Performing Load Calculations in Airtight New Construction

Foam Around Window in New Construction

This article is Part 6 of our series on Manual J load calculations for new HVAC installations.

There’s something we really love about a lot of new homes going up around Atlanta, and it’s not the open floor plans or the quartz countertops. It’s how incredibly airtight they are.

Compared to homes built before the mid 2000s, today’s new construction does an overall better job of keeping conditioned air in and outdoor air out. That’s a good thing for reducing HVAC wear and tear, keeping occupants comfortable, and slashing energy consumption.

But as Jerry Garcia reminds us, every silver lining has a touch of gray. Airtight homes, as great as they are, can be tricky when it comes to performing a load calculation.

All homes need fresh air, and yet…

...very few of them get the right kind. Older, leakier homes don’t get much “fresh” air at all. They get air infiltration. Outdoor air enters the home through gaps and cracks in the envelope. This air isn’t filtered, conditioned, or controlled to meet the occupants’ ventilation needs.

Newer, tighter homes don’t get nearly as much air infiltration, which is good, but they might not get any fresh air ventilation either. When they don’t, humidity levels have a tendency to rise beyond comfort thresholds. There are two reasons why:

  1. Occupants generate humidity through cooking, showering, and other activities. Since the home is airtight, this humidity tends to stay inside the home.
  2. The lack of air infiltration keeps the air conditioner from running very often. That’s great for reducing energy consumption, but it can prevent the system from removing enough humidity.

What does all of this have to do with load calculations? Well, your Manual J results might not point you to a system that’s going to remove enough humidity from your airtight home. The professional performing the load calculation needs to account your ventilation situation so that you get a system that keeps you comfortable.

Here’s what can happen in existing homes.

If you own an existing high performance or “green” home and you experience high indoor humidity from spring through fall, one of two things has probably happened. Either the HVAC contractor working with your builder didn’t do a load calculation when they chose your AC or you did get a load calculation but it didn’t consider how atypically tight your home is.

Here’s what that means in practical terms:

You might have an air conditioner that’s too big for your home.

This is a common outcome when HVAC contractors don’t do a load calculation. Your oversized AC might be short cycling, which means it turns on and off for short periods of time. It will rarely run long enough to remove much humidity.

However, if your AC isn’t too big, you might have this problem:

Your AC might be the right size, but it might not be the right type.

To remove more humidity, you might need a variable speed air conditioner. If you’re not in a position to replace your existing system, you might need to install a whole-house dehumidifier to pick up the slack for your AC.

So, what should you do?

These problems can be avoided when your load calculation contractor considers air exchange as part of the process. To do so, he or she needs to perform a blower door test, account for fresh air system design, and confirm that the results help you choose the right size AC and the right type.

Here’s what can happen in new construction.

Designing the right HVAC system starts with a load calculation, and it’s a great idea to perform one before a home is built. Benefits include:

  • Infrastructure: When you know what kind of AC and furnace the home needs, you can accommodate the necessary infrastructure (space, ductwork, etc.) now instead of later, after the home is finished.
  • Fresh air: HVAC design can address the home’s need for fresh air via mechanical ventilation. In our climate zone, this normally involves installation of an ERV or ventilating whole-house dehumidifier.
  • Humidity control: AC and furnace sizes and types can align with the home’s unique requirements for humidity removal, taking into account any ventilation equipment and exhaust systems, such as bath fans and range hoods.

Of course, your load calculation contractor will need to know what your home’s attributes will be after it’s built. Otherwise, he or she might not know about certain conditions that will affect the load. Ventilation (or a lack of it) is a major one.

The takeaway? Make sure your load calculation contractor understands your home.

Today’s homes aren’t like homes built twenty years ago. They have different humidity management requirements that can affect the load calculation. Your contractor needs to understand those requirements and be able to account for them.

Ideally, you’ll end up with an airtight home that has mechanical ventilation and a right-sized air conditioner capable of removing enough humidity to keep you comfortable. A lot of high performance homes aren’t equipped with those things, but a properly executed load calculation can set you on the right path.

An experienced practitioner – one who understands the unique challenges of new, airtight homes – is the best person to do the job