Build tight, ventilate right

In Medellin, virtually all houses are passive houses, though not precisely built according to the German Passivhaus standard. In a wonderful temperate climate where the temperature hovers between 15 degrees and 28 degrees (centigrade), designing a house for passive heating and cooling is a lot easier than in Central Europe. Here are some characteristics of residential buildings in and around Medellin:

  • Passive solar design: Medellin is located 6 degrees above the equator with little seasonal temperature variation, thus designing for passive solar gain is rarely a priority. Buildings do experience unwanted solar gain on their East and West facades and low-e window coatings are fairly common, though reducing the glazed area by design isn’t a normal practice.
  • Superinsulation: predicated by the Passivhaus proponents is non-existent in Medellin. Residential buildings are constructed with bricks or concrete with no insulation whatsoever.
  • Advanced window technology: a couple of days ago the CEO of a window manufacturer told me that I was the first person ever to ask for aluminum frames with a thermal break! The standard window found in every house in Medellin, from the poorest to the most luxurious, has single glazing and a regular aluminum frame.
  • Airtightness: modern multi-family housing projects all feature what appears to be a tight building envelope made of masonry. However, sliding windows and glass doors are the norm and leak some air.
  • Ventilation: passive natural ventilation is the rule in Medellin, mostly wind-driven (i.e. I have never visited a building locally that was designed to take advantage of buoyancy-driven ventilation). Bathrooms rarely include an exhaust fan: an operable window, with or without louvers, is the norm. The same goes for kitchens, although a hood/extractor is generally fitted. There’s no standard like ASHRAE 62.2in place to define a minimum level of ventilation.
  • Space heating: central heating is non-existent in Medellin, but so is heat recovery ventilation. As opposed to Bogota, homes never include a fireplace.
  • Lighting and electrical appliances: consumers have felt compelled to buy compact fluorescent lamps (CFL) and are often interested in LED lighting but deterred by the high purchase price. Energy Start certified products are readily available in Medellin, though their adoption rate is unclear.

There’s a stark contrast between Passivhaus recommendations and traditional building practices in Medellin. Are local builders lazy or worse, demented? Or is Passivhaus irrelevant in the mild climate of Medellin? What best practices apply?

Is mechanical ventilation needed?

The LEED for Homes 2008 standard contains a whole section devoted to Indoor Environmental Quality (IEQ). IEQ section 4 Outdoor Air Ventilation provides an exemption for homes that qualify under the ASHRAE 62.2–2007 climate exemption. For our project, the relevant exemptions are:

  • Buildings in zone 3B or 3C of the IECC 2004/2007 Climate Zone Map
  • Buildings with no mechanical cooling that are located in zone 1 or 2 of the IECC 2004/2007 Climate Zone Map

The IECC is a US-centric organization and no climate zone classification exists for Medellin, or Colombia for that matter. Fortunately, there is a method to determine the climate zone for any location on earth with a weather station, based on Heating Degree Days (HDD) and Cooling Degree Days (CDD). The relevant figures for zone 1, 2, 3B and 3C are:

  • Zone 1: 9000 < CDD 50 degree F
  • Zone 2: 6300 < CDD 50 degree F <= 9000
  • Zone 3B: 4500 < CDD 50 degree F <= 6300 and HDD 65 degree F <= 5400
  • Zone 3C: HDD 65 degree F <= 3600

Now the challenge is to find the HDD and CDD data for Medellin. Obviously, no such data exists for our project site in La Estrella, but the Jose Maria Cordova airport in Rionegro has a weather station code, SKRG and so does the Olaya Herrera airport in Medellin, SKMD. We will be using the SKRG data since our lot is located nearly 700 m. higher than Medellin, almost exactly at the same altitude as Rionegro (though minimum temperatures aren’t as extreme in La Estrella).

Next, using the excellent Web site, it is possible to calculate the HDD 65F and CDD 50F yearly averages for the last 4 years:

  • SKRG HDD 65F: 1544 (< 1% estimated)
  • SKRG CDD 50F: 4392 (< 1% estimated)

Remarkably mild San Francisco is located in the 3C zone for which the ASHRAE 62.2–2007 exemption applies, yet California Title 24 Part 6 references ASHRAE Standard 62.2–2007 as California’s state ventilation code. This is worth further investigation.

Two arguments in favor of mechanical ventilation apply equally well to San Francisco and Medellin:

  • During mild weather with little wind, little air moves naturally in and out of a home. The level of indoor pollutants can rise, and inside air can become stuffy, stagnant, moisture-laden, and unhealthy. This is especially true in the temperate and fairly humid climate of Antioquia. Buoyancy-driven ventilation in the absence of wind isn’t an effective solution in temperate weather.
  • Wind-driven ventilation likewise suffers from major drawbacks, such as the unpredictability and difficulties in harnessing due to speed and direction variations and the fact that it may create a strong draught, discomfort.

What the ever-progressive Californians have discovered is that although temperate weather may considerably aleviate the need for active heating or cooling, especially when building to the Passivhaus standard, there’s no substitute for good ventilation.

Additionally, in any context it is worth keeping in mind security requirements, and leaving windows or doors open at night in order to ventilate is a practice that impacts the safety of the house and its inhabitants.

What ventilation rate and building airtightness?

In that sense, LEED for Homes is justified to allocate two points for projects that deliver Enhanced Outdoor Air Ventilation in section IEQ 4.2. While LEED for Homes follows ASHRAE 62.2 rather religiously, it is worth checking other international ventilation standards and see whether they agree with ASHRAE 62.2.

I have extracted the following table from the excellent thesis of Judy Alice Roberson, entitled A Critique of ASHRAE Standard 62.2–2003:

We can see that most countries require an ACH rate greater than 0.35. ASHRAE 62 was in line with that requirement, but was updated to specify a continuous whole-building ventilation at a rate of 7.5 cfm per person plus 0.01 cfm per sq ft of occupiable area. For our 268 sq. m. house (2884 sq. ft.) with 4 bedrooms we would require 75 CFM of continuous ventilation:

Alternatively, using the formula, the calculation yields 0.01*2884+7.5*5 = 66 CFM. Interestingly, this calculated value is lower than what is specified in the above table.

Now let’s calculate the building volume, which equals one air change (AC): 2884*10 = 28,840 cubic feet (AC). What is the fan capacity required to move 0.35 air changes per hour? 0.35*28,840/60 = 168 CFM.

These results are a reminder that ASHRAE 62.2 assumes a fairly average building envelope airtightness (i.e. with outside air infiltration at a rate of 2 CFM/100 sq ft) and even offer infiltration credits that lower the required ventilation rate for “leaky” homes. In a simulation using CONTAM, Roberson found that even with no mechanical ventilation the average house in Houston, Texas, mostly meets the 0.35 ACH requirement:

On the left we see a normal house (NL55) with no ventilation (V0), 75 CFM (V75) and 126 CFM (V126) ventilation. In the middle, a more airtight home (NL31) under the same conditions and on the right an airtight building (NL23). Since the 126 CFM calculation was ACH-based while 75 CFM was the ASHRAE 62.2 formulaic recommendation, Roberson concludes that ventilation for modern airtight houses must be sized according to the 0.35 ACH goal.

Based on Roberson’s thesis, for our project, we will be looking to achieve 168 CFM of continuous ventilation, in a fairly airtight building with an ACH50 lower than 3.0 (for reference, Passivhaus calls for an ACH50 lower than 0.6). How are we going to build such an airtight home? This will be the subject of a later posting.

Stove and range hood

We are looking to install a beautiful Stuv 21/75 DF stove between the living and the dining room. The issue is that if a home is under enough negative air pressure, a stove can spill toxic combustion products into the room. As stated in the ventilation code of California, the safest way to avoid backdrafting problems is to replace natural-draft combustion appliances with direct-vent, sealed combustion appliances. When this is not possible, ASHRAE 62.2 provides a prescriptive requirement to avoid backdrafting: the combined exhaust of the two largest fans in the house cannot exceed the equivalent of 15 cfm per 100 sq ft of living area (0.15 cfm per sq ft).

For our home, the Faber Cylindra Isola range hood is by far the largest fan, rated at 600 CFM, though a CFM reducer kit is available to reduce ventilation to 300 CFM for makeup air environments. Doing this calculation, 2884*0.15 = 432 CFM, which corresponds to the capacity of the range hood and a typical 110 CFM kitchen or bathroom fan.

Through this exercise, we have determined that we need to add a direct-vent for the stove and reduce the CFM of the range hood, to prevent any back-drafting risk when adding logs into an existing fire.

Kitchen and bath ventilation

ASHRAE 62.2 specifies that local exhaust ventilation be provided for:

  • Kitchens: 100 CFM or more if intermittently operated or 5 ACH of kitchen volume if continuously operated.
  • Bathrooms: 50 CFM or more if operated intermittently or 20 CFM if continuously operated.

We plan on using exclusively the Faber range hood for ventilating the kitchen, which is rated at 300 CFM with the reducer installed, far exceeding the ASHRAE 62.2 requirements.

For each of the 4 bathrooms and the laundry room, we are looking to install in the false ceiling a Broan ZB110H ULTRA Series Humidity Sensing 110 CFM Multi-Speed Ventilation Fan, ENERGY STAR Qualified, which is designed to operate continuously at 30 CFM until crossing a preset Relative Humidity point, e.g. 60% RH, causing the fan speed to increase to maximum airflow, 110 CFM, with a time delay return to 30 CFM. The unit is virtually silent, rated at 0.3 sones.

The astute reader will at this point have noticed that we’re paving the way for an exhaust-only ventilation strategy, but would it meet our requirements?

Exhaust-only ventilation

Having asserted the need for mechanical ventilation, we find that there are 3 ways to keep indoor air fresh:

  1. Exhaust-only ventilation
  2. Supply-only ventilation
  3. Balanced ventilation

An exhaust-only ventilation is appropriate for our project, based on the following reasons:

  • Single-point systems offer the least expensive first-cost approach to adding whole-building ventilation
  • Using only a few state-of-the-art bath fans, the installation and operation is straightforward and requires no intervention from HVAC experts who aren’t familiar with residential housing projects.
  • Generally the least costly system to operate at 7.7 CFM per W
  • According to Green Building Advisor, research shows, that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well
  • Arguably, fresh air inlets are only effective if the house is very tight. Green Building Advisor refers to a study from 2000, A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont, which concludes that installing passive fresh air inlets in the walls is a waste of time and money. Based on Roberson’s thesis, we believe that installing passive fresh air inlets works in tight buildings.
  • Given the mild weather in Medellin, we aren’t concerned that incoming air is not tempered (e.g. by an HRV or ERV system). Ventilation won’t be perceived as draft, as it is the case in very cold climates.
  • Contrary to supply-only and balanced systems with no HRV/ERV, no duct-work is required for exhaust-only ventilation. We envision our house to be essentially ductless, except for short runs from the bathroom fans to the walls.
  • Since the house is located next to a large protected forest extending for miles, drawing outdoor pollutants into the home is of little concern to us.
  • Though depressurization may cause backdrafting of naturally vented combustion appliances, our Stuv 21/75 stove is fitted with glass walls and will be equipped with a direct-vent.
  • Finally, compared to cutting-edge decentralized HRV/ERV products (e.g. Lunos e2), an exhaust-only system doesn’t require any fan in the bedrooms and should hence be virtually silent.

Our initial design includes 5 air exhaust fans (30 CFM each, continuous) shown in red on the plan below, and 5 or 6 passive vents, depicted in blue:

Although bedrooms should be well ventilated, the living room and dining room area is a cause for concern: there is simply no wall surface to install a passive vent across from the kitchen. We may have to investigate window air vents products or adopt the thin vents suggested by our architect, located between the window frames and the roof. The drawback of those is the difficulty of filtering incoming air.

Passive inlet vents

Roberson mentions that inlet vents were developed in the extremely airtight houses of Sweden, whose building code sets an upper limit on air leakage corresponding to 3.0 ACH50 (our airtightness goal).

Unless a house is tight enough to be significantly and constantly depressurized by continuous exhaust fans, these vents impair rather than facilitate the effectiveness of exhaust ventilation. This is because when stack effect is stronger than the fan, air is as likely to exit as to enter a house through these (or any other) vents, depending on their location relative to other leakage sites. Even vents with a damper designed to ensure one-way flow (outdoors to indoors) can leak when closed in response to stack effect, which is strongest when ventilation is least needed.

Fortunately, in a mild climate like ours the stack effect is negligible and shouldn’t limit the effectiveness of passive inlet vents.

The Airlet brochure specifies that inlets should be located in bedrooms and main living areas such as living, dining, and family rooms. Within the room, they should be installed high up on exterior walls, within 6–8 inches from the ceiling.

Here is a non-exhaustive list of passive inlet vents suitable for our exhaust-only ventilation project:

Indoor air circulation

Quoting Roberson once again, even in a tight house, movement of air from each room to the exhaust fan can easily be disrupted by closed interior doors, unless the doors are deliberately and substantially vented and/or undercut.

From experimental results, it has been demonstrated that in an extremely airtight house, stable ventilation almost unaffected by an internal external temperature difference can be achieved by a nonduct exhaust-only ventilation system with a single exhaust fan continually operated, if suitable door undercuts are provided.

At a minimum, we will take care to undercut all the doors of wet rooms where air exhaust fans are located (see green rectangles):

This should provide unimpeded air circulation from rooms equipped with fans to rooms with inlet vents.

Decentralized HRV/ERV

To be honest, before adopting the exhaust-only approach, I spent hours investigating decentralized HRV/ERV products as they are called in Germany, or Single Room HRV (SRHRV) as they are named in the UK. These systems are ductless, which makes them easy to install in any house.

As shown before, an HRV or ERV is hardly needed in a temperate climate. A study by John Semmelhack highlighted that typical passive house HRV/ERV systems do not appear to be particularly cost-effective in the milder climates (e.g. for San Francisco).

Since I couldn’t find any good list of manufacturers online, I’m including it here for all home builders living in more extreme climates.

  • SEVentilation (site only in German)
  • LTM (site only in German)
  • Schuco offers the intriguing VentoTherm HRV built-into a window (site available in English).
  • bluMartin manufactures the highly regarded and expensive freeAir100 HRV (site only in German). 90% heat recovery and ultra-silent.
  • Meltem produces a whole range of KNX-compatible intelligent HRVs (site available in English)
  • Maico sells the WRG-35 decentralized HRV (site available in English)
  • InVENTer has several HRV models, including one tailored for “thin” North American walls (site available in English)
  • Helios offers a comprehensive range of fans and HRVs (site available in English)
  • Viking House an Irish company that will launch a high performance HRV in February 2014.
  • Lunos their e2 HRV boasts 90% heat recovery efficiency and 16 dB noise level at 17 m3/h. The units work in pairs, alternating exhaust and intake every 70 seconds. They are available for sale in the US. Their German site features a comprehensive download section where under “Berechnungshilfen” you’ll find a useful Excel spreadsheet for sizing your Lunos ventilation system. Their pricelist in EUR is available online. Highly recommended: if money was no object and Medellin as Zurich, this is what I would get.


Smart ventilation and airtightness are only two facets of a well-designed, comfortable, Passive House that requires no central heating or air conditioning for cooling. I haven’t addressed here the issue of air dehumidification. Together with insulation, these are topics that I would like to discuss in an upcoming post.

In the meantime, for more information on strategies and techniques used in Passive Houses in Europe, I recommend Sustainable Solar Housing by Robert Hastings and Maria Wall which I found to be nothing short of fascinating.

Originally published at on December 09, 2013

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