My vote would be the crawl space if there is sufficient room and easy access for service. The attic would be fine if the equipment and duct work were installed in conditioned space. If you have vaulted ceiling, then you will need to install some registers near the ceiling on one of the walls. Ideally you should have a return on the opposite wall for good air circulation.

You've narrowed your choices down to two unconditioned spaces, neither of which is ideal for mechanical systems or ductwork. In the attic, you'll lose heat to the environment in winter. In summer, the system will work harder due to heat gain and there is the potential for condensation to occur. If there is a problem with the drain system you risk damage to a ceiling directly below the unit. It will likely result in less heat gain if the airhandler and duct work are in the crawl, but you elevate the potential for condensation, mold/mildew and the attendant risks to the health of the structure and occupants.

A better option for the equipment, and the health of the structure and its occupants is to make whichever space you choose a conditioned space and put the unit and ductwork in it. I'd lean toward a conditioned crawl because of the sum total of benefits by making the space conditioned. The cost to make a crawl conditioned is typically less than for an attic, too.

I'd lean toward a conditioned crawl because of the sum total of benefits by making the space conditioned. The cost to make a crawl conditioned is typically less than for an attic, too.

this is the answer. :-)

Much nicer to be able to service / maintain anything, as well. Hopefully your crawl is sufficiently tall so you are not having to slither into the crawl to change a filter.

Crawl space if hard metal ducting OR if the crawl space is well sealed from critter intrusions. Reason - less extreme temperatures than attic. I wouldn't condition it.

A space can be insulated and sealed, but have no HVAC supply registers. That could be considered as unconditioned space.I would be surprised if anyone was building new construction with an uninsulated crawl space.

If we put the HVAC in the crawl space we will probably seal it off within a year. To do it right now will add another $7k to construction costs and its a project we can tackle on our own in short time

We have dual HVAC systems in our new construction, one for each zone. One is in the mechanical room in the conditioned finished basement and the other is in the unconditioned attic. We've had no issues. The one in the attic is suspended from the ceiling in a horizontal orientation, and has a pan with a plumbed drain under it to prevent any potential ceiling damage from moisture to the room below. That's my preferred setup. It's one thing to install HVAC in a crawl space during construction, but 15-20 years in the future after your flooring above that crawlspace is installed, it's going to be tough to replace a HVAC unit in a 36" tall crawl space. I'm willing to trade the relative inefficiency of operation in an unconditioned space for easy access for servicing and eventual replacement.

@Craig,

We've been designing and building conditioned crawl spaces in our new homes since 1999. We typically install the air handler serving the 1st floor in the conditioned crawl space. It doesn't get installed until after the home is "dried in." All new equipment goes through the same crawl space access door that replacement equipment will pass through when it's time for replacement. There's simply no need to trade off 15 +/- years of efficient operation to improve access when it's an event that occurs only every 15 years or so.

@Elmer J Fudd, @mike_home,

The building code here in Virginia requires either a fully vented crawl space or a conditioned crawl space. The code prescribes a couple of alternatives for meeting the conditioning requirement.

Does fully vented mean uninsulated? If the crawl space is uninsulated, then is insulation added to the flooring above the crawl space?

Yes, a "vented" crawl space has been a requirement of the prescriptive code for years. The option of a conditioned crawl space as an alternative was only recently added to the prescriptive code. Prior to that we had to apply for a code modification request to construct a conditioned crawl. For the vented option, insulation of the floor above is required. Typically fiberglass batts are used which provide a poor air seal, tend to hang down over time, and the binder is food for mold. It was never a good choice--particularly here in the south.

@ Charles Ross Homes,

Fair enough. I picture crawl spaces having tiny little access doors and not much space to work in. I'm not sure what the code is here in NJ but we met it and passed inspection. I do know the attic is fully vented via rafter/soffit, gable, and ridge vents. There is no crawl space in our build, as the elevation results in the entire foundation being habitable with an 8 foot ceiling. I'm certainly no expert - I don't know if in a situation like mine, one can put both units downstairs and have one unit pushing air two flights up as an alternative. That certainly would have been preferred if possible.

"Does fully vented mean uninsulated? If the crawl space is uninsulated, then is insulation added to the flooring above the crawl space?"

In my mild weather area, where basements have not historically been that common, vented crawl spaces are the norm. Slab on grade is seen with high volume (lower quality) developers and some modern styles but is not that common.

Newer builds will have insulated floors, older ones often don't. My house (a custom build around 1970) doesn't have floor insulation. About 5 years ago when I was having extra insulation put into my attic by the area's leading insulation contractor, I asked if floor insulation should be added and the response was "I wouldn't if I were you. It's really not worth the cost or the trouble on an existing house but we'll do it if you want." In colder climates, it's probably more common and more important.

As far as access, because my house is on a slope, the access is a door under the deck that opens to a portion of the crawl space that has 10 feet of headroom. For a small portion of the house uphill, it is a more normall crawlspace with perhaps 4 feet of headroom.

Building in NC with an unconditioned crawl space is not wise. I would insulate your crawl space and run the ducts there. Cheap, easy, smart. No brainer.

Now I did (in triangle NC), a slab, ducts between floors and then ducts under blown attic insulation. 2 story house. We don't need to run the heat upstairs because the house is so tight. We run 2-3 hours of evening a/c at 2 tons typically upstairs. So that is $.30 a night for 3 months.

So you do need to consider the penalty of whatever plan you do. They aren't much.

Build tight and at least semi condition the ducts and units. And the cheapest way to do that is in the crawl. For a one stony house in our climate, there is no discussion.

Put it in your conditioned crawl space.

Is this house already built or are you finalizing the plans? If its still in planning, then a conditioned crawlspace is definitely the way to go. The reason I ask is that a conditioned crawlspace will usually have additional insulation and sealing under the floor, compard to a typical thin slab poured in a vented crawlspace.

Our architect specified the insulation thickness, moisture barrier and other details for under the slab. We then added spray foam insulation on the walls. You do have to decide between closed cell (better insulation but costs more), and a couple types of open cell foam. I'm in Virginia, so using open cell Icynene gave us enough R-value that we could use the same foam in the crawlspace and the upper floor 2x4 walls. Closed cell would have also worked, but was more expensive.

Bruce

The current prescriptive code in Virginia requires R-15 in walls. You'd need to use Icynene MD 200 @ R= 6.75/inch to meet current code requirements.

Are basements uncommon where you live? If not cost-prohibitive, I would much rather have a basement for mechanicals. It’s also nice to have unfinished storage, exercise and play spaces, and I would prefer not to have to crawl or stoop to maintain or clean things. I agree with everyone above who said that the furnace should be in a conditioned space. I had the furnace moved from an attic in a rental, but the ducts are still in the attic where they lose a lot of heat.

A/C in the attic is terrible. I had a new house built back in 2000 (I know, long time but it’s not done any better now) in Dallas where this is the default location. It was so hot up there in the summer that it was dangerous to spend more than 10 minutes in that heat. Besides the potential for air leaks, the flimsy duct “insulation” was a joke. What’s the point of 1/2” of some crappy insulation if the temperature difference is 50F between cold air from the coil and the attic temperature?

As a result, the massive two unit 7 ton total system had troubles cooling the house below 78F and the monthly electricity bill was in excess of $650.

I sold that money trap and now have A/C in conditioned space and that cools the whole house to 70F during 100F weather for around $100/month.

Will this crawl space have a dirt floor? What does the builder add to seal the crawl space for an extra $7,000?

I'm beginning to think the advised placement of the unit is heavily dependent on the climate where you live, the quality of the build, and whether you have a basement. I hear the arguments against attic installation and I get it - but I wonder whether they apply to everyone. I'm in NJ and the efficiency concerns don't appear to be an issue. My build is very tight and our attic is not extremely cold nor extremely hot in our milder summers compared to the south. The insulation on the ducts up there is also very thick. I'm heating and cooling 4000 finished Sq ft with two units at 75 degrees in summer and 72 in winter, and have never had a bill much over $300. So I'm thinking a well done attic installation is less of an issue in milder climates. I know that I am willing to trade efficiency for quiet. There's no way I'd be happy with a noisy unit in a closet on my second floor where we sleep. And I suspect most spec builders aren't incurring the expense of running ducts from a unit below grade level up two stories in homes with full height basements.

The attic, if unconditioned, is not a great place.

Of course mild climates make the issues less.

Basement installs with duct chases is very common in cold climates.

For Craig - is your attic sealed? Because it should be over 110 right now at least - maybe 130. And $300 is a lot if it could be $150 (my last house at 5,000 sqft).

For Mike - all crawls have dirt floors in NC (of course not all but 99% I would guess)

David - You are right. In my area of NC a majority of homes are built with traditional vented crawl spaces. Basements are rare but we do have them. I see a good portion of track home developments with slab foundations but that has been a newer trend. I've only learned about conditioned crawl spaces recently in my quest to learn more due to building

The house we are building is a complete custom build. About 2400sqft. Gently sloped lot on sandy soil base. For my builder it's a $7k upcharge to condition the crawl space. The neighborhood I'm building in has all custom homes and I'm only aware of one home that has a conditioned crawl space. (All homes built within the past 12yrs in the $450k to $900k price range).

Sounds like the crawl space location is the way to go. We may seal and condition the crawl space at a later time if it's worth the investment.

I also considered placing the air hander in the garage but not sure If that does us any favors since the garage is off to one side of the home and I understand that its best to try and place the air handler as close to center in the home as possible.

Appreciate all the feedback! Lots of decisions to make during the build and I just hope I make the right calls for the longevity of maintaining the home.

Our heater is in the attic, and the A/C is outside, beside the house. I think this is normal for southern California, and I do not know how to compare our climate to where you are in NC. We seldom use the heater, and we did not have A/C when we bought the house but are glad we have it now. We use the A/C more than the heater. Before, we had to open windows and turn on ceiling fans for cooling.

Do you get ocean breezes?

@David, I'm not sure if my attic is sealed or not. I don't know what that would entail. Keep in mind that electricity costs are vastly different amounts in different parts of the country. It's expensive in NJ. What I pay is cheap relatively speaking. My previous home was a 1300 sq ft condo and my typical summer bill was $250. So spending $300 to cool 4000 sq ft is a steal relatively speaking. Friends in much smaller older homes are spending in the $400-600 range.

We've been moving more units to the crawl space if there is room. CA has increased its attic insulation requirements and calculates ducts in an insulated vs uninsulated space. We are using an additional layer of insulation under the roof deck as well as R30-38 at the ceiling level. If you climate is hot in summer as is our central valley, try to get it out of the attic. Explore dedicated furnace closets also.