Don’t Forget the V in HVAC

November 30, 2006

V stands for ventilation and if you don’t control it you are in big trouble.

“Ventilation is the one thing that if you truly understand it, you can save yourself from that dreaded callback in ten years.” Al CobbThis goes for the owner of a SIPs house as well as the installer. The Department of Energy’s National Laboratories did a study and found that SIPs walls are 15 times tighter than stick-framed walls.  This is great news for the performance of SIP walls but if the HVAC contractor hired for the job doesn’t fully understand just how well a SIPs house can perform, how tightly it controls airflow, big problems can develop.

how wind hits a house

This is a recurrent theme. If we don’t understand the emerging technologies used in our own homes and how it relates to bulk water management, the homes we live in will fail and bite us in the butt as well as the pocketbook. Cobb says that the common practice of designing our HVAC systems for the worst-case scenario is stupid. What’s worse is that HVAC systems that are designed for worst-case stick-frames homes are routinely applied to the best case performance of SIPs.In gross terms this means a 10-ton ventilation system will be applied to a house that demands only 3 tons. In real terms this means you might have a machine in your mechanical room that resembles a semi truck when all you need is a Volkswagen.

big truckhttp://www.djdoboy.com/misc/pics/vw_bug.jpg

Bear with me as I take the deep dive.


When an oversize system tries to cool a small space (or a large space that is well sealed) it cycles on for a only short time before it reads that the air is at the target temperature and shuts off. But what it doesn’t do, what it doesn’t have time to do, is condition the air. In rough terms, conditioning the air means bringing it to the proper humidity. This means that while the air is cooled to the proper temperature, the humidity continues to rise out of control.

If the system is properly sized this doesn’t happen. Instead of cycling on and off, a right-size system works at a lower capacity for longer intervals. This gives the system time to remove the moisture from the air. So instead of creating a room that is wet and clammy at 67 degrees it creates a room that is drier and more comfortable at 72 degrees.

This allows for a whole host of benefits of air quality.

ASHRAE chart

Enough of science. Here is a flipbook of the fun we had today.

slabcornerswallsmore wallsgablehow many guys?almostridgewhat time is it?

Beer 30



  1. I wish that i could use material handling equipment like the picture can the panels be smaller with the same r – factor and is it better to use more people by the look of this install . and what would it take in time to install this size of home

  2. David – this project could be manufactured with 4′ wide panels that could be manuevered on the right site by a crew of four without the aid of heavy equipment. The students we are training are professionals and the motto of the week is work smarter – not harder. With the number of students we have, we could easily set the wall panels by hand, but when on an actual job site they will not have the luxury of a dozen workers with strong backs. The r-value does not change with the size of the panels, however there are at least twice as many joints that have to be sealed properly. Additionally, by week’s end all certified installers are also OSHA certified to operate that equipment – so they need exopsure to the equipment to show proficiency in it’s operation.

    With a properly trained (certified) four man crew on a decent site, that project could easily be assembled and sealed up and ready to turn over to the other trades within three days. What you see in the pictures above took a crew of nine with many stops to explain proper techniques and alternative details about 4 hours to set the wall panels and ridge beam. The roof panels went on the next day in about the same time. Keep in mind that since this building will be disassembled and re-built on a Habitat for Humanity site, there are no nails or glue or any other type of sealing.

  3. Good article. People tend to thinkbigger is better, not knowing the result

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