House Ventilation

Air, Water, Food, Shelter are basic requirements for human life.  They sit at the bottom of Maslow’s hierarchy of needs (for an explanation with a nice kick at Freud, whom the realist abhors [but I digress] look here).  The quality of air inside the house is probably one level up from basic need, but needs to rank well before self actualization.  The realists argument is that you can’t self actualize if you don’t have proper ventilation!

Like everything else to do with energy efficiency the arguments about ventilation get complicated.  To steer through the complications there is a wealth of detail available on the net and in books (e.g. Paul H. Raymer, “Residential Ventilation Handbook, Ventilation to improve indoor air quality”, McGraw Hill, 2010)

What follows is a simple summary highlighting some key concerns and ideas.

Why is Ventilation Important?

  • Bring fresh air in, get stale, “used” air out
  • Get indoor pollutants out:
    • Carbon Monoxide
    • Radon
    • Outgassing from fabrics, plastics, paints
    • Cleaners
    • Combustion gases
    • Cigarette smoke
    • Human, pet and insect debris (odor, skin flakes, hair)
    • Outdoor pollutants sucked into the house – car exhausts, pollen, smoke etc.
  • Control moisture – “Without ventilation a green house is a moldy house”
  • Become comfortable – live in our narrow band of temperature, humidity and air-flow

All of which is captured by the term Indoor Air Quality (IAQ)

Natural Ventilation

In the US most houses rely in “natural ventilation” to provide the needed air exchange between inside and out.  Obviously it’s OK – we’ve survived fine so far.  The problem is that natural ventilation is not controlled and the venting takes place through paths that you may not want, like pulling fresh air through the garage.

The amount of ventilation is also uncontrolled.  In the US the largest percentage of existing houses is very leaky – i.e. unintended air flow between interior and exterior approaches one air change per hour (1 ACH).  So the entire volume of air is changed every hour – all day and all night, maybe more when the wind blows hard.  All the heating and cooling and moisture conditioning is lost every hour.

The “standard” is set at 0.35 ACH, and where there is airflow below 0.245 ACH mechanical ventilation is required by code.

Even very new houses rarely get to standard.  With the best will in the world it’s very hard to avoid small gaps when site assembling houses from sticks and panels.  Even if the architect and building contractor are full of good will, it is hard to think air tightness when you are a subcontractor working up on a ladder, with a storm coming, on a fixed price and a deadline.

Now this leads to some muddled thinking.  Somehow a house that is “too tight” becomes a bad thing.  And yes, it does, if there is no added ventilation.  Traditional builders will swear that the house must “breathe” – because they have seen too many rotted and moldy attics and walls.  But the rot and mold are caused by badly planned ventilation and poor understanding of moisture control.


I find moisture still difficult to explain after years of trying to understand it.  Water as water is obvious.  But water as water vapor is invisible.

Warm air can hold more of it than cold air.  As air rich in water vapor touches a cold surface the water vapor condenses out as liquid water.  If the condensation is on a visible surface it’s mostly a nuisance, like a steamed up window.  But if the condensation is on an invisible surface, like inside a wall, or in the attic, or inside ducting then we have a problem.  The condensed water, together with a food substrate, like paper or wood, will encourage the growth of fungi such as mold and rot.

Fear of mold and rot makes builders insist that the house must breathe.  And it is the same fear that drives air change standards and most ventilation design.

Moisture, i.e. humidity, levels are also a major comfort factor.  Too dry and we get itchy eyes and throats, too wet and it feels clammy.  We have a very limited band for comfort.

In the Northeast US the climate makes things more difficult.  We have, without doubt, a “heating climate”, with cold, dry air outside all winter.  But in summer we have a real “cooling climate” with a hot, humid environment.  Why do we even live here? [I digress again]

Exhaust Ventilation

Some areas of the house need to have special ventilation to quickly get pollutants and moisture out of the house.

  • Kitchen cooking areas
  • Bathrooms, toilets
  • Clothes dryers
  • Chimneys for wood and gas fireplaces
  • Radon removal system
  • Attic ventilation
  • Garage ventilation

In each case there is a fan that runs when needed and is connected directly to the outside.  In many cases the vent exhausts, either intentionally or by accident, to some place inside the house, often the attic, with bad results.  The remedy is obvious, but often difficult to implement.  It may mean opening up walls and running new ducts.

We have to remember when running these exhausts they draw air out of the house and that air has to be made up from somewhere else.  If there is no special make-up supply the air will enter by all the “natural” (i.e. leaky) pathways.  If you are a gourmet chef with an industrial strength range hood over your industrial strength gas range and oven then the volume of makeup air can be very large, and needs some professional advice.  Most HVAC (Heating, Ventilation and Cooling) specialists are very good at H and C, not so much on V – so check it out carefully.

Integration of Ventilation with HVAC

In a house with heat, and probably cooling, delivered through air ducts the obvious way to provide ventilation is to add a controlled venting inlet and outlet to the main system.

It is not the best way to do things.  First, the ducts are probably leaky creating all sorts of problems with house pressure in relation to the outside and possible introduction of pollutants – see some more detail and diagrams here.

Then there are all sorts of problems of integration.

The system does not run all the time, there are long periods in spring and fall (autumn) when there is no/limited need for heating or cooling.

Air is not an efficient way to transport heat, so the ducts and fans are much larger and noisier than those needed for ventilation.

Make-up air can hit cooling coils at the wrong temperature and condensation can shorten the life of these coils.  Many HVAC suppliers and installers do not like integrating ventilation with the HAC system.

In summary, integration of venting with existing HVAC ducts is not ideal.  However it may be the only affordable way to add controlled ventilation to an existing house.

HRV’s, ERV’s

Controlled ventilation has one major drawback; in winter heated air is vented out and cold air drawn in, so wasting energy.  In summer, cool, dehumidified air is exhausted and replaced by warm, moisture laden air.  So the heating and cooling systems need to work harder, use more energy, waste more money.

Special ventilators are available to overcome that problem.  Heat Recovery Ventilators (HRV) include an air exchanger to use the warm exhaust air to preheat the incoming air.  Efficiency of these devices is up to 80% saving a considerable amount of energy.  ERV’s are similar in approach but use a moisture absorbing medium to capture the incoming moisture and to cool the air coming in.  Filters are built into the devices and they provide a very pleasant interior environment.

H/ERV’s are complicated devices compared to a normal fan, and the cost is significantly higher.

HRV’s (and ERV’s) can be used centrally or come in small room units.   The pictures below show a high end unit manufactured by Meltem in Germany.  (These units are not available in the US at present.  Let us know if you would like them to be introduced)

Wall mount HRV - Meltem

Inside a Meltem HRV showing heat exchanger and filters

Here is a listing of ventilation devices from the Home Ventilating Institute (Section 3 lists HRV and ERV units.  It is not a complete list of available products, but has those tested by HVI)


Combustion safety testing needs to be carried out before, during and after air sealing and any other “tightening” activity on the house envelope.  The same applies when upgrading exhausts.

HRV’s and ERV’s are typically low volume devices and not designed to make up bulk air for volume exhausts, like those for a high end range hood.


Costs are in 2 categories; installation and operation.

Installation will vary greatly between new construction and retrofit, and then between retrofit in a house with existing ducts vs. a new duct system.

Operating costs are low.  Maybe a dollar a day for whole house venting, plus supplies like filters.

Installation will start at $2000 and go up from there.  A single room unit like the Meltem above, will start at $1000, but can be “hung” on the wall minimizing installation costs.

Costs of course need to be balanced against energy savings, comfort and health.  Rule of thumb: energy savings will not (usually) pay for the ventilation system.

The Ideal

Here’s my ideal solution, remembering that I live in a stick house built in the 1850’s.

  • Air seal, air seal, air seal
  • Direct vent, high efficiency furnace – feeding into under-floor hydronic or baseboards (we have baseboard now, I don’t like ducted air distribution at all)
  • Direct vent hot water system (in a new house I would go with instantaneous systems located close to use points like the bathrooms and kitchen)
  • Bath and kitchen exhausts (I’m not a gourmet, so have no gas flames inside the house at all.  Makes sizing of the kitchen exhaust relatively simple)
  • Mini-split air conditioning
  • HRV ventilation.  Because the house is so old retrofitting a central ducted system is not economic.  I would go for individual units in the main living and sleeping areas.

In our search for an ideal can we learn anything from Australia and Germany?

Australia – not so much.  Because of the more benign climate, compared to the US Northeast, Australian houses can be more leaky even than the US.

German houses tend to be closer to my ideal:

  • Tight, either during construction or by retrofit,
  • High efficiency heat
  • Air distribution of heat is rare because very few German houses have central air conditioning
  • HRV’s are becoming common.  In new construction this is central via small diameter ducts.  In older houses retrofitting of a ducted system is often impractical because of the predominant masonry construction, so decentralized units are popular for existing houses (and small commercial sites).

To sum it all up:  IAQ = “Gemütlichkeit” = Comfort

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