Many still believe that a house needs to have air leaks in order to breathe through “natural ventilation”
The myth that “a house needs to breathe” is still considered a fact by many. 100 years ago, houses did not have thermal control via insulation. Back then, air movement in the building helped dry the structure out in the wet season using natural ventilation. This was 100 years ago, and today our standards for comfort and health are much greater. We no longer burn wood or coal as a primary heat source. We also don’t want to give half our paycheck to the utility company to have conditioned air and hot water.
This pandemic has brought to light many building science principles include ventilation and air filtration. The experts recommend that we wear a mask that filters the air before it reaches our nose or mouth and gets transported to our lungs when pathogens or harmful particulates are in the air. OSHA even requires this for our workers. Especially when working around materials such as drywall that have silica that can cause lung damage. Most prefer a tight-fitting mask to one that allows particulates to enter your lungs.
Education is the key
Several weeks ago, I engaged with several builders on a forum. One of them brought up the antiquated concept that that building must be leaky to breathe. I was polite and stated that the current consensus is that we must “build tight (no holes) and ventilate right.” Not to mention that the energy code also requires insulation and air sealing to reduce the energy needed to operate the house and keep it comfortable.
Like your body, you want to control where the air comes into the house. A mask that has holes in it is not effective. When you bring “fresh air” into the built environment, you want to control where it is coming in. Air coming in from a musty crawlspace or attic that is full of coal residue should not be considered FRESH.
Current building codes and green building standards state that air should enter the structure through a controlled ventilation system. The air will be filtered (for pollutants), and the air could be conditioned as well. Fresh air brought either directly into the HVAC system OR a balanced air system such as an Energy Recovery Ventilator should be installed. Cold air in the Winter or “hot and humid” air coming in during the Summer will increase the heating or cooling needed to condition the air. Conditioning air for humans to be comfortable is one of the top demands on our energy supply.
Top Ventilation Solutions
Most of the top green building programs require “whole house ventilation” systems to be installed. Some newly constructed homes may reap the benefits of updated building codes and new building science techniques recommended by green building programs. We build to the standards set by EarthCraft House and LEED for Homes. These same whole building ventilation strategies should be implemented when design HVAC systems for existing homes as well. We recommend finding and sealing all air leaks, improving the insulation, and then designing the ventilation system. If these steps are completed out of order, then the HVAC system may not work properly.
Balanced Ventilation- Neutral Pressure Systems
Neutral Pressure Ventilation Systems bring in fresh air from the outside while expelling polluted air from the inside. Systems such as Energy Recovery Ventilators can exchange the energy from the conditioned and humidity. Hence, the “fresh air” comes into the structure at a similar temperature and humidity level as the house’s existing air. I have an article and several videos listed below that explain the science behind ERV.
Balanced Ventilation- Positive Pressure Systems
Positive Pressure Ventilation Systems bring fresh air from the outside directly to the HVAC system’s return side. This air is filtered and conditioned as it mixes with the existing conditioned air in the house. If the humidity levels are not ideal, humidity control systems can either add or take away humidity from the incoming air depending on the time of the year.
Let us help you fix your home [holes & ventilation]
Our team can help make your house more comfortable, healthy, and energy-efficient, whether designing a custom new home or renovating an existing structure.
Contact us today to get a free phone assessment of your needs so we can schedule the appropriate next steps to solve your problems.
Here is an article that I wrote for a Green Building Website called Rate It Green that explains ERV in more detail:
Indoor Air Quality, aka “IAQ,” has received lots of attention recently. The world has just started exploring the quality of air and how to make it better. Green building programs have been aware of this for decades and have some good building science principles already in place. We don’t have to “reinvent the wheel.”
Thought Leadership and Green Building
We’ve been renovating and building for over 40 years in the Atlanta market. SawHorse was the first firm to offer design + build for remodeling in Atlanta. We were the first contractor to partner with Southface to renovate homes using the EarthCraft checklist. Our promise to our clients is that we will maintain a culture that is forward-thinking and constantly improving.
A major part of green building is making sure the built environment occupants are safe and healthy.
Commitment to improving your home’s Air Quality
One major cause of health issues in houses is bad indoor air quality. The EPA says that the pollution INSIDE the house can be 2-5 times worse than outside air. For that reason, we launched a campaign called FRESH AIR FRIDAYS. We will share a new video each Friday dealing with Indoor Air Quality (IAQ).
Here is a link to our IAQ video playlist created to help you keep your family safe and healthy.
If there is a topic that you would like for us to cover, please leave a comment below OR leave a comment on our YouTube channel.
For dealing with Covid-19 and Indoor Air Quality in Your Homes and Workplace
Update: We posted this right as the pandemic forced us to stay at home. Many of these suggestions are still valid post pandemic to keep your family healthy.
The CDC, EPA, and other agencies give us practical ways to prevent the spread of the Covid-19 virus, such as social distancing and washing hands. While these are essential, there are additional steps you can take to improve indoor air quality. We took a look at some of these last week in a short video on how to design for situations like these; however, the immediate feedback was, “What can we do now?” So our team created a list of “Things to Do NOW,” but we discovered just as many “Things NOT to DO” that isn’t showing up on current lists.
Our list is based on advice from the CDC, EPA, and OSHA with extra green building tips that comply with the building code. Green building is not just “energy efficiency,” it also targets the occupants’ safety and health. Indoor Air Quality is just as important as disinfecting, and both need to be considered during this current crisis.
I’ve been listening to town hall meetings, watching webinars, and doing additional research to make sure we are getting the latest information to our clients and partners. Even with that effort, NEW information will come up as we learn more about this particular virus and its behavior. Much of the advice we received was based on how the flu behaves, and the assumption is that Covid-19 will behave similarly.
I am a licensed contractor with a focus on high-performance and healthy buildings. My company, SawHorse, Inc., has completed hundreds of home performance assessments in the Atlanta area and has renovated hundreds of homes using our building science expertise. Even with decades of experience, there is still much to learn, and this article will continue to evolve as we learn about Covid-19.
Disclaimer: This is information gathered for you to understand the importance of thinking through the situation. I’ve provided reference links to items as needed to support these claims so you can do your own research as well.
1. DO- Self-Quarantine with ANY sickness.
This is pretty obvious and all over the news to do; however, I still see LOTS of people in public coughing then contaminating surfaces by touching them. Covid-19 is not the only virus out there. We still have the flu, common cold, and other pathogens that we can contain in addition to Covid-19.
…for cleaning hands thoroughly (20 seconds or more) and for cleaning most surfaces. If you plan to disinfect, then you need to clean the surface with soap and water first, or the dirt may soak up all of the disinfect so it will not be able to attack the pathogens.
Save the hand sanitizer for “on the road” in situations where you don’t have access to soap and water. You still need to use soap and water to CLEAN your hands when you get access. The sanitizer only kills certain pathogens but does not clean. ALSO- it does NOT kill certain microbes you might pick up from the bathroom, so always try to use soap and water.
…(after cleaning)- and follow the instructions. The key here is to use only ONE disinfectant on the “high touch” surface to avoid unwanted chemical reactions. See NOT TO DO LIST below. It would be best if you also observed the “Dwell Time,” which is the amount of time the surface must remain wet to be effective against the pathogen. If you spray the surface and immediately wipe it off, you probably did NOT disinfect the surface. Most disinfectants have a dwell time of 10 minutes, so 10 seconds will NOT give you the desired effect.
READ THE INSTRUCTIONS ON THE DISINFECTANT AND COMPARE TO MANUFACTURER’S “RECOMMENDED CLEANERS’ LIST FOR BEST RESULTS
Additional reading for office disinfecting through 3rd parties:
NOTE: If you live in a humid climate DON’T leave the windows open now due to humidity which can cause other indoor air quality issues such as mold. Allergy season is upon us as well.
6. DO– Leave shoes at the door.
It seems obvious, and most of our parents told us to do this growing up, however many of us are not doing it at home right now. If you are still not convinced, shoes can track bacteria, toxins such as herbicides + Lead, and dirt, which means you have to clean again.
Covid-19 can live 1 hour to 1 day on cardboard. You can open carefully and leave the cardboard outside or spray with Lysol and bring it in several minutes later. Even if it could live on the package, it is most likely NOT on it. The contents are probably safe as well, unless they were packed hours ago.
9. DO- Change the HVAC air filter
…(some of us are in pollen season). Hopefully, your HVAC tech sold you at least a MERV-13 filter. If not, you can still get decent 1” filters that take out some of the larger particles floating around in the air. Consider upgrading to a 4” or 6” pleated filter with a certified HVAC company since it is NOT a DIY task.
It seems counterintuitive when we’re asked to stay home, but walking or biking away from crowds is safe. The air is also cleaner now since the dramatic reduction in-car use and manufacturing since this pandemic. Take advantage of the cleaner air if you don’t have allergies.
11. DO- Change out your hand, kitchen, and bath towels daily.
These are breeding grounds for unwanted microbes.
Don’t Ever Do
1. DON’T Mix Chemicals
Chlorine + Ammonia OR Chlorine + Alcohol can cause unwanted chemical and toxic reactions. If you are not sure, don’t use bleach to be safe in combination with other cleaners. Contact the manufacturer’s website or call their technical centers for clarification.
Here are a few toxic combinations that are avoidable: Dawn dish soap (Denatured Alcohol) + Clorox Bleach (Chlorine) OR Windex (Ammonia) + Clorox Bleach (Chlorine)
These chemicals used on the same surface even hours apart are not recommended since the first application’s residue may react with the 2nd chemical.
NOTE: We are NOT stating to avoid these brands. Windex can be “Ammonia-Free,” and Clorox can be “Bleach Free.” The products with Ammonia and Bleach can still be used, just not near each other. Please ventilate since they are not good for you to breathe or touch with your skin.
…after you wash them by touching doors or faucets.
Here is the recommended approach to washing hands:
Before I go over this step by step, I will mention that water conservation IS important; however, not all faucets will allow for this without recontamination. Only faucets that have levers where you can use wrists or elbows will work. Otherwise, leaving the water on it is the best option.
Wet hands with water
Apply soap and scrub hands and wrists (20 sec or so)
Grab a towel and turn off the faucet
Dry hands. Pathogens thrive on damp hands.
3. DON’T Wear common dust masks
…if you are NOT sick- they are not effective. Do wear if you are sick to help spread your pathogens to others. Plus, workers who actually need them cannot work safely to protect themselves in dusty environments if there is no supply for them.
Ozone is considered an air pollutant on most indoor air quality charts and, at certain levels, will harm your lungs, skins, and surfaces in your house. The best way to clean air is with a fan + filter combination.
If the humidity is too low or below 40% Relative Humidity (RH), pathogens like the flu thrive. The sweet spot for humidity is between 40% and 60%. I personally prefer around 49% in my environment. RH above 60% creates ideal environments for mold, dust mites, and some pathogens as well.
…if you have to leave your house or office while the heat is still on- turn it to cooling mode so the AC can kick on so you won’t return to a moldy environment. This only applies for an extended period of time.
Every cleaner or disinfectant that makes a claim MUST register with the EPA. The EPA confirms that the product is safe for YOU to use and meets the packaging’s claims. NO ONE CAN CLAIM TO KILL COVID-19 BECAUSE THEY HAVE NOT TESTED THEM YET. Everyone is only assuming that these products will work against COVID-19 since they work against similar strains.
VOC stands for Volatile Organic Compound. If the name does not scare you, then knowing that they can cause damage to your health and home should be reason enough to avoid them. Many products used to clean and build houses have VOCs and are easy to avoid if you look at the packaging. For instance, you can get NO or LOW VOC paints offered by most paint manufacturers.
In 1776, Thomas Paine stated that “THESE are the times that try men’s souls. The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country; but he that stands by it now, deserves the love and thanks of man and woman.”
I have seen so many people already helping each other, and as I read stories on social media about how we are helping each other out. Our connections and willingness to help out demonstrate that the human spirit will prevail and help us through this crisis.
1. Help your neighbors that cannot get out
2. Support a local business that is still offering services in a safe manner
3. Help the technically challenged with getting Skype and other communication technology set up
4. Call friends or family that you know are alone to check on them
5. Only enact “social distancing” in the physical environment- Be kind and help others through this crisis.
6. Donate to local food banks to help those in need if you are able
Picking out windows and doors for your home is a big decision. If you make the wrong choice, then you will be replacing them sooner than later. There are many manufactures on the market and in this post we will explore the options for Marvin Windows and Doors.
New Names for Old Windows Lines
Marvin has recently decided to change up the names for their windows to there is less confusion. The original line of windows and doors was just called “Marvin Windows and Doors.” These included their solid wood doors and windows and the wood core windows and doors with aluminum cladding.
They also has a line of called Integrity Windows and Doors that had a fiberglass core with wood cladding on the interior. They also had a line that was solid fiberglass with no wood classing called Integrity All-Ultrex.
In 2019, they rebranded bringing all of the lines under the name Marvin. They have renamed the types of windows and doors and added a few new lines as well.
Marvin Signature Collections
Marvin has 2 different lines within the Signature Collections. The “Ultimate” Collection is the more traditional and in line with the old Marvin Windows and Doors brand. They have also added a “Modern” collection to the Marvin Signature collections as well.
The Marvin Signature Ultimate collection is an wood core for the windows and doors within the collection. One options is for an all wood window or doors with no cladding- just the wood. The option that we recommend is the wood core with aluminum cladding to protect the sash and the frame from the elements. The aluminum cladding is extruded with a durable finish coated with Kynar.
The Marvin Signature Modern Collection is the same as the Ultimate collection, except that it is designed to look more modern. The intent was to create a sash and frame that looks like a store front window. This line performs better than store front windows since they are more durable, energy efficient and DON’T LEAK! They have also increase the maximum dimensions of the Modern collection to give architects and designers more flexibility with their designs.
Marvin Essential Series
The Marvin Essential Series is an all fiberglass window and door. It was previously called Marvin All-Ultrex. Fiberglass is much stronger and more durable than other non wood options such as PVP and Aluminum. Fiberglass and expands and contracts at the same rate as the glass in the window which helps the lifespan of the glass and prevents glass failure.
Marvin Elevate Series
The Marvin Elevate Series is fiberglass like the essential series, however there is an option to add wood cladding on the interior. This gives you the look of a traditional Marvin product with the durability of the fiberglass core. The fiberglass windows also rate slightly better energy wise than the wood with aluminum cladding.
Which window and door is best for your project?
That is a good question and it really depends on the design. For the projects that we work on our “go- to” window is elevate since it is price competitive and worry free when is comes to maintenance. We also learned from feedback from our clients that they want the option to change of the color in the future. The fiberglass is easier to prep and paint than the Kynar finish on the Signature collection.
Even though you can paint the fiberglass, you don’t need to. The Marvin Elevate, Essential and Signature all come prefinished so no paint is needed on the exterior of the window. The essential is all fiberglass and the color is consistent on the interior and the exterior of the window.
If you need help with the design or construction of your home, contact us today so we can share even more options with you.
Thank You! Houzz just awarded us “Best of Houzz 2020” for customer service. We were not expecting this award and we got it because of people like you. We are honored that you all had such great things to say about us. The SawHorse staff will keep working to exceed your expectations moving forward as well.
Most awards require a submission package or nomination from a 3rd party. In this case, these requirements . Houzz awarded us “Best of Houzz 2020” based on your projects, reviews and engagement with our profile.
How we use Houzz.com
We have been using Houzz as a tool to help you all organize your projects and share ideas that you like with us. We’ve also created some ideabooks like the one listed below to give you all idea about what is possible for your home.
In addition to projects portfolios and ideabooks, Houzz also has a place to leave testimonials. They “review the reviews” and this will take several days to vet them before they populate. Unlike other social media sites, this extra step insures that you all are reading real feedback from real people that have worked with SawHorse.
Houzz.com provides tools that help you share ideas with us and we created a video to better explain how to use this function. The “Ideabook” allows for you to organize your thought and ideas and share them with us with through Houzz.com. For instance, when you plan to renovate a room in your house, you create an ideabook for that space. If you plan to renovate multiple rooms, you have the ability to create multiple ideabook.
Let us help you get started
If you need assistance with your projects, contact us so and we will get you started with these tools. Our team will begin with collecting your thoughts and ideas and assist you to project completion.
Sandy Springs is the 6th largest city in Georgia and second largest in metro Atlanta according to its website. In 2005, Sandy Springs incorporated and started to provide some of its own services. This included providing building permits and inspections. SawHorse enjoys working with our clients in Sandy Springs. We are very familiar with the requirements to get permits and inspections for your new custom home, custom renovations or light commercial projects.
The city of Atlanta has a reputation for strict guidelines and standards that need to be met in order to get a permit. Compared to the City of Atlanta, getting a permit in Sandy Springs used to be much easier. It can be argued that Sandy Springs is more challenging that the City of Atlanta with the adoption of its latest zoning ordinance in 2017.
Since 2005, several things have changed in their permitting and enforcement process. Here are some items you need to be aware of if you still working off the old permit checklist:
New Zoning changes in 2017
Survey and Site Plan Requirements for your permit
You will need a full “tree and topography” survey for any addition that you work on. Check the date on your survey. You might need a new one because the zoning has changed in September of 2017. A comprehensive survey is standard for all municipalities now, not just Sandy Springs.
You will need to survey your property and parts of your neighbors property. Most surveyors only survey your property, however here are several items on the checklist that need to be added to the survey or site plan:
Sandy Springs officials are very helpful and willing to meet with you/ us before submitting the permit documents. They take appointments every Thursday to review any questions and offer guidance on how to navigate the new codes. We have set up meetings for each of our current and prospective clients. We have come away from them with a better understand of how they will enforce the zoning and building codes.
Other permit considerations
Pools– their checklist states that “Pool Permit will be required prior to SFASP approval. See Pool Permit Checklist…” NOTE– we got a pool permit first on one of our projects according to this checklist, however in a plan review they asked why we got a pool permit separate from the building permit. Next time we will apply for both at the same time. The note on the checklist since there is internal confusion at this time.
The Chattahoochee River runs through Sandy Springs. If you are planning a project within 1000 feet of it you need to get special approval. Contact ARC- Atlanta Regional Commission.
Neighbors Trees– Must create an escrow account to hold funds for trees that could be damaged on neighbors property as stated in 9.3.8 E:
Where the City Arborist determines that due to approved construction or land disturbance activity an applicant may remove a tree pursuant to the terms of this Article, and the applicant is required to pay for the lost tree canopy of the removed tree, the applicant may, at its election, propose alternative construction or site design methods to attempt to preserve the continued viability of the tree. Should the City Arborist determine that the proposed alternative construction or site design methods will reasonably result in the survival of the tree, that portion of the funds required to pay for the lost tree canopy of the tree pursuant to this Article must be paid into an escrow fund maintained by the City.
We can help you
We will continue to edit this as we learn more and get feedback on on different projects. I’ll also create a list of items to be expected in ALL municipalities based on state codes.
Don’t let all of these items dissuade from doing your project. If you do not have a design team yet, please contact us. We will guide you through the design and permitting process. If you already have a set of plans can can still help with permitting and construction.
Most argue that green homes cost more without looking at the performance and value benefits. Studies have shown that green homes not only use less energy and water, they also sell for more.
This home has high-performance systems and renewable energy, which helps the operating costs stay very low throughout the year. If you are building a new high-end home, the house’s structure and systems must be upgraded, not just the finishes.
In this green home, we upgraded the insulation and designed a geothermal HVAC system. We also added a solar thermal system since that was the best option at the time for water heating. To help with irrigation, we captured all of the rainwater and stored it in a 5000-gallon underground cistern.
We have been promoting high-performance green homes for years. Now there are many more of them on the market. This is a large enough sample size to show that they do indeed sell for more. This is good news for SawHorse clients. All of our homes are built to higher quality performance standards. This means you get the benefit of lower operating costs during the time you enjoy the home. You also get a higher ROI if you sell your home.
The national association of home builders put out an article on their website to help support our claim. Here are some of the key points taken directly from their article:
Property value analysis rated homes sold for 2.7% more than comparable unrated homes.
Better-rated homes sold for 3-5% more than lesser-rated homes.
From the loan performance analysis, the default risk of rated homes is equivalent to unrated homes, once borrower and underwriting characteristics are considered.
Loans in the high debt-to-income (DTI) bucket (45% and above) with ratings, however, appear to have a lower delinquency rate than unrated homes.
Whether designing new custom homes, expanding existing homes, or renovating existing spaces, the house’s mechanical systems must be considered part of the final design. Most houses that we renovate have standard efficient HVAC systems or 80% AFUE furnaces before we get started. This is not ideal.
What is AFUE, and why should you care?
From a recent post:
AFUE– is the measure of “Annual Fuel Utilization Efficiency” of heating equipment per year. For instance, in a typical 80% furnace, 20% of the energy produced by natural gas is expelled as exhaust directly to the exterior. 90% is considered “high efficiency” and 95% AFUE is pretty common among most new gas-powered furnaces. We recommend at least a 90% efficient furnace.
When choosing a gas-fueled furnace, you can either choose an 80% standard or 90% and greater “High-Efficiency” Furnace. Our standard is a 90% AFUE furnace with a variable speed motor at a minimum. 95% is pretty common and not that much more.
100% efficiency (which does not exist for natural gas) means that for every therm that you purchase 100% of that energy stays in your home. The 80% AFUE furnace wastes 20% of the gas since it is exhausted through your roof. Even worse, the efficiency decreases over time so you are wasting even more money!
Top problems with 80% AFUE furnaces as part of HVAC system
Upgrading to a 90% AFUE furnace or High-Efficiency Heat Pump solves many problems, including the ones listed below.
80% AFUE furnaces require combustion air which can cost more to install.
The high-efficiency versions allow you to bring this air directly to the unit from the outside. Special considerations are needed whenever you have open combustion appliances such as standard gas furnaces and standard gas water heaters. You have to have combustion air either from the surrounding area if the space is large enough. If not, you must bring it in from the outside.
When combustion of natural gas occurs to create heat, the byproducts can be harmful
80% AFUE furnaces are “open combustion,” meaning that their design can allow for some of these harmful gases, such as CO- Carbon Monoxide, to enter your home, which can cause illness or death.
80% AFUE furnaces must have their own room
If you chose to install the standard option, your savings would build a space for the furnace and water heater. High efficient furnaces and water heaters can vent directly to the exterior with a seal combustion system. This means that they do not need a special room or sealed closet with the combustion air. They can get their combustion air directly from the exterior of the house. The cost to retrofit this in an existing house is not worth it. We recommend the upgrade to a high efficient furnace or heat pump. The difference in construction costs will be less than the difference to upgrade from standard efficiency equipment.
Open combustion HVAC and water heating appliances can cause fires if flammable items are off-gassing in the same area.
You should never store gasoline or other harmful chemicals in your home due to contaminating your breathable air. This a health issue. It can also become a safety issue if the flammable gases reach your equipment. The open sparks from some of these systems could ignite and cause a fire in your home, destroying your investment.
So what should you tell your HVAC installer?
If you prefer ALL electric, then a ducted mini-split is a great option. They are very energy efficient. If you plan to go “off the grid” with renewable energy, electricity is a must. Some of these can go up to 42 SEER, which is extremely efficient.*
If you like the benefits of gas, I recommend at least a 90% AFUE with a variable speed ECM fan and 14 SEER (code in GA) system. If you can afford the upgrade- 95% AFUE with a variable speed ECM fan and 16 SEER (or greater) with a variable speed condenser.*
Regardless of the system, make sure they design the ductwork to be 100% within the conditioned space of your house. Otherwise you can have additional energy penalties if the insulation or sealants on the duckwork fail over time.
*I’ll update these specs based on feedback from our installers and manufacturing partners.
So what’s scarier than ghosts and goblins on Halloween? Radon in your Atlanta house since, like ghosts, you cannot see it or smell it. Most other indoor air pollutants such as mold and toxic chemicals are easy to detect with your nose. Radon is an odorless gas; therefore, you have to test for it.
Even scarier, Radon is the 2nd leading cause of lung cancer in the US. Atlanta has some of Radon’s highest rates in the region. The good news is that there is a solution to fix the problem if your house tests at high levels.
Radon Testing and Mitigation
The first step is to get your house tested. There are some DIY kits available. If you suspect radon is in your house, then a long-term test by a professional is recommended.
If you are building a new house in the Atlanta area, then definitely plan for Radon vents during construction. They are much cheaper than a retrofit later. All of the new houses we have built in the last few years have had passive Radon vents installed. They start at the slab, and the vent goes to the roof of the house.
Radon causes lung cancer and like avoiding cigarette smoke, avoiding radon is possible as well. Testing for radon, then coming up with a plan to mitigate it is really all you need to do.
Other Radon mitigation ideas
There are specific designs for getting rid of higher levels of Radon through underground and roof vents. What about the radon or other toxins in your air? Good mechanical ventilation can solve this. Devices such as Energy Recovery Ventilators can remove “bad air” while bringing in “fresh air.”