Basement Ductwork vs. Attic Ductwork
A Design Memo
Let’s say you have a 2-story house, with a basement and and attic. Should you duct the 1st floor or should you just have mini-splits?
Our general take is that mini-splits are probably easier on the 1st floor, and that ductwork from the basement has several complicating factors that attics don’t face.
Things Can Fall Down the Registers
The most obvious difference between running ductwork from the attic versus the basement is the position of the supply & return registers.
If you have ductwork in the attic, coming down to the second floor, the return and supplies are typically on the ceiling. Things cannot fall up these registers.
On the other hand, if your ductwork system is in the basement, your registers will most likely be on the floor of the first floor. This means that things can fall down between the grills.
You can mitigate against that by placing the supplies and returns far from any place where you may drop things, such as the kitchen counter, but ultimately, attic ductwork systems will have fewer issues to do with materials going into the ducts.
It is also worth mentioning that returns close by to door. Entrances sometimes will suffer from people placing wet shoes over the grill. Ultimately, the water trips down and corrodes the duck work.
Furniture on the 1st Floor Can Block Floor Space
When it comes to placement of the supplies and returns, second floor ceiling space offer more or less complete freedom because you have very few things on the ceiling. Sometimes, there is a ceiling fan or lights, but it's very little compared to the amount of furniture on a first floor taking up the floor space.
If you have a new house where you haven't moved furniture in yet, then maybe this is not as much of a problem. But it's useful to consider that placing furniture in certain locations on the first floor may block airflow. I suspect this is another reason why contractors prefer to simply install mini-splits on the first floor.
Basement Ceilings Are Tough to Work With
Consider the amount of things on the ceiling of the basement floor, versus on the floor, an unfinished attic:
In attics, you just simply have a lot more freedom because you generally do not have a bunch of pipes running across the ceiling backing obstruct ductwork.
Additionally, with a basement ceiling, you’ll need to suspend ductwork. When you suspend ductwork, you need to attach straps to the joists, and it’s tough to figure out where the joists are (or if there are pipes or electrical wires running through the bays) when you have plaster on basement ceilings. I’ve personally tried to strap a few ducts to the ceiling, and it’s hard work. This doesn’t happen with attic ductwork.
Unless the bays are open, you can generally anticipate more work to be done in basements.
Condensate Pump is Likely Necessary
This is a condensate pump.
The ductwork in the basement is connected to a basement air handler. This air handler will produce condensation in the process of cooling, like any other indoor unit.
Above ground air handlers (e.g. in the attic) or mini-splits have a tube that runs outside, and since they’re above ground, you can run the condensate lines sloping downwards and the water will end up running to the outside, where you want it to go. This is called gravity draining.
If your air handler is in the basement, however, you’ll need a condensate pump to pump the water upwards. If this condensate pump breaks, you’ll have a leak.
While condensate pumps are fairly standard devices, it’s just one extra element to deal with that you wouldn’t need to consider for attic ductwork. You’ll also need an electrician to get an outlet down to the basement so you can power the condensate pump.
You Lose Basement Space
Ductwork takes up substantial space. If you’re using the basement for other purposes, you should take that into consideration.
Basement Ductwork is More Efficient, However
The one downside to attics? They’re super hot. Which means you lose a lot of heat in winter as the air goes through duct, and vice versa during summer.
In contrast, basements are naturally fairly cool. For example, the discharge temperature here from the basement is around 52 degrees, while discharge temps we typically see for attic ducted systems are around 60 to 65:
So, for what it’s worth, basement systems will be able to cool better. Plus, in the winter, even if you have some heat loss in the ductwork, you end up with a little bit of heating for the basement, which might be useful for ensuring the pipes don’t freeze.
Conclusion: Basement is More Difficult
For these reasons, we think contractors prefer to use mini-splits on the first floor of most places with a relatively open layout for the first floor. Can you install ductwork in the basement? Yes, but it will be more difficult and required more skilled design. It will also come with a maintenance/longevity tradeoff.
Personally, I would recommend the mini-splits. Then I wouldn’t have to ever worry about spilling or dropping anything in the ductwork!