knob on the fan itself so that it can be adjusted to meet the ASHRAE 62.2 ventilation rate.
At EnergyLogic, our biggest frustration has been finding these fans incorrectly wired to operate as designed. The wiring process is different than a standard bath fan and to complicate matters, different from one brand to another. Quite often we test fans that cannot be adjusted to meet the ASHRAE requirements and either over ventilate or under ventilate the home. Each time, the fans are not wired per the manufacturer instructions. (We have even seen oversized fans that are modified with an inline damper to try to adjust the flow to meet ASHRAE. This limits the effectiveness of the fan and drastically increases the noise. A loud fan is a fan the homeowner will turn off.)
By understanding how these fans are supposed to work and properly wiring them, builders can be assured that their investment in a high quality, multi-use fan will perform as designed, providing fresh air to the homeowners and running quietly in the background. As Raters we strive to be knowledgeable in the ventilation systems our clients are installing and to help their trade partners understand the technical aspects of the systems. We encourage our builders to contact EnergyLogic directly for access to our tech bulletins on exhaust ventilation wiring and installation or to get help navigating the maze of various products and configurations available to meet the new ventilation requirements.
By Rusty Buick
Field Services Manager
Building scientists always recommend creating a gap between your siding and your drainage plane to promote drainage and drying. One of the best ideas I’ve seen a builder implement this past quarter was as follows. They created a gap by using sill seal installed vertically on the outside of the framed wall and behind the siding on top of the house wrap. This simple and low cost idea creates a break between the siding and the house wrap/drainage plane. As we know, moisture will bypass any siding product that is installed on the house. The drainage plane is designed as a built-in redundancy to ensure that water drains down and away from any opening in the house. What a simple, low cost, and elegant way to ensure that the space is maintained and that drainage behind the siding is ensured.
The worst idea of the quarter goes to jurisdictions that oddly enough have begun to once again require a plastic poly vapor barrier in our wall assemblies. We have learned from our building science friends that a vapor barrier is designed to stop moisture migration into our wall assemblies by the process diffusion. Physics tells us that diffusion is a very slow process and only has the ability to move small quantities of moisture. Physics has also taught us that latex paint substantially stops the process of diffusion and is recognized as the only vapor management strategy that has been codified. A class one vapor barrier – Poly, class two vapor barrier – Kraft faced batt or a product like MemBrain™, and a class three vapor barrier – latex paint.
Why should you care? More Moisture Moves with air than by the process of diffusion. We install plastic so poorly that it tends to trap moisture in the assembly once air has bypassed it and dropped off the moisture it was carrying. Luckily, we have found that jurisdiction have issued variances to builders who are concerned about this issue. So if you believe strongly, like I do, that plastic in the wall is actually detrimental and causes more builder risk than benefit. Let me know and we can work on a variance request to ensure that you can continue building forgiving assemblies that have great drying potential in both directions.
Quite often I step backward and look outside the badge of a BPI auditor and put myself in the shoes of a homeowner. A homeowner whose home I’m about to pick apart. For the most part homeowners are happy to have us there. We’re there to help, not to sell. They expect us to make recommendations telling them what should be improved. They expect, “Fix or replace this, and your house will be better.” What they don’t expect is the education that comes with it. With an understanding of how these recommendations make a difference, homeowners could save thousands.
On a recent assessment of a home I was told the main interest in having an audit was to confirm the need to replace an induced draft furnace with a sealed combustion furnace. The homeowner was replacing a sensor that was failing every few months. An HVAC contractor suggested buying a sealed combustion furnace. Ta Da! Problem fixed. Not so fast.
It should be noted that the furnace was located in the crawl space; and a vented crawl space to boot. This was a big factor in several issues, one of which was the routine replacement of the sensor, the comfort in the rooms above the crawl space and poor indoor air quality. The thinking was that the sealed combustion furnace would reduce or eliminate the likelihood that the sensor would need to be replaced often and that the air flow would be better to the rooms above, increasing comfort.
As you know, a new furnace doesn’t address the root of the problem. This is where educating the homeowner is valuable. We need to make sure they understand why we recommend the improvements we do. We sat down to discuss why the problems existed in the first place and how a new furnace will still leave them with their current problem.
First, let’s look at the sensor problem. The dry, dry dirt in the crawl space was being pulled into the furnace housing and collecting on the sensor. What he needs is to install a sealed barrier over the ground of the crawl space, preventing premature failure of the furnace. More importantly, this would improve indoor air quality. Next was to address the comfort issue in the floor above the crawl space. With a new furnace and increased air flow to rooms, the vented crawl space would still allow cool air to directly impact the floor above. To truly fix this would be to seal the crawl space vents, air seal and insulate the rim joist and insulate the walls. This was an easier fix and less expensive than replacing the furnace. The homeowner understood and agreed, then decided to pursue improving the crawl space.
In the end, recommendations for improvements are a lot more beneficial if the homeowner has basic understanding of the science behind those recommendations. Too bad there is not a Cliffs Notes of basic building science for homeowners.
Steve Byers and his family are on vacation in Turkey this month. While traveling he is continually reminded of the fact that building science is, to quote the 1980’s British band James, “…like a disease without any cure.” In other words, no matter where you go, building science goes with you.
Direnkuyu is a town in the Cappadocia region of Turkey. In Direnkuyu, there is an underground city, one of some at least forty such troglodyte habitations in the area (so psyched that I am able to use the word “troglodyte” in a post!). These cities were built as refuges for sheltering from attackers across the centuries. To give you a sense of scale, these are not holes in the ground, these are truly cities. The Direnkuyu city could house 20,000 people and their associated livestock. It extended 60 meters underground and had a tunnel connecting it to a nearby city in another village that ran for 8 kilometers. I was fascinated by the entire thing. I banged my noodle about six different times on the ceilings of tunnels out of shear excitement about what lay around the next corner…
In case you think concern about ventilation is a modern development, check this out: The city had ventilation shafts that ran the entire depth of the structure. The hundreds of family caves were all connected via small tunnels to the main tunnel so that each was ventilated. All cooking was done on the upper level to keep smoke from inhabitants.
One additional function of all these shafts was to enable easy and instant communication around the community. Apparently an even softly spoken, “Help, we’re under attack!” would carry to every level of the complex allowing large circular stone doors to be rolled into place sealing the structure off to invaders. Learning this, we made an immediate connection to our EnergyLogic training center where we have a Duct Music Aire. Never heard of that? Well, the Duct Music Aire was a speaker you installed into your duct system to pipe stereo (quadrophic possibly?) sound around your home. My guess is it worked less well than Direnkuyu’s ventilation shafts.
Which, by the way, don’t leak at all…Steve Byers CEO of EnergyLogic, Inc. RESNET HERS Provider and Trainer – EnergyLogic Academy
Over the holidays, I had the pleasure to try and wrap a toy pony for my daughter. I kept thinking, “I need to put this in a box.” Yes, I know – this is why the whole gift-bag industry exists, but it’s not really “unwrapping” when you just pull out some tissue paper, is it? Anyway, boxes are so much easier to wrap than a pony — legs, tail and head all vying to poke through my paper.
Houses are the same. It’s pretty easy to wrap a boxy house and make it air tight and well insulated. So why are houses so often drafty or just cold? Well, part of it is that no one really likes to live in a totally boxy house. We like it when a house has contour – windows and overhangs that stick out on the outside; drop-down or vaulted ceilings on the inside; interesting angles and visually appealing lines. All of these add style and often practical use to a home — think built-in book cases — but can also contribute to a loss in energy efficiency if not done correctly.
At EnergyLogic we’ve done thousands of home energy audits. While some windows need to be replaced (I’m looking at you pre-1960s single-pane aluminum-frames) it’s far more often the basement rim joists, bay windows and drop ceilings that may be the real energy culprits. These are the legs, tail and head making your boxy home not so boxy – and they are potential areas of energy loss. They’re not impossible to wrap, but it does take extra time and a little bit of effort.
When we do a home energy audit where the homeowner has comfort issues – that is, it’s too hot or cold in a particular area – 95% of the time it’s an issue with air sealing and insulation. Someplace in the home there is a gap that is allowing air to move freely in/out causing loss of heating or cooling, or, there is insufficient or missing insulation to control the movement of conditioned air.
The good news is that most of the leakage issues are relatively easy to fix and won’t require expensive retrofit work. Once these areas are taken care of, you’ll be able to wrap that pony in comfort and take all the time you need to do it right.
COO of EnergyLogic, Inc.
How does air move through a piece of duct? Try to imagine yourself as a molecule of air traveling through a piece of duct work with millions of your closest friends. It might feel like driving down I-25 at rush hour through the tech-center when everyone is staying in their lanes; five lanes of traffic moving in unison in one direction basically in one straight line. If the cars were air it would be called laminar flow; air moving smoothly though a straight piece of duct work without any obstructions, elbows, or eddies.
Now imagine an accident in the center lane of I-25. All the cars would have to slow and go around the accident disturbing the straight stream of traffic. If it were air, or any fluid, moving around the center lane accident turbulence in the flow air would occur. The resulting chaotic swirling lessons the amount of air that can flow through the duct just like the accident in the center lane restricts the number of cars that can pass and slows the traffic or the number of cars that can travel down the straight piece of highway.
Now imagine traveling down the 5 lane highway at rush hour and it changes from 5 lanes to two lanes to pass through a tunnel. The flow of traffic would slow and back up cueing for its turn to pass through the tunnel as the tunnel and the restriction of lanes is a resistance to the steady flow of traffic of the five lane highway. Sharp corners or curves in the road, passing though toll booths, and roundabouts are other examples of things that resist the flow of traffic. In the same way airflow can be restricted. Elbows, small diameter duct, poorly designed diffusers; all resist the flow of air in the duct and change the flow form efficient laminar flow to chaotic swirling turbulent flow. In duct work this is measured by the static pressure in the duct which is a measurement of the resistance to the flow of air, the greater the resistance the less air measured in cubic feet per minute can flow through the duct.
Now that we are measuring the flow of air through ducts on a regular basis, whether it be HVAC duct or bath fan duct, we are realizing that smooth straight properly sized duct is desired to meet the performance requirements of programs or code. This is especially true for bath fan duct as the termination is a vent cap instead of a register cover. It turns out that most vent caps act like the car accident in the center lane, dramatically restricting the flow air, while a duct register cover is like a car pulled over on the side of the road changing a tire, causing curiosity slowing but not impacting the flow of traffic dramatically.
As we move forward with EnergyStar V3 we are learning that the vent cap choice is as important as fan choice, duct layout, and upsizing the duct in order to ensure that fans are able to push the required amount of air out of the house. We are building tight so we need to ensure that moisture management systems are not only installed in homes but that they are performing as they have been designed. We know that the standard 50 CFM builder grade bath fan is not pushing nearly 50 CFM. That most often the louder the fan the less air it is actually moving. However, we were surprised when high quality Energy Star qualified fans that have been upsized in both CFM and ducting where not able to push 50 CFM. The culprit, it turns out, is the vent cap.
Below are pictures of a very typical vent cap that we see in our market. As you can see the screen portion surrounding the 4” diameter duct has about 1” of free space between the top of the duct and the top of the cap. The air crashes into the cap and cannot exit to the outdoors and flow through the fan is impinged increasing the static pressure or the resistance to the flow of air through the system.
Ventilation experts like Paul Raymer with Heyoka Solutions have studied the impact of different vent caps on the flow of air and more information can be found on his and others websites. In general search of a vent cap that allows air to smoothly leave the duct on its way to the outdoors. Just as it is difficult and dangerous to navigate a car traveling at 60 MPH around a 90° corner air can’t be expected to be able to flow smoothly into a wall (vent cap) and then exit to the outdoors. Remember it is there and does it work – all pieces of the building science puzzle are important when it comes to performance.