Multi-family blower door testing is consistently becoming more common in all Colorado counties. Just as with a regular single family home, a multi-unit building will also need to have each unit pass a blower door test. This is typically required regardless of whether you are building a duplex or a 40-unit apartment building.

On top of that, the smaller the unit square footage is in an apartment building, the more difficult the test can be. Smaller footprints make achieving a 3ACH rating harder than a larger unit.

It is important for builders to think about their blower door testing requirements before construction starts. Once many of the air leaks are covered up with drywall, there is no way to seal them in a multi-unit building.

Because of this, we compiled some tips to help you with your next multi-family blower door test.



1. Exterior wall insulation:

We typically still see most builders using fiberglass batts during the construction of multi-family units. While this is a fine insulation product, it does very little to block air leakage.

We recommend using high-density blown-in products like cellulose in the walls. The packed insulation will provide a better air seal than batts. If possible, closed-cell foam is best, but this is a much more expensive product and is not always in the budget.Multi-Family Blower Door Test air leaks in walls
In this picture, you can see that air is moving through the wall cavities and in through the outlets and bottom edge.

2. Insulation between floors:

Between the drywalled ceiling of one unit and the floor of the above unit, the rim joists that meet the exterior walls and party walls should be sealed as tightly as possible. This is a commonly overlooked area as insulation is not typically required on the rim joists on large buildings.

However, the exterior wall coverings and wrap cannot stop all the air from getting into these floor joists. If not stopped, air typically will flow into the walls and through ceiling penetrations.

Many builders will spray-foam this section, which reduces exterior air moving through the floor cavities.

Be sure that you consider this when framing, because once the drywall is up, there is very little you can do about this large air leak.

Are you in need of a blower door test for your construction project in Colorado? Learn more about our services here!

3. House wraps:

Typically, house wrap is required on the exterior of the framing in Colorado, but not all wraps are created equal!

Plastic wrap on the exterior of the home can create a nice air barrier but non-ridged systems can rip, tear and be sealed incorrectly.

Consider using an exterior sheathing that claims to be a ridged air barrier such as Zip Wall System.

We have seen great improvements in homes that use this system top to bottom on the exterior of the home. This typically performs better than a Tyvek wrap.



4. Party Wall Mitigation:

Between units, a party wall with an air gap in between is required to prevent fires from spreading between units quickly. This is an important construction technique for fire safety, but can significantly contribute to air leakage.

Because the party wall runs the height of the building, the wall cavity is typically open at the top of the wall in the attic or elsewhere. This creates a large cavity for air to move from the vented attic, and into the unit through outlets, baseboard gaps and more.

See the image below for a visual representation of the top of the party wall.

These areas are usually insulated, but that often does not do enough to stop the air from entering the unit.

Sealing the top of the party wall with spray foam insulation will greatly reduce air flow between the shared wall sides of the units.




This way, air cannot run down from the attic, into this wall cavity, and through outlets and cracks. We also recommend sealing the areas where the party wall meets the exterior walls just like in tip 1.

Also, adding a plastic wrap over the insulation, similar to the exterior walls, can help these areas.

We cannot stress how important this step is especially in duplexes. This one fix can easily make you pass your multi-family blower door test.

Multi-Family Blower Door Test part wall air leak
The wall with the outlet in this photo is the party wall. Air is moving through this wall because of the air gap required for fire protection. Seal the tops and the sections where the air gap meets the exterior walls with closed-cell foam.

5. Sealing plates and bottom edges:

Many insulators will now offer air sealing packages where they seal the bottom and top plates around the exterior walls.

The gaps between the wood and the concrete or steel beams can be a source of air leakage.

After drywall, we highly recommend sealing the bottom edge of the drywall to the floor before baseboards are installed. Also, sealing the top plate of framed walls to the drywall should be done as well.

Sealing these sections will reduce air leaks around the edges/joints at exterior walls. This should typically be done before wall cavity insulation goes in.

This is still important to do if the unit has another unit above it. As we talked about in tip 2, floor joists can be prone to air leaks as well, but that air can be blocked by sealing these gaps at the top plate.

Multi-Family Blower Door Test bottom plate air leak
Here is a good shot of air moving through the wall cavity and into the unit through the bottom edge of the walls. Since all other areas of the wall are taped and sealed, this is a week section. Seal the plate and the drywall.

6. Sealing heat supply boots, fixtures, and ceiling penetrations:

Since most of your heat ducts run through the upper floor joists and attic, sealing the edges of the boots, junction boxes, and recessed lights that penetrate the ceiling drywall is very important.

We constantly see leaks around the edges of supply boots that run through the ceiling.

This is especially an issue on top floor units where the attic above is fully vented. Ensure that the top plates that meet that attic are sealed as well.

Multi-Family Blower Door Test leak through the ceiling drywall
Here you can see around the edge of the J-box there is leakage. Also, you can see the cooler ceiling indicating air is likely moving through this area.

7. Bath Fans:

The bath fans pull in air when blower fans are run. While most bath fans have dampers that pull closed, it is typically not a perfect system.

Higher-end bath fans will reduce this issue, but if you are installing 100+ in a big apartment building, this might not be the most cost-effective solution.




We recommend adding an electronic or cape style damper within the vent line to prevent back-drafting of air through the fan.

These dampers work like check valves on water lines where air can only run one direction. Here is an example of the product.

Multi-Family Blower Door Test air leak through the bath fan
This photo shows the air leaks back-drafting through the bath fan.

A Final Note.

If you are building a multi-family unit in which all the units will need to be tested, it may be in your best interest to do a pre-blower door test before drywall.

Once the windows and insulation are in, we can come and perform a pretest to see where some of the bigger leaks are occurring before they get covered up with drywall.

Doing this on a few of the units can save you a lot of headaches in the long run. We will take thermal images and talk you through areas that need sealing. It is better to be proactive than reactive.

If you have further questions about multi-family blower door testing, our energy specialists would be happy to take your call.