Mechanicals-Ductwork in Attic

A couple weeks ago I conducted a blower door test on a home with ductwork that left the conditioned space of the home and was installed in an unconditioned attic.  That home failed the code required blower door test, as a matter of fact, of the two new home’s I’ve tested that failed the test, both had ductwork in the attic.  So, what’s the big deal about ductwork in an attic?  First off, lets look at the installation.  Several large holes must be cut in the finished wall covering and probably the air/vapor control layers to install the ductwork.  Sealing these holes can be tough.

A hole cut in both the vapor and air control layer by an HVAC contractor.

Because of the stack effect, warm, moist air will move through the unsealed hole and end up in the attic.  If the attic is unable to clear the moisture by way of attic ventilation, the warm air will condense on the first cold surface it encounters.  In my climate, that could mean frost in the winter and possibly water damage in the spring.

Secondly, any ductwork that is not air-tight and located within the attic space can have an effect on the air pressure within the home.  Depending on where the leak is located, in the supply or return duct, the air pressure within the home will become positive or negative.  Positive pressures push warm air into roof and wall cavities where there is the possibility of a cold, condensing surface.  A negative pressure can create a radon gas issue and is also a great way to create a comfort issue during a Minnesota winter.  The negative pressure within the home will try to equalize by pulling cold, outside air into the home where ever possible.

Lastly, any metal ducts located outside the building envelope can become hot or cold depending on the temperature in the attic space.  As the air handler begins moving air through the ductwork, the temperature within the ducts is moved into the living space, possibly causing a temporary comfort issue.

The building codes that deal with ductwork and air handling systems have something to say about ductwork located outside the building envelope.  First, the ductwork needs to be insulated.

R403.2.1 states:                                                                                                                Supply ducts in attics shall be insulated to a minimum of R-8.  All other ducts shall be insulated to a minimum of R-6.                                                         Exception:  Ducts or portions thereof located completely inside the building thermal envelope.

R403.2.2 Sealing (Mandatory). Ducts, air handlers, and filter boxes shall be sealed.

Duct tightness shall be verified by either of the following:

  1. Post construction test:  Total leakage shall be less than or equal to 4 cfm per 100 square feet of conditioned floor area when tested at a pressure differential of 0.1 inches w.g. (25 Pa) across the entire system, including the manufacturer’s air handler enclosure.  All register boots shall be taper or otherwise sealed during the test.
  2. Rough-in test:  Total leakage shall be less than or equal to 4 cfm per 100 square feet of conditioned floor area when tested at a pressure differential of 0.1 inch of w.g. (25 Pa) across the system including the manufacturer’s air handler enclosure.  All registers shall be taped or otherwise sealed during the test.  if the air handler is not installed at the time of the test, total leakage shall be less than or equal to 3 cfm per 100 square feet of conditioned floor area.                                                Exception: the total leakage test is not required for ducts and air handlers located entirely within the building thermal envelope.

The building codes are telling us that we must air seal, insulate, and test all ductwork that leaves the conditioned space of the home.  I’ve heard of several forced air heating and cooling systems failing the post construction duct leakage test.  (Commonly called a duct blaster test.)  Fixing the ducts after the home is complete becomes difficult and expensive.  Most contractors have turned to Aeroseal, a vaporized caulking blown into a pressurized duct system that seals all the holes from the inside.  This technology has been around for a couple decades and eventually led us to the Aerobarrier (whole house air sealing) process.

Above is a good thermal imaging photo showing leaky ductwork and an unsealed duct penetration leading into the attic space.  This home was nearly complete when it failed the required blower door test.

Ok, so it’s a good idea to keep ductwork out of the attic space.  How do I do that when building a slab on grade home?  Careful planning!  The photo below shows a truss plan that will be used for a slab on grade home that will be constructed the summer of 2019.

This truss design is called a plenum truss.  The chase for the ductwork has a 16 inches x 8 feet notch in the truss.  The air/vapor control layer will follow the notch and insulation will be blown over the top allowing for the ductwork (along with plumbing and electrical runs) to remain inside the building envelope.

Some designs uses soffits build under the air/vapor control layer to be used as chases for the ductwork.  This plan works best if the ceilings where the duct runs are located are taller, usually 9 foot.

Another option is to scrap the ductwork and move to a mini-split air source heat pump and electric baseboard heaters as a back-up.  The cold weather versions of the air source heat pump efficiently create heat when the temperatures are as low as the teens below zero.  There is no ductwork required.

A popular choice in my area is to install a hydronic in-floor heating system.  Most will install a mini-split system for air conditioning and spring and fall heating needs.

Have I blower door tested homes that have passed with ductwork in the unconditioned attic spaces.  Yes.  The HVAC contractor carefully air sealed all the ductwork and in most cases, the insulating contractor sprayed closed cell spray foam, encapsulating the entire duct system.  The cost of the extra materials and labor could have been easily reduced by planning an alternative solution.

This entire blog has been about ductwork in an attic space, much of this information will also pertain to ductwork located in an unconditioned crawl space.

My advice, find a way to keep the ductwork inside the conditioned space.

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