How to Evaluate Home Infiltration Levels

One of the trickiest inputs in a Manual J load calculation is infiltration. Get it wrong and your entire load calculation skews in a direction that could lead to oversized or undersized equipment. Get it right and you're designing systems that actually perform the way they should.

The challenge is that most HVAC contractors don't own a blower door. And without one, figuring out how leaky a house is can feel like guesswork.

Here's how to make an educated assessment instead of just picking whatever makes your load calc look the way you want it to.

The Right Way: Blower Door Testing

The gold standard for measuring infiltration is a blower door test. You pressurize or depressurize the house to 50 Pascals and measure how much air is leaking through the building envelope. That gives you CFM50, which you can convert to air changes per hour (ACH).

Once you have the actual ACH, Manual J provides a straightforward chart. Based on square footage and measured air changes per hour, you select the appropriate construction quality: tight, semi-tight, average, semi-loose, or loose.

Quick note on code requirements. If you measure below 5 air changes per hour, mechanical ventilation is required. Tight houses need fresh air brought in intentionally because they're not getting it through cracks and gaps anymore.

When You Don't Have a Blower Door

Most contractors don't have access to blower door equipment on every job. So what do you do?

Lawrence Berkeley National Laboratory conducted extensive studies on air tightness in U.S. homes. When you cross-reference their findings with Manual J tables, you get a solid starting point based on construction era and building characteristics.

Here's how it breaks down:

Loose: Homes built before 1950 that haven't been weatherized. These houses were built before anyone thought much about air sealing. Gaps around windows, doors, and penetrations let air move freely through the envelope.

Semi-Loose: Homes built between 1950 and 1994. Building practices improved somewhat during this period, but air sealing still wasn't a primary concern. Most homes from this era fall into this category unless they've been upgraded.

Average: Homes built after 1995. Building codes started requiring better construction practices around this time. A typical house from the mid-90s forward, built to code but nothing special, lands here.

Semi-Tight: Energy Star certified homes. These houses were built with air sealing as a specific goal. If a homeowner mentions their home is Energy Star rated, this is your starting point.

Tight: Net zero homes, passive houses, or homes with spray foam insulation throughout. If you hear any of these terms and it's new construction, select tight. These buildings were designed to minimize air leakage as a primary performance metric.

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What About Weatherized Homes?

Here's where it gets interesting. A house built in 1880 would normally be loose by default. But if it went through a weatherization program, the story changes.

Weatherization crews often perform blower door tests as part of their work, especially if they're doing combustion appliance zone testing. Many times they'll leave the results posted near the electrical panel. Ask the homeowner if they have weatherization records, or take a look yourself.

If the home was weatherized but you can't find test results, bump it up one category from where it would normally fall. That 1880 Victorian that would be loose without any work done might be semi-loose after weatherization.

If you can find the actual blower door results from the weatherization work, use those numbers directly. Real data beats educated estimates every time.

Visual Inspection Tips

Beyond construction era, look for physical evidence of air leakage during your walkthrough:

Signs of a leaky home:

• Visible gaps around window and door frames
• Daylight showing through exterior penetrations
• Dust accumulation patterns near outlets and switches on exterior walls
• Drafty feel, especially on windy days
• Homeowner complaints about dust despite regular cleaning

Signs of a tight home:

• Sealed penetrations with caulk or foam visible
• Weather stripping in good condition
• No drafts felt near windows and doors
• Modern windows with intact seals
• Spray foam visible in attic or crawlspace

Why This Matters for Your Load Calculations

Infiltration accounts for a significant portion of heating and cooling loads, especially in older homes. Selecting "average" for every house because it's the middle option will give you the wrong answer more often than not.

An older home that you mark as average when it's actually loose will end up with undersized equipment. A new construction home marked as average when it should be tight will end up oversized.

Neither outcome is good for the homeowner or your reputation.

Take the extra minute to assess construction era, look for weatherization evidence, and observe the physical condition of the building envelope. Your load calculations will be more accurate, your equipment recommendations will be better justified, and your customers will end up with systems that actually perform.

Conduit Tech pulls in property data to help default infiltration levels based on building characteristics, then lets you adjust based on what you observe on site. See how it works.