Infiltration/Ventilation
Infiltration, to some degree, is both a bad thing and a good thing: bad because it let in outside air that the HVAC system has to heat up (or cool down), and good because it also brings in needed fresh air. Early attempts at building tight houses occasionally resulted in severe mold problems, and sometimes damage to the building, but newer construction techniques that take into account how water vapor moves in buildings has virtually eliminated the problem1. Sick building syndrome, also related to tight buildings, is due to incorporating materials that off-gas toxins and/or using toxic products in the building: leaky building provided a cover-up for these products.
The motto of green HVAC design is "build tight, ventilate right". The theory behind this is that it takes less energy to ventilate a house than it does to supply the additional heating/cooling energy to compensate for air leakage. This is true because for two reasons: ventilation air can be supplied for relatively low energy, and air leakage reaches its maximum during the harshest weather (for some background, see the infiltration section under heat loss calculations)
Essentially the problem with leaky buildings is that on the coldest, windiest day infiltration will be high, while on very mild days it will be very low, yet the fresh air requirements are the same throughout the year. During the cooling season, heat gain due to infiltration is not as high as during heating because the temperature difference is generally not as high, because there is no stack effect air leakage, and because it is generally not as windy. In general, tight buildings are probably not as important in mild climates as they are in cold climates (ie 4000HDD and up).
Ventilation requirements: there are varying ventilation recommendations, the issue being how much air is necessary to providing dilution air for pollutants, odors and water vapor (see health section for further discussion), not due to keeping the oxygen/CO2 level correct. For commercial buildings, 15CFM/occupant is considered adequate, while a typical residential building recommendation is .35ACH. Unfortunately a small house at .35ACH yields only 40-50CFM, while a large house would be 150-200CFM, so it is better to stick to using 15-30CFM/person (depending on lifestyle introduced pollutants).
How tight? To some degree this is dependent on what climate you in, and in particular how much cold wind there is. If measured in ACH, it also depends on how big the building is, or rather how many square feet of space in the building per occupant. Air leakage is not a reliable source of fresh air, although when the leakage amount goes down due to mild weather, windows can be opened to provide additional fresh air. Historically, houses have been so leaky that there is still enough fresh air infiltrating even when the pressure difference between inside and out is nearly zero.
From a practical standpoint, tightening is easy up to a point, and then gets progressively more difficult unless the construction technique is inherently tight (eg SIP, spray foam). It is fairly easy to achieve 3ACH50, and not terribly hard to get to 2ACH50, but beyond there will require some work (at least until there is a standard practice on how to do so). The passive house standard of .6ACH50 will both challenge builders, and result in standard practices that allow create a clear path to tight buildings (assuming the level of interest in passive houses stay high).
Ventilation Energy .vs. Reheat Energy
The following chart compares a 1500SF (16,000ft3) house built to three different tightness levels: a loose house at 15ACH50 (which is on the good side for homes that weren't sealed during construction), a moderately tight house at 2.5ACH50, and a very tight house at .6ACH50. Since infiltration rates measured in CFM vary with house size, larger or smaller homes will result in different comparisons. Because we want to compare these homes during different weather scenarios, the standard LBL model for converting the measured infiltration rate (for background, read this from Home Energy) into a "natural" rate (ie under actual weather conditions) does not apply to all of them, so the charts are constructed using a range of conversions: 1/20 for moderate weather (40F), 1/15 for cold weather (20F), and 1/10 for very cold weather (0F). For mechanical ventilation, the assumption is that an 80CFM fan uses 25W, and a 100CFM HRV uses 100W and is 80% efficient. A required ventilation rate of 50CFM is assumed (approximately the recommended amount for 2-4 occupants). All of these are all ballpark numbers and some have been rounded to simplify calculations.
For each situation, if the natural ventilation rate is lower than the assumed 50CFM requirement, two options are considered: using a simple exhaust fan to increase the rate to 50CFM, and using an HRV instead.
| House type | Vent rate (cfm) | Infil. loss (btu/hr) | Fan energy (btu/hr) | Exhaust heat loss (btu/hr) | HRV energy (btu/hr) | HRV exhaust loss (btu/hr) | Total loss - Fan | Total Loss - HRV |
|---|---|---|---|---|---|---|---|---|
| Very Cold (0F, ACHnat=1/10) | ||||||||
| Very tight | 12 | 907 | 40 | 2873 | 130 | 575 | 3820 | 1611 |
| Tight | 50 | 3780 | 0 | 0 | 0 | 0 | 3780 | 3780 |
| Leaky | 300 | 22680 | 0 | 0 | 0 | 0 | 22680 | 22680 |
| Cold (20F, ACHnat=1/15) | ||||||||
| Very tight | 8 | 432 | 45 | 2268 | 143 | 453 | 2745 | 1029 |
| Tight | 33 | 1800 | 18 | 1260 | 57 | 252 | 3078 | 2109 |
| Leaky | 200 | 10800 | 0 | 0 | 0 | 0 | 10800 | 10800 |
| Moderate (40F, ACHnat=1/20) | ||||||||
| Very tight | 6 | 194 | 47 | 1426 | 150 | 385 | 1667 | 630 |
| Tight | 25 | 810 | 27 | 1890 | 85 | 378 | 2727 | 1273 |
| Leaky | 150 | 4860 | 0 | 0 | 0 | 0 | 4860 | 4860 |
The first obvious conclusion from the chart is that building tight always saves energy, and since the savings is quite dramatic, the assumptions behind the values would have to be very far off for this conclusion to be wrong. The second obvious conclusion is that fan energy isn't that significant, even in the very tight house during moderate weather. It also seems obvious that using an HRV makes sense, even during mild weather, although incorrect assumptions may be making the gap seem bigger than it is.
Exhaust Fans & Depressurization
In a tight house, the use of exhaust fans (especially downdraft cooktops) can result in depressurization of the house, which can make combustion devices backdraft (vent to indoors instead of outside). Typical shower fan use (90CFM for 30 minutes), in a moderately tight house probably won't cause much depressurization, but in a very tight house (especially a small one), or with a more powerful fan (like a 300CFM range hood), depressurization is likely. (Solutions to be added in the future).
Air Sealing Techniques
Air leaks mostly occur at the joints between materials in the envelope: around windows and doors, around holes drilled for pipes, wires and ducts, and in joints between sheets of plywood, in the rim joists, etc. Dense insulation like spray foam, or to a lesser degree dense pack cellulose, reduce some joint infiltration, and air barriers like Tyvek or asphalt building paper reduce the pressure on joints due to wind. All joints need to be sealed with weather stripping, caulk, spray foam or any equivalent product. Most of them can be found by a careful visual inspection after framing and before insulation is installed. Using a blower door to reduce house pressure, air tightening specialists use smoke sticks to find leaks.
Some building techniques result in inherently more air tight buildings than others. An analysis is beyond the scope of this document.
For more on blower door tests, see
http://www.energysavers.gov/your_home/energy_audits/index.cfm/mytopic=11190
OR this article from Home Energy
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/940110.html
Notes
1: The issue was the in very leaky homes, the high leakage rate resulted in very low indoor humidity, so the leakage didn't cause mold. When the leakage was reduced a greatly, the indoor humidity wouldn't be so terribly dry, but yet there would still be a leak or two (thru can lighting, or electrical outlet holes etc), and as the moister air runs thru those leaky spots, it now condenses, when it used to be too dry to condense. Essentially the problem was that they were tight, it was that they weren't tight enough (or rather that they had leaks in places that could cause condensation. If the excess air were flying out a bath fan duct, it might go right out without condensing on anything. Air moving thru insulation, and then out a crack between sheets of plywood on their other hand can leave a lot of moisture on the interior surface of the plywood.)