As rising energy costs caused home owners to focus on increasing home heating/cooling efficiency, residential construction advances tackled the problem of infiltration, which had been the accidental fresh air supply for many centuries before anyone knew it was important. This airtight building approach was great for reducing unwanted heat loss/gain, uncomfortable drafts, dust, pollen, bugs and more. However, for better or worse, we need fresh air to live. As above ground homes get tighter, building codes and organizations like ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) have developed standards to take some of the guesswork out of what is required (Specifically, ASHRAE-62). These same ventilation standards are useful and required for Earth Sheltered home builders. As ASHRAE says, “If you build tight, you must ventilate right!” Another detailed document
Codes allow most rooms to have natural ventilation provided by windows that can open to at least 4 to 8% o the floor area. Where this is not available, a more intentional system for ventilation is required. Since you may not always want to open a window (and let all your heat or cool out), a well designed ventilation system that doesn’t rely on windows is usually a good idea. In fact, ASHRAE 62.2 recommends that windows are not actually sufficient to control the moisture and cooking by-products (grease that sticks to walls, etc.) generated in an average kitchen.
In earth sheltered homes, it is common for bathrooms and other rooms to be on the “back” of the house without the option of opening a window. These back rooms are also against cool earth, increasing the probability of problem causing condensation and making adequate ventilation even more important.
The following table gives a high level overview of ASHRAE-62.
Living Areas include habitable space such as living rooms, play rooms, dining rooms, bedrooms, etc. (pretty much anywhere you sit or lay down). It does not include storage rooms, mechanical rooms, large closets, laundry, etc. Generally the 15 cfm per person dominates the requirements. It is a judgement call as to how many people can be expected in a room, but as a rule of thumb, common rooms should be sized for the total number of expected residents. Bedrooms should be sized for at least 2 people.
As an example calculation, lets assume a living room that is 15’x15’x10’=2250 ft^3. If we must do 0.35 ACH, then that is 788 ft^3/hr or 13 CFM… Even for a hermit, that is less than our minimum of 15 cfm per person. Lets say we have 4 people and need 60 cfm. Most forced air furnaces run at about 700 ft/min (8 mph). These are driven by a fan and can only function for a relatively short distance, so lets assume that our more passive systems operate at a more leisurely rate of 175 ft/min (2 mph). This would require nearly 50 square inches of inlet, which would require either four 4 inch pipes, or two six inch pipes or one 8 inch pipe… Of course, we could make the room much larger (up to 32 x 32) and not need any larger duct work… In many cases, the living room, dining room and kitchen essentially share the same space without walls to separate them, so we can consider them one living area for the purposes of the ventilation requirements.
Other smaller, but separate, rooms will each need to support 15 CFM per person expected to occupy them. If we assume 30 CFM per bedroom, we could use a single 6 inch duct to carry fresh air to each room. There are no exhaust requirements for these living areas, but air must be allowed to escape as fresh air enters or a pressure differential will occur. It can escape thru return air vents or under doors. In many pasivhaus designs, used air from the living spaces is exhausted thru the bathrooms or kitchen. This promotes whole house flow.
The exhaust requirements for bathrooms are often specified as 8 ACH (Air Changes per Hour). This is not a continuous requirement, but is typically applied when sizing the exhaust fan. Some local codes require the exhaust vent to be place at or near the ceiling. This makes sense because the warm moisture laden air you are trying to remove is typically buoyant and rises to the ceiling. The code also requires that the moisture laden air be exhausted to outside; it does not permit exhausting to internal duct systems that may lead back to an air to air heat exchanger or earth tubes.
I have heard of local codes that specifically require the exhaust to vent thru the roof. This can be problematic for an earth sheltered home builder who would rather run this thru a wall than penetrate the waterproofing layer. Where this rule is enforced, it is an attempt to prevent the builder from inadvertently dumping this humid air under the eves where it could be sucked back into the soffit vent and degrade the insulation above the ceiling. If your earth sheltered design has no soffit vents, and it would be easier to run it thru the wall, you may be able to get permission to vary from the code (a variance).
A code variance or modification considered when there are “practical difficulties or undue economic hardship involved in meeting a construction code requirement.”
If you must (or would prefer to) run these exhaust vents up thru the roof, I recommend running them up thru the skylight curb (A skylight is a pretty standard feature in an earth sheltered bathroom) to reduce the number of roof membrane penetrations. If the tube can be run up the sun-ward side of the skylight curb, they may be some potential for a solar chimney application.
As a quick calculation, a 5′ x 8′ bathroom (about the smallest layout that “works”) with 10′ ceilings has about 400 cubic feet of volume. With 8 ACH, we are looking at exhausting 3200 cubic feet of air per hour or about 53 cfm. Velocity is important. Most bathroom fans are rated between 50 and 120 cfm, so it would be no problem to install one of those, or you could use more passive methods and a more leisurely pace, but you would need more or larger pipes. Two 6 inch pipes at 175 ft/min would easily clear 68 cfm. Fortunately, the “continuous” ventilation provided by a passive system would only require less than half of that and could be served by a single evacuation tube, even at low temperatures. I plan to design for a passive system, but go ahead and install the standard bathroom fan with a switch anyway.
Bathroom fans are important for removing moisture, but many people refuse to turn them on due to the awful racket they produce. It is possible to use a remote fan to suck the air out of the bathroom from further away to reduce the noise. On the other hand, a friend of mine was a construction contractor and was often asked to install very high end bath room fans and then called back because the fans didn’t provide the comforting (and tinkle sound covering) noise that the home owner expected. He would take it down, bang it up with a hammer and re-install.
In addition to the basic ventilation needs for the kitchen (listed as 100 cfm intermittent or 25 cfm continuous or an operable window), there is also the need for a range hood to deal with the moisture and odors of cooking. Many homes exhaust this air directly to the outside and the rules for exhausting this air (along with its moisture and suspended oils) are very specific. However, many home fires have resulted from oil coated ductwork catching fire. It also seems very wasteful to eject heated air from your home at a rate of 150 cfm. More modern range hooks have a charcoal filter to remove the cooking oils and other particles from the air and then return it to the room, with any wasted heat. In fact, if the heat from cooking is simply filtered and returned to the room, it can be considered an internal heat gain when performing a home heat loss calculation.
IRC 2003 – RANGE HOODS
Range hoods shall discharge to the outdoors through a single-wall duct. The duct serving the hood shall have a smooth interior surface, shall be air tight and shall be equipped with a back-draft damper. Ducts serving range hoods shall not terminate in an attic or crawl space or areas inside the building.
Exception: Where installed in accordance with the manufacturer’s installation instructions, and where mechanical or natural ventilation is otherwise provided, listed and labeled ductless range hoods shall not be required to discharge to the outdoors.
If I had a gas range (officially referred to as a “combustion appliance”), particularly if it was a large gas range, I would strongly consider venting to the outside. But with my electric range, I am happy to have any heat generated be filtered and returned to the room.
Another interesting consideration is ventilation for your refrigerator. A refrigerator is really a decent sized air to air heat pump that extracts heat from the inside and pushes it to the outside (behind). Without proper ventilation, it can warm up the region behind the refrigerator, which makes it less efficient as it tries to dump more heat into the same space. Most refrigerators have a built-in air-intake on the bottom front of the refrigerator, but assume you will leave space behind and above for the heated air to exit again. Be careful not to surround your fridge with cupboards, and if you do, make sure to provide adequate ventilation. I am considering adding a duct from the basement to the the space behind the fridge so that it can draw and heat basement air and add it to my living space as it cools my food. Perhaps this would be a good entry point for an earth tube.
Clothes dryer exhaust is the main concern here. A clothes dryer works by heating up room air to lower its relative humidity, and blowing the warm air thru the tumbling wet clothes where it can pick up the moisture. The dryer exhaust contains a lot of moisture and often lint along with the heat.
You can get a natural gas clothes dryer that uses electric motors for the fans and rotating the drum, but a natural gas flame to heat the air. These can be cheaper to run over the long term (assuming the price of gas stays low relative to electricity), but they are more complicated so they cost more to purchase and need a professional installation. There is always a potential for problems with any gas appliance, including leaks of the methane its self or combustion by-products such as carbon monoxide, nitrogen dioxide (linked to asthma), VOCs, fine soot, and many other trace chemicals, so I don’t recommend them for well sealed earth sheltered homes. If you do go with gas, make sure you add what ever extra ventilation you need to stay safe, plus a bit more.
In the cold dry winter, the moisture and heat (and perhaps the smell of fabric softener 😉 ) may be welcome additions to the home. It is a shame that most Americans pump this “enriched” air outside. You can purchase a simple dryer heat diverter for less than $10 that will divert this air back into the room. Unfortunately, if this exhaust air, already saturated with moisture, is sucked back into the dryer, it will not be able to pick up much more water and the dryer efficiency will be reduced. Ideally, the exhaust air can be diverted into another room that needs it instead of being sucked back into the dryer. Reusing the dryer exhaust in winter is like giving the dryer a second job as a heater/humidifier.
In hot humid summer, it is very important that any hot humid dryer exhaust exit the home as quickly as possible. If the duct is long or constricted or contains many turns, the back-pressure increases and the (built in) dryer fan is not able to push as much air thru the system and the efficiency of the dryer is reduced. Dryer booster fans can be purchased to compensate, but because they are designed to manage the heat, moisture and (worst of all) dryer lint, they are expensive. A much cheaper solution is just to design for as short and direct an exhaust duct as possible. This can be a bit more difficult in an earth sheltered home where the laundry is often against a back wall where the earth is thickest, but make sure to keep it under 25 feet and count any 90 degree turns as an extra 5 ft.
Another way to make it difficult for the dryer is to limit the amount of incoming air. Air must be able to get into the laundry room before it can be blown thru the dryer. If not, the pressure in the laundry room will drop and the shortage of air will reduce the efficiency of the dryer fan causing it to work harder and take longer. Most homes are not built tight enough for this to be a serious problem, and ASHRAE guidelines actually exempt the laundry room from all ventilation requirements, but earth sheltered laundry rooms can get pretty tight. It may not be worth ventilating directly, but consider adding a transfer grill or jump duct to let in air from the hall, or you could trim the bottom of the door to let air in that way.
Many home designs keep the laundry away from the bedrooms for a variety of reasons, including separation from unpleasant laundry room noise and vibration. My heavy cement house shouldn’t let much sound or vibration between rooms, so I have placed my laundry close to the bedrooms where the dirty clothes are ;). I need to let air in for the dryer, but I don’t want the sound getting out. I have a false ceiling in my hall that I use to run duct work and electrical to the master bedroom. I also plan to put a transfer duct above the hall ceiling into the laundry so air can get in, but the sounds of the laundry room will not have a direct route into my hall.
A piece of string makes a great passive solar clothes dryer and is probably the cheapest passive solar component you can add to a home… Dryers use a lot of electricity, so it can make a big difference. 😉
My garage is essentially a buried Quonset hut, so I do not have the sort of heat gain problems associated with above ground garage structures. ASHRAE garage ventilation requirements are 100 cfm per car. Since it doesn’t make sense to run this sort of fan all the time, I will probably put mine on a switch or on a motion sensor. I don’t recommend starting your car before opening the garage door anyway.
Do not use one of those rotating vents, it will run all the time and over ventilate your garage.
If you have a shop in your garage, you may need to consider that ventilation also. If it is basically one room, meeting the ventilation requirements for the cars can count as your shop ventilation also. Again, providing a switch so you can turn the ventilation on only when you need it is probably a good idea.
ASHRAE-62.2 is very specific about how duct work and air handling equipment that passes thru the garage should be well sealed to avoid carrying contaminants often found in garages into the rest of the home.
Many passivhaus designers recommend using earth tubes to bring fresh air into main living areas and then exhausting this air thru moisture laden (wet) areas such as the bathrooms or kitchen. Exhausting this moisture directly to the atmosphere runs contrary to the “Camels nose” design that many earth sheltered home designers would rather implement, but I suspect that trying to draw moist bathroom air down into earth tubes is probably not a good idea. I have seen some HRV web sites that do recommend running bathroom exhaust back thru the HRV (or HEV) to recover this heat (and moisture) in winter.
Passive stack (Bernoulli) and Solar chimneys are a great way to passively draw air flow thru a building. While rare in the energy-rich USA, passive stack is very common in Europe, particularly in overcast England where it is found in 90% of single family homes. Optimizing a stack design can require a study of the local weather patterns and home design, even the surrounding neighborhood structures and trees can affect the flow patterns. However, as a general rule of thumb, taller stacks work better, especially if you add a rotating cap that works in any wind direction. Larger diameter pipes work better than smaller diameter ones which have higher flow resistance. Careful design may also be needed within the home to ensure that all rooms are equally well ventilated; this is more of a concern with passive homes because of the lower driving pressures. Passive systems may need to be “augmented” to increase their effectiveness as necessary. Modern “active” ventilation fans often come with an intelligent control system that can dial back the amps when the passive methods are working well. As any kite flier knows, its not windy every day. However, due to the way weather is generated by hot or cold fronts, calm days are usually moderate and good for opening windows.
If you are lucky enough to live in one of the sunnier areas of the world, you may want to add a solar chimney to add passive solar heating to the passive stack effect.
Fans can be purchased based on their specific cfm ratings needed for the various rooms, or whole house ventilation systems can be used (typically, 50 cfm is sufficient for a whole house ventilation fan system). If these systems draw in fresh air directly from outside, you could end up in a situation where you are continuously heating/humidifying or cooling/dehumidifying fresh air and then dumping it outside along with its energy. Earth tubes are one way to moderate this incoming air, but those above ground dwellers may have a cheaper/easier way… Many well sealed above ground homes use an ecomonical “balanced ventilation” system such as an HRV or ERV.
A balanced ventilation system is one that controls the ventilating airflow in and out of the home, typically using a pair of fans. These units usually pass the two airstreams next to each other so that some of the thermal energy can be transferred from one to the other. They can do this thru a cube shaped heat exchanger or thru a turning wheel. These are typically known as heat recovery ventilators (HRV) if they only transfer heat (Or as an ERV if the interface can also transfer moisture). HRV/ERV systems usually have several fan speeds and range from 50 to 150 cfm, but at lower speeds, the heat exchange is more efficient (more time for transfer, less dynamic resistance), so running constantly at the lower speed is better than running 20 minutes of every hour at the highest speed. Many also include timer switches that are installed in the bathrooms to boost incoming flow temporarily. Underground passive solar homes can use the same mechanical systems, but often prefer to use more passive methods, such as earth tubes to temper the incoming air and a solar chimney to draw the air thru a home and out the top.
I believe that all systems should start with a good passive design, but in many cases, adding some “active” assistance can dramatically improve performance for a very reasonable cost. Cost wise, an HRV costs little more than the price of the two fans it contains, but provides a lot more value. These units typically contain filters which also help improve air quality. They almost certainly cost less than an earth tube system.
It may be that an ideal design would use a combination of earth tubes and an HRV or ERV. Unlike an earth tube system, an ERV or HRV does not have the mass to store energy. An earth tube system can store the warmth of summer and return it in winter. Even a relatively short Earth tube can at least help moderate day to night fluctuations. An HEV can only transfer energy from one stream to the other. However, this can significantly reduce heat loss/gain while ventilating. An ERV can also reduce moisture loss/gain, which can be helpful in an earth sheltered home.
The choice of an ERV vs HRV is more complicated than the “climate zone map” that the manufactures show. They are interested in making the sale as quickly as possible and a complicated decision would just slow the sales down. As an informed customer, you should think about what the equipment does and make your own decision about what would work best in your design. Also, ERVs tend to be 10 to 20% more expensive than the equivalent HRV.
Using an earth tube upstream of an HRV should prevent frost buildup in the unit, which means you could purchase a lower cost HRV that does not include the “Re-circulation Defrost” option (should save about 10% of the unit cost). This defrost option is typically the most complicated (and failure prone) part of the system and works by occasionally blocking the fresh air intake and allowing the warm air from the home to defrost the unit, in many designs, this causes cross contamination between the inflow and outflow air. At best, this re-circulation option puts the ventilation out of balance and wastes heat when operating.
You may want to consider an ERV (Energy Return Ventilator that exchanges heat and moisture), you may see that many of the manufacturers (such as the Lennox web page) only recommend ERVs for warm climates, but never explain why. Digging further, I found that these are not recommended for cold climates primarily because of the risk of freezing the moisture exchanging core. This is not a problem when fed with an earth tube, or you could pay a little more to get the defrost option. The benefit of going with an ERV is that it is able to transfer incoming humidity to the outgoing air stream. The advertisements make it look like a complete transfer, but it reality the heat exchange is the same as for an HRV, except across a moisture permeable interface. This happens best if the hot humid humid outdoor air is cooled while warming (transfers its heat to) the cooler dryer exhaust air stream. Relative humidity changes force moisture out of the cooling air and it is picked up by the warming air on the other side of the membrane. It is important to understand that this moisture transfer only works if the outgoing air is much cooler or dryer, which usually only occurs if you cooled and/or dehumidified it. An ERV is NOT a dehumidifier or an air conditioner, but it can help you retain your conditioned air during ventilation.
Earth sheltered homes don’t tend to need AC equipment which would automatically dehumidify the air as part of its process. An ERV will not dehumidify the air, most earth sheltered homes will still want a “whole home dehumidifier”. However, using an ERV will help reduce the summer humidity load. Cold air can not hold much moisture. During cold winter ventilation, cold dry air entering the home will pick up moisture from the inhabitants as it warms. If it is expelled from the home with an HEV or exhaust fan, it takes that moisture with it. Using an ERV in a cold climate gives that moisture a chance to jump onto the incoming air as it is warmed up in the core and could actually help prevent your home from drying out (process reversed from summer).
The exchange also works best if the system is in balance. The outgoing flow should roughly match the incoming flow if you want to recover heat or moisture. During the summer, I plan to use a solar chimney to draw the air out of my home… Incoming air will still pass thru my HRV or ERV, but I wouldn’t expect any “recovery”, and I may even want to remove the core (pressure and dynamic losses) and turn off (or redirect) the outgoing fan.
I have read a number of articles (such as this one) on the proper installation of these HRV or ERV units and here are the top concerns.
- Use a dedicated duct system. Some try to feed their forced air system and end up picking up the moisture off their condenser coil. This increases the humidity and causes other problems.
- Exhaust high moisture sources (bathrooms and kitchen) directly outside. It is not worth trying to recover this heat or moisture. You will just be spreading too much moisture around.
- Separate the outdoor intake and exhaust ports to avoid drawing the exhaust hair back into the unit
- Balanced flow is key to good recovery. Make sure to check the flow rates in both directions and make adjustments if necessary. If you intentionally unbalance the flow, be smart about it.
- Use with a dehumidifier or AC unit in summer. Don’t count on a ERV to remove moisture (unless it is a combo ERV/Dehumidifier), it can only help reduce the burden of fresh air on your other systems.
- Reduce the number of twists and turns to a minimum. I recently saw an installation whose intake ran thru flex pipe along two walls and for more than 40 feet. It included two tight 90 degree turns before a 180 degree turn into the unit. I expect those dynamic and frictional losses will dramatically reduce the airflow rate.
Using the above information, it is possible to size the earth tubes and fans necessary for healthy ventilation.
(obviously still in progress… I’ll get back to this later)