Ventilation
BUILDING SCIENCE – DOES MY BUILDING NEED TO BREATHE?
The simple answer is no, only living creatures like you, your family & pets do. The catch is that we also have to consider that not only do we need air but we have to have a way to eliminate or exhaust the contaminants from the space. For example;
  • Not only do we, but also our pets sweat & release odors we also exhale carbon dioxide & moisture into the air when we do breathe.
  • Taking showers, having plants inside, cooking, and other activities can add to not only the moisture in the air, but even some more interesting odors.
  • For those with combustion appliances (or worse yet a vent-less heater) inside the house, now we also have a device that not only wants to consume the same oxygen we need but it produces even more moisture, carbon dioxide & occasionally carbon monoxide (aka the silent killer).
  • VOC’s & others odors come from not only everyday items like pens & hairspray, but also certain building materials off gas for a while… 
Is it any wonder that many people consider the outdoor air as fresher than indoor air, or that in many cases it really is?
 
WHAT DOES MY BUILDING NEED TO DO?
 
While your house does not need to breathe, it does need to be able to dry out. As you can see above almost all the byproducts that happen inside an occupied house relate to moisture being released. Not only does one have to worry about this moisture, but also what the weather is like outside.This leads us to an interesting issue as many will point to historic homes and state that one of the big reasons that they are still standing is due to “natural ventilation / their ability to breathe.” To top that off, many will claim that by us rigidly air-sealing everything up, we have turned our houses into huge petri dishes & invented sick-building syndrome.
 
In some ways they are right, as many older homes are still standing because they had the ability to dry out quickly. While leaving things open in older homes was perfectly fine for a house built back then, it simply won’t fly with all our modern conveniences like doors that fully shut, running water, air conditioning, steam showers, commercial style cooktops, or many peoples acceptable level of comfort. It always helps to remember that when you change one thing in a house it affects a whole host of other items.
 
ATTIC VENTILATION
 
At first it may seem odd to add insulation for warmth and then purposely allow cold air to enter the attic through vents, but this combination is the key to a durable and energy-efficient home. Here's why: in the winter, allowing a natural flow of outdoor air to ventilate the attic helps keep it cold, which reduces the potential for ice damming (snow that melts off a roof from an attic that is too warm and then re-freezes at the gutters, causing an ice dam that can damage the roof). Proper insulation and air sealing also keeps attics cold in winter by blocking the entry of heat and moist air from below. In the summer, natural air flow in a well-vented attic moves super-heated air out of the attic, protecting roof shingles and removing moisture. The insulation will resist heat transfer into the house.
 
The most common mistake homeowners make when installing insulation is to block the flow of air at the eaves. NEVER COVER ATTIC SOFFIT VENTS WITH INSULATION — use rafter vents and soffit vents to maintain airflow.
 
Attic Fan Ventilation
Attic fans are intended to cool hot attics by drawing in cooler outside air from attic vents (soffit and gable) and pushing hot air to the outside. However, if your attic has blocked soffit vents and is not well-sealed from the rest of the house, attic fans will suck cool conditioned air up out of the house and into the attic. This will use more energy and make your air conditioner work harder, which will increase your summer utility bill.
 
*You don't want your unfinished attic cooled by your air conditioner. To prevent this, follow the air sealing and insulation strategies and make sure the attic is well-ventilated using passive vents and natural air flow.
 
Baffle (Rafter) Vents
To completely cover your attic floor with insulation out to the eaves we need to install rafter vents (also called insulation baffles). Complete coverage of the attic floor along with sealing air leaks will ensure you get the best performance from your insulation. Rafter vents ensure the soffit vents are clear and there is a channel for outside air to move into the attic at the soffits and out through the gable or ridge vent. Rafter vents come in 4-foot lengths and 14-1/2 and 22-1/2 inch widths for different rafter spacings. Rafter vents should be placed in your attic ceiling in between the rafters at the point where your attic ceiling meets your attic floor.  Once they are in place, we can then place the batts or blankets, or blow insulation, right out to the very edge of the attic floor. Note: Blown insulation may require an additional block to prevent insulation from being blown into the soffit. A piece of rigid foam board placed on the outer edge of the top plate works very well for this.
 
BATHROOM VENTILATION
 
Bathroom ventilation systems are designed to exhaust odors and moist air to the home's exterior. Typical systems consist of a ceiling fan unit connected to a duct that terminates at the roof. Bathroom ventilation fans should be inspected for dust buildup that can impede air flow. Particles of moisture-laden animal dander and lint are attracted to the fan because of its static charge. Ventilation systems should be installed in all bathrooms. This includes bathrooms with windows, since windows will not be opened during the winter in cold climates.
 
The following conditions indicate insufficient bathroom ventilation:
  • moisture stains on walls or ceilings;
  • corrosion of metal;
  • visible mold on walls or ceilings;
  • peeling paint or wallpaper;
  • frost on windows; and
  • high levels of humidity. 
The most common defect related to bathroom ventilation systems is improper termination of the duct. Vents must terminate at the home exterior.
 
The most common improper terminations locations are:
  • mid-level in the attic. These are easy to spot;
  • beneath the insulation. You need to remember to look. The duct may terminate beneath the insulation or there may be no duct installed; and 
  • under attic vents. The duct must terminate at the home exterior, not just under it.
 
Improperly terminated ventilation systems may appear to work fine from inside the bathroom, so you may have to look in the attic or on the roof. Sometimes, poorly installed ducts will loosen or become disconnected at joints or connections.
 
Ducts that leak or terminate in attics can cause problems from condensation. Warm, moist air will condense on cold attic framing, insulation and other materials. This condition has the potential to cause health and/or decay problems from mold, or damage to building materials, such as drywall. Moisture also reduces the effectiveness of thermal insulation.
 
Even though mold growth may take place in the attic, mold spores can be sucked into the living areas of a residence by low air pressure. Low air pressure is usually created by the expulsion of household air from exhaust fans in bathrooms, dryers, kitchens and heating equipment. Ventilation ducts must be made from appropriate materials and oriented effectively in order to ensure that stale air is properly exhausted.
 
Ventilation ducts must:
  • terminate outdoors. Ducts should never terminate within the building envelope;
  • contain a screen or louvered (angled) slats at its termination to prevent bird, rodent and insect entry;
  • be as short and straight as possible and avoid turns. Longer ducts allow more time for vapor to condense and also force the exhaust fan to work harder;
  • be insulated, especially in cooler climates. Cold ducts encourage condensation;
  • protrude at least several inches from the roof;
  • be equipped with a roof termination cap that protects the duct from the elements; and 
  • be installed according to the manufacturer's recommendations.
  • The following tips are helpful, although not required. Ventilation ducts should:
  • be made from inflexible metal, PVC, or other rigid material. Unlike dryer exhaust vents, they should not droop; and 
  • have smooth interiors. Ridges will encourage vapor to condense, allowing water to back-flow into the exhaust fan or leak through joints onto vulnerable surfaces.
Above all else, a bathroom ventilation fan should be connected to a duct capable of venting water vapor and odors into the outdoors. Mold growth within the bathroom or attic is a clear indication of improper ventilation that must be corrected in order to avoid structural decay and respiratory health issues.
 
HEAT RECOVERY VENTILATOR (HRV) OR ENERGY RECOVERY VENTILATOR (ERV)
 
Whether natural or mechanical, homes need ventilation. They are no longer built to leak heat and moisture the way they used to be; we now build them as airtight as we can. This makes mechanical ventilation essential in a high performance home.  How much fresh air is required and the best way to provide it are important issues. Energy recovery from exhaust air is becoming common place in cold regions, and two types of equipment can do this- an HRV (Heat Recovery Ventilation) and an ERV (Energy Recovery Ventilation).
 
Both HRVs and ERVs are somewhat new to mainstream home construction, and can often be confused. In an effort to clear that up, we will first explore why ventilation is so crucial, then explain the options and their best applications.  Up until the last few decades, houses were so leaky that sufficient cold dry air seeped in to meet the needs of occupants, and ensure homes had no moisture damage. These houses were said to 'breathe', but that would be like breathing through your skin instead of through your nose.  It meant that cold, dry winter air would need to be warmed as well as humidified, while hot and humid air would enter in the summer.
 
Nowadays, in the name of energy efficiency, houses are built to much higher standards of air tightness, so a mechanical ventilation system is essential for the following reasons:
  • To provide oxygen for occupants since people deplete oxygen as they breath. In a reasonably airtight home with no ventilation you would feel the effects of that in quite short order.
  • To remove contaminants – because along with the toxins emitted by the human body (ammonia, benzene, carbon monoxide and methane to name but a few), chemicals in building materials and furnishings continue to off-gas for many years after installation.
  • To remove the excess humidity generated by normal human activity in order to ensure building durability and efficiency in heating. 
HOW MUCH FRESH AIR IS ENOUGH?
 
It is very difficult for humans to detect low levels of contaminants in their air, even when they represent a health hazard. An ideal ventilation system would include sensors that could detect the presence of excessive humidity and all harmful agents in order to provide fresh air accordingly, but no such system exists yet.
 
Therefore, our best option at present is to err on the side of safety, and provide a minimum fresh outdoor air supply at all times. Most building codes rely on the *ASHRAE standard 62.2 (or some variation of it) to establish ventilation norms for homes.*ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) is the most respected and authoritative source for interior air quality standards. ASHRAE 62.1 and 2 are the recognized standards for ventilation and indoor air quality (IAQ).
 
CHOOSING BETWEEN AN HRV AND AN ERV
 
Heat Recovery Ventilation (HRV) is a system that uses the heat in stale exhaust air to preheat incoming fresh air. This reduces the energy required to bring outside air up to ambient room temperature.  Similar to the human breathing system as mentioned above, this exchange of air is performed in a single area of the home, the lung of your home, your ventilator core.  Note that outgoing air and incoming air never mix in the heat recovery process; they simply pass in separate channels in the ventilator core, allowing an exchange of heat through conduction.
 
The 'efficiency rate' of an HRV unit determines how much energy will be saved by using that particular device. Although it requires the operation of a fan on a continual basis, the energy recovered from the inside air is many times that of the energy required for the fan.  Typical efficiencies range from 55% to 75%, but some extremely efficient models are rated as high as 93% efficiency. At present, these latter units are significantly more expensive and only available from Europe.  However, when you factor the value of energy savings over the unit’s full life cycle, shipping these costly units across the ocean can still make it a financially and ecologically sound investment.
 
Energy (or Enthalpy) Recovery Ventilation (ERV) goes a little further than the HRV scheme, as this type of system also captures some of the humidity in the air to keep it on the same side of the thermal envelope that it came from.  So in winter, the system transfers the humidity from the air being extracted to the incoming fresh (and dry) air to help keep the ambient humidity level at a reasonable value (between 40 and 60%) at all times.
 
In summer, the humidity transfer reverses and the humidity in outside air is removed before it is injected into the home. This saves energy by reducing the load on your air conditioning system and/or dehumidifier. A high efficiency of humidity transfer would be around 70% but this value depends on the actual humidity on either side of the envelope.
 
One important note is that whatever you choose for your needs, there will always be a power on/off switch. If your system is too noisy, you will likely turn it off for long periods of time even if you really need it. Ensure you have a quiet system and that it is installed properly to avoid the temptation of turning off a piece of equipment that represents both a financial and health investment.
 
WHICH ONE IS RIGHT FOR ME?  HRV OR ERV?
 
The best option between an HRV and an ERV depends on your climate and specific needs. If your house is too humid in winter (above 60% RH) then an HRV is the better choice, as it would surely get rid of excess humidity while an ERV would tend to keep it at a high level.
 
If the opposite is true and your house is too dry in winter, then an ERV would be a better choice as it helps retain humidity, eliminating the need (and cost) for you to generate it through other means.  In summer time, the use of an HRV will usually increase the humidity level inside your home, so an ERV is better in hot and humid zones. But a dedicated dehumidifier will likely do the trick much better. At the very least, the ERV will lower the load on the air conditioning system, even if it can’t keep up with the high humidity level on the outside.
 
So in the end, there is not one right choice. It depends on your climate, your lifestyle and your home. In a perfect world we would have one of each, short of that we are left to make a choice.
 
One thing is for certain though, whichever you choose, an airtight home with an ERV or HRV is an evolutionary leap beyond the leaky houses of the 20th century, so if  you are building or have a reasonably airtight house, don't lose sleep over which one to get – just get one.

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