Ready your envelope – Part 4

Ready your envelope – Part 1 is an overview of the role of a tight building envelope.

• In Part 2 and Part 3 we looked at the movement of heat and water.

Finally, let’s consider pressure differences.

Why it matters: With pressure differences, air and everything it carries (heat, moisture, contaminants) is moving from higher pressure to the lower pressure.

• Tighter buildings resist the movement because air leakage requires a pressure difference AND a pathway (like an actual hole or an unsealed penetration).

 Repeat after me for review

 Hot moves to cold

 Wet moves to dry

 High pressure moves to low pressure

 Zoom in: 3 factors can create pressure differences —> wind, stack effect and mechanical systems (like your HVAC or large vent hood)

 Dig deeper with this great explainer by Dr. Allison Bailes.

Here’s one example you might relate to:

If you have a bedroom door that tends to close when your AC kicks on, the AC is creating negative pressure in that room.

• The attic above that bedroom is now at a higher pressure —> and the air, moisture and pollutants in the attic will try to move into the bedroom through any leaks.

 For your Smart Homeowner list

• Start observing your home with this idea in mind.
• Do you feel cold drafts in the winter?
• If you have a two-story house, is your second story always hotter in the summer?

Best,

Travis