Quote:
Originally Posted by bilbao58
Oh please. I was born in Houston in 1958. Spent some of my childhood, most of my teenage years, and most of my adult life there. Houston is still almost totally dependent on cars and, if electricity ever becomes too expensive for the average person to afford air conditioning, the city will empty out faster than you can say “I’m gonna die if we don’t get a Blue Norther soon!”
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That would be true if everything were built to earlier energy code standards. But modern energy codes are slowly but surely trending towards
Passive House standards or similar low-energy high-occupant-comfort building standards.
Though I've never been there, Houston is notorious for having extraordinarily hot and swampy summers due to its proximity on the eastern coast next to the ocean and near the path of many tropical storms. For me, the combo of high humidity and high temperatures is the worst.
So how would Passive House standards help Houston? Passive House standards focus on decreasing a building's energy usage in regards to HVAC while also focusing on occupant comfort.
Decreasing HVAC usage is obvious and important. Overall, a Passive House building should use anywhere from 50-90% less HVAC energy than a normal code-standard building. That translates to significant energy savings, especially if you add solar panels or other on-site clean energy generators.
Occupant comfort is based on the science of what humans perceive as comfortable. All humans are different, with some preferring cooler temps while others prefer warmer. However, there is a general range of temperature, air humidity, and air velocity that humans will deem as comfortable. To meet Passive House standard criteria, the building should be designed so the indoor conditions fall within these general ranges on comfort.
Luckily, both HVAC energy savings and occupant comfort go hand-in-hand. The general gist of the design is as follows:
1. Have enough wall, roof, and floor slab insulation. People generally understand wall and roof insulation, but find floor slab insulation to be a foreign concept. Floor slab and foundation wall insulation is very important in cold climates, where the winter soils and suck out a lot of heat from a building.
2. Air and vapor seal the entire envelope of the building. This is especially important in a humid climate like Houston's. Humid air, like all fluids, will always try to normalize. If your building's indoor air is drier than the outdoor air, the humid outdoor air will seek every crack and cranny to try to enter you building. Passive House standard requires air-sealing a building until it meets 0.3 air changes per hour (an air change is equal to the total volume of indoor air).
3. Install high quality windows, doors, roof hatches, or any other openings. This is important for two reasons.
- One, windows generally have the worst insulation performance of all parts of an exterior wall. Studies have shown that without a decently-insulated window (or in industry parlance, a less thermally-conductive window), increasing the wall insulation even to crazy high R-values will have diminishing returns if bad windows are used.
- Generally speaking, double hung and sliding windows are considered too leaky (see point 2), so only fixed, casement (including hopper and awning), and tilt-and-turn type windows can meet Passive House standards. Windows and doors play a very important part in air-sealing a building.
4. Because the Passive House-standard building is so airtight, occupants will experience
"sick building syndrome" without proper ventilation. To combat this, Passive House buildings use
Heat Recovery Ventilators or Enthalpy Recovery Ventilators.
- These ventilators have been gradually improving over the years to increase efficiency. They are designed to constantly run at low energy and low fan speeds 24/7, and bring in fresh filtered outdoor air while exhausting out stale indoor air.
- To reduce the energy demand, the system is designed to exchange heat depending on the season. During a cold winter day, the stale warm indoor air being exhausted out transfer some of its heat to the cold fresh outdoor air coming in. During the a hot summer day, the hot fresh outdoor air transfers some of its heat to the cold stale indoor air being exhausted out.
- Enthalpy Recovery Ventilators (ERV) also allow humidity to be transferred between the supply and exhaust air. During the wintertime, it helps retain the indoor humidity by transferring to the dry outdoor air coming in. During the summertime, it helps remove the moisture of the fresh outdoor air by transferring it to the stale exhaust air.
We now know that Passive House buildings reduce ongoing operation costs (HVAC), and they also increase occupant comfort. What other benefits do they have?
1. They require less complex and less expensive HVAC systems. This helps save on up-front mechanical equipment and design costs.
2. They can extend the lifespan of a building. There is often invisible mold and mildew building up inside wall cavities. This can be either due to a water leak, or moist air condensing on the colder building components and not drying out. When this is discovered, the affected building components must be disposed of and replaced with new building components. With Passive House designs, this problem is essentially eliminated.
3. They can drastically improve the indoor air quality. Normal buildings are either ventilated through the HVAC system or through the outdoor air seeping its way through every crack and cranny in the building envelope. In Passive House design, the air is ventilated almost exclusively through the HRV or ERV system, which includes a filter to pick up any pollutants and dust from the outdoor air.
One other note:
1. Whereas traditional HVAC systems are focused on sensible cooling (dry air temperature), latent cooling (moisture and humidity management) is more important in Passive House buildings. This is because the good insulation and fenestration mostly takes care of the sensible temperature part. On the other hand, even with good air-sealing and a good ERV, the humid Houston air will tend to find a way into the indoor space (just at a much lower rate than most buildings).
- The way most A/Cs work is that they remove moisture by blowing the warm moist air over the cooling coils. The cooling coils condense the moisture to be drained away, and the cooled air is drier. The problem is that the air is only dried if the cooling coils are operating and remain cold. This is especially true for "oversized" cooling systems, which cool a space too quickly to dry out the air.
- To overcome this flaw, A/Cs are being developed to send the air through a condenser that turns on if the cooling coils aren't on in order to continue to dry the air.
So if you are living or working in a building properly built to Passive House standards, unless you throw your windows and doors wide open during the swampy summer, the only discomfort you should feel is when you are outside in the hot muggy air. You should feel little discomfort inside. And you should feel little pain when it comes to seeing your monthly electricity bill (at least in terms of HVAC costs - your other electrical costs will depend on your usage).
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