Passivhaus Envelope and HVAC


If you talk to any Passivhaus designer, invariably, he/she will tell you that there are two crucial components that makes a house a Passivhaus.

1. Envelope

In order to drastically reduce active heating and cooling of a house, the envelope is the first thing that needs to be optimized. A houses envelope includes walls, roof, foundation walls, slab, windows and doors. Generally speaking, passive house walls will have much higher insulative value than a code home. According to IECC, a new home in Arlington, Virginia needs to have R-16 walls, R-38 roof and the window U value of 0.49 or less. Our home is designed to have an R-42 wall, R- 50 roof and window U value of 0.11.  This tells you that a passivhaus is insulated about twice as much as a conventional home.  The numbers will change depending on the climate, in some very temperate regions, passivhaus performance does not require a significant departure from code requirements.

Another aspect of Passivhaus is the air-tightness of the home. Passivhaus requires an air-infiltration rate of 0.6 ACH at 50 Pascals or less.  This makes the house extremely air-tight. This requirement is both a quantitative and a qualitative one. Such air-tightness means no drafts and minimal radon infiltration.  This level of air-tightness is also an important indicator of building quality (it’s difficult to build that air-tight without paying extreme attention to every detail).

What do we achieve by building such a robust envelope? We minimized heat loss. By reducing heat loss to next to nothing, the temperature inside of the home responds very slowly to exterior conditions.  This means on a hot day, the interior heats up very slowly and on a cold day, interior temperature drops very slowly.  This also means drastically reduced need to actively heat or cool the home.

Here are some pictures of what our envelope will look like.

thermal envelope

2. HVAC

The typical HVAC system is designed to deal with heating and cooling needs first.  This is necessary in a conventional home largely because it does not have a robust enough thermal envelope. When approaching a home, the HVAC designer frequently assumes the house is not well insulated and not built tight. Without further data to properly engineer the HVAC system,  the engineer “over-engineer” the heating and cooling loads to avoid insufficient heating or cooling, this leads to unnecessary over-sizing of equipment.  This sounds too much like guessing to us.

Additionally, in conventional HVAC design, ventilation is even less precise.  The reason is the builder “intentionally” built the house not air-tight to allow the house to breath.  This “breathing” is done through the various cracks, gaps, holes, etc. in the house’s envelope, as I wrote in a previous post, breathing though your skin.  Others in the Passivhaus movement have called this”breathing through your underwear.”

The Passivhause HVAC design approach is completely different.  First, we assume the envelope is air-tight.  Then we figure out the ventilation strategy. An adequate ventilation strategy is very important in providing the highest indoor air quality especially in homes so tight. A dedicated Energy Recovery Ventilator (ERV) is typically utilized to continuously remove stale air and bring in fresh air.  Additionally, as the two streams of air are exchanged, temperatures are transferred from the outgoing to the incoming, eliminating the need to completely reheat or recool the replacement air.

Even with the robust thermal envelope and ERV, on the hottest and the coldest days, a little active heating and cooling are still necessary in many regions of the U.S.  Designers have come up with many different solutions to satisfy these needs.  In our design, a point source 1.5 ton reversible Mitsubishi MiniSplit (15000 Btu/h) is used to meet the heating and cooling needs.  Our HVAC guy (Mike Bonsby of Michael Bonsby HVAC) told us that in a conventional home of the same size, a system that delivers 150,000 Btu/h would be necessary to heat the house.  Our modeling indicates to us that when built, the active heating load will only be 10% of the equivalent conventional home.

Mike and us also devised a simple strategy to eliminate the potential for temperature stratification when bedroom doors are close.  We will install transfer fans to blow air between different rooms and floors.  Link to transfer fans:http://www.tjernlund.com/Tjernlund_Aireshare_Lit_8500730.pdf

Here’s what our HVAC design look like

A-241

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2 responses to this post.

  1. It’s great to see some really knows about heating and air condition. HVAC system if not done correctly can cost a home owner or business a lot of money. Great posting.

    Reply

  2. Hi Adolph,
    Thanks for your kind words. We are lucky to work with people who know what they are doing and willing to keep an open mind about HVAC.

    Reply

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