Building Science 101: Ventilation

INDOOR AIR QUALITY is important– we need clean, fresh air to remain alert and healthy. Traditionally, houses were usually built with enough air leaks that we didn’t have to worry about where fresh air was coming from – more than enough would get in, especially in cold weather, while heat escaped just as quickly. Today building scientists recognize that insulation and air tightness are important for comfort and energy efficiency, and we need to get clean, fresh air in another way.

Whole-house ventilation

Many people in the high-performance building world cringe when we hear, “a house has to breathe” or “don’t build too tight.” Houses don’t have to breathe, but their occupants do –preferably with clean, fresh air at a comfortable humidity level. Providing this air with a leaky building envelope, as my designer friend David Foley likes to say, is akin to stabbing someone repeatedly in the chest to improve their lung function. Yes, some air will get in, but not in an efficient or effective manner.

Can’t you just open a window if you want fresh air? Yes, of course – when the weather cooperates. If you want fresh air on cold or rainy days, or hot and humid days – it’s more practical to have a mechanical system provide it.

The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) researches how much ventilation is needed and the best ways to provide it. They have found that the floor area and the number of occupants are the most important factors in determining how much to ventilate, and it tends to be roughly equivalent to changing all of the air in a house every three hours.

ASHRAE Standard 62.2 is the standard most building codes reference, including Maine’s. It is updated every three years. Maine is currently using the 2013 standard, which includes tables that list the ventilation rates for different house sizes and occupancies – for example, a 2,400 sf home with three bedrooms should be ventilated at 60 cubic feet per minute (cfm). A 3,200-sf home with five bedrooms should be ventilated at 90 cfm. A 900 sf, 1-bedroom house should be ventilated at 30 cfm. Alternatively, if you’re a building science geek like me, instead of using their tables you can use this formula: Ventilation rate cfm = ((0.03) (floor area square footage)) + ((7.5) (number of bedrooms + 1)).

ASHRAE 62.2 isn’t the only standard; the Passive Housebuilding energy standard includes ventilation rates calculated to maintain ideal indoor air quality. The result is usually not far off what building codes require for minimum rates; it’s just calculated differently. Older versions of ASHRAE 62.2 allowed lower ventilation rates, which some building scientists prefer to the newer, higher rates; they maintain that higher rates waste energy – you are sending conditioned air directly outside, after all – without significantly improving indoor air quality. Other experts say that higher rates are needed to ensure that all homes have acceptable indoor air quality.

How is this fresh air provided? There are three categories of mechanical ventilation systems: exhaust-only, supply-only and balanced. Each has pros and cons.

The Lunos E2 is a single-room energy recovery ventilation model that provides a low-profile solution for small spaces where ductwork is not feasible.

Exhaust-only ventilation

The most common approach is exhaust-only ventilation. This can be as simple as a bath fan set to run continuously on low speed. (ASHRAE and the building codes provide adjustment factors if you would rather have intermittent ventilation.) The advantages of this type of system are that you need some sort of bathroom ventilation anyway, so there is essentially no up-front cost to whole-house ventilation. One downside is that while the bathroom may be very well ventilated, we spend most of our time in other rooms – especially bedrooms, where air quality can get low while occupants exchange oxygen for carbon dioxide during sleeping hours. Another downside is that exhausted air must be replaced with outside air, but you don’t know where that air is coming in.

In tight homes, exhausting the air without providing makeup air can depressurize the interior, leading to problems such as back drafting of wood stoves and other combustion appliances, increased radon and carbon monoxide levels, and stormwater can be drawn into the building. If anything, it’s better to slightly pressurize a building, but maintaining equilibrium with the outdoor air pressure is the safest approach.

Another form of exhaust-only ventilation is an attic fan. Many people swear by them, but building scientists usually swear at them – they depressurize all but the leakiest homes, while drawing potentially damp air in through random gaps and cracks, where it can lead to mold and fungal growth.

One solution to the problem of depressurizing the interior is to add passive makeup air – essentially a hole in your wall with a damper to block most air movement except when pressure balancing is needed. The obvious downside is that it’s still an uninsulated hole in your wall.

Supply-only ventilation

Supply-only ventilation is less common, but the concept is simple: force outdoor air into your house using a fan system. The downsides are clear: forcing cold air directly into your home is not very comfortable, and it takes a lot of energy to heat or cool the supplied air.

Balanced ventilation

As you might expect, in the high-performance building world we generally prefer balanced ventilation – the outgoing air is replaced with an equivalent amount of fresh, incoming air from the outside. A heat recovery ventilator (HRV), allows most of the heat in the outgoing air to pre-warm the incoming air during cooler months (in summer, outgoing air cooled by an air conditioner cools the warm incoming air). The incoming air also passes through a filter, available up to HEPA level filtration, for indoor air that is cleaner than outside air.

A variation on this system is an Energy Recovery Ventilator (ERV). ERVs allow both heat and moisture to transfer between air streams; this can be a good thing with dry winter air or dehumidified summer air, but in airtight homes they can lead to excessive moisture build-up in winter months. HRVs and ERVs are sometimes called air to air heat exchangers.

Ducted or point-source

Ventilation of any type can be provided either by a ducted system or a local ventilation (single room) system. Ducted systems are usually preferable, because they allow more flexibility in where the extract, supply and fan motor are located, which helps to ensure every room has fresh air and sounds are minimized. Local ventilation systems can save money, and they are even available with heat recovery. (Lunos makes the best-known and highest-efficiency local systems readily available.)

Regardless of the type of system, ductwork should be designed by someone experienced in ducted system design. Many systems suffer from ductwork that was not designed or installed properly, starving the system for air.

If your heating system is provided via ductwork, it is possible to add either supply air or balance ventilation to the system. But in most cases you will be better off with a dedicated ventilation system because it allows ventilation when your furnace is not running: you can dial in the ventilation rate and have more control over which rooms get their air extracted – typically smelly or damp areas – and which rooms get supplied with fresh air, typically sleeping and living areas.

Bathroom exhaust

A common question is whether a balanced ventilation system (HRV or ERV) can be used for ventilating bathrooms and kitchens. The answer in both cases is a qualified “yes.” Ventilation systems running at normal speeds are usually not enough to clear steam and odors from bathrooms in a timely manner. (While you may care more about bathroom odors, it’s the steam that building scientists are concerned about.) HRV/ ERV should be designed to run on low speed most of the time, with a “boost” mode available to help clear bathroom air. But that boosts the ventilation throughout the house, not just where it’s needed, and it can still lead to moisture build-up, which can then lead to mold growth.

In most homes, a more forgiving approach is to use a balanced system for whole-house ventilation, and to also install dedicated bath fans for quickly and efficiently clearing the bathrooms. Note: In the tightest homes, this approach could lead to depressurized interiors—those homes should either be on a well-designed, well-installed, properly-operated balanced system, or include a makeup air system. But in most homes the short run times won’t lead to problems. In the tightest homes, the risk of depressurized interiors requires careful consideration during the planning phase, and some equipment options will be limited.

Kitchen exhaust

Exhausting kitchen cooktops and ranges to the outdoors is required by the building code, as a safety measure, when the fuel source is natural gas or propane. But cooking releases a lot of steam and particulates that are best sent outdoors as quickly as possible, regardless of the fuel source. Entry-level range hoods typically run at 125/250 cfm (low/high), but they are available in higher power – the most powerful one I’ve spec’d was 1,200 cfm, for a restaurant-style gas cooktop. If I had that in my 1,400 sf house, it would replace all the air in the house in less than ten minutes. For range hoods 400 cfm and higher, Maine’s building code requires a source of makeup air. The best makeup air systems use a pressure-activated switch to open a damper when the hood is running, and they bring the makeup air into the cabinet toe kicks adjacent to the range.

For situations where range hoods can’t vent to the exterior (and the fuel source is electricity), or when the highest levels of energy efficiency are desired, you can install a recirculating range hood, which helps filter cooking particulates. Recirculating hoods do nothing for fresh air, though. A balanced ventilation system can’t be used in place of a range hood, but it can augment a recirculating hood – just keep the extract register at least ten feet from the stove to minimize grease accumulation.

Dryers exhaust about 100 to 200 cfm when they’re running, which is enough to depressurize tight homes. You can open a window a few inches when running the dryer or install a makeup air system. Or use a ventless dryer, which uses technology to turn the moisture in clothes into liquid condensate that is sent down a drain, instead of sending heated air outdoors.

My experiences lead me to think that ventilation is the least understood and most poorly implemented system on most construction projects. Yet it is one of the most important details to get right for occupant health and comfort. It can be hard to find a qualified system designer, so check out the suggestions from Christy Crocker, director of the Maine Indoor Air Quality Council.

How to Find an Energy Recovery Ventilator Installation Contractor