HVAC system mistake, outdoor ducts energy efficiency

#1 Most Common HVAC System Mistake Revealed!

Foreword by Ian Thompson, Editor

Is your HVAC system vulnerable to a common mistake that many designers and builders commit? Often, these mistakes arise from a lack of necessary building science knowledge. With recent building code “upgrades” in countries like New Zealand, the focus on constructing air-tight homes has increased. But what happens to the moisture (produced from cooking and breathing) when it can’t escape because the building can’t breathe properly? I suspect we’ll soon see a wave of new buildings with increased mold levels causing a range of health and damage issues. The reason? Incorrectly specified or improperly installed HVAC systems. I hope I’m proven wrong.

Join our regular expert host, Matt Risinger, as he highlights the #1 HVAC mistake that affects buildings across various US climates: exterior ductwork!

From water pooling to leakage issues, discover why routing ducts outside crawl spaces leads to efficiency disasters and air quality problems. Learn from the smart strategies used up North, and understand why the South relies on the vulnerable slab on grade setups.

Get insights into the energy efficiency dilemma that comes with breaching air barriers, leading to a 30% heat loss! Learn about the microbial problems caused by exterior duct condensation. Equip yourself with tips on insulation techniques, high-efficiency furnace recommendations, and fool-proof remodeling using PVC venting.

Don’t let your HVAC system fall prey to this easily avoidable design mistake! Ensure you insulate and seal ductwork within conditioned spaces. Let’s aim for high efficiency and zero preventable problems!

Over to you, Matt!

#1 Biggest HVAC System Mistake Revealed!

Video Transcript

On the Build Show today, the number one biggest HVAC system mistake. This is part rant, part building science education, and probably part apology too, because honestly, I’ve made this mistake before. Let’s get going. Here we go.

The number one biggest HVAC system mistake is this: ducts on the outside of your house. I mean this is ridiculous. Why would we possibly put duct work on the outside of the house? Now before you say, “Of course I would never do that Matt,” how many times have you seen this before?

New construction, fluffy stuff on the ceiling, and then some silver ducts outside of that. I’ll be honest, this is a little bit more of a southern issue than it is a northern issue, but for the last, I don’t know, six or seven decades, here’s some, I don’t know, this 50s house, 60s house, they’ve been doing this all over the place.

Now the other place we see this a lot though is in the South, in crawl spaces, not just attics. Crawl spaces where the fluffy stuff’s at the floor line but then the duct work is below that.

Not only that, I mean with this fluffy stuff up here and this duct right here, this is just as bad, especially because we’ll often see pooling water and all kinds of other nastiness in a crawl space. I mean I can’t tell you how many disgusting crawl spaces I’ve been in, in the Northeast as well.

Now the northern builders, you do get a bit of a pass on this sometimes by, uh, happy accident, by having fulling round basements, by having that full basement, putting your mechanical systems in the basement.

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All of your duct work is inside the house, it’s inside the envelope, it’s inside the air barrier, it’s within the fluffy stuff on the house, and that makes a huge difference, both for having some mechanical space but also I’m going to tell you in a second, the building science behind why it’s really dumb to put ducts outside of your conditioned space.

But let’s back up for a second and say why this happens in the South. In the South we often do slab-on-grade construction and that’s for two reasons: Number one, we have a lot of rock down here, but number two, we don’t have the frost line requirements like you guys in the North do.

I mean frost lines, depending on how far North you go, could be as low as 36 or maybe even, I think the lowest is 42. I may be wrong, connect me, correct me in the comments if I’m wrong.

In the South here we don’t have that frost issue and we also have a lot of rock which means crawl spaces and slab-on-grade is typically how we build throughout the whole South and a little bit into the California, Pacific Northwest, even on brand new construction today.

Now let’s look at why ducts out of the condition space is a problem. Check out this drawing. Here’s a house, the green line here is representing the outer envelope of the house. So this is the line that’s the air barrier and then obviously the fluffy stuff is going to go on top of that air barrier, or sheetrock on the ceiling, and inside a typical house you’re going to see, let’s say 70° and 50% relative humidity.

Attic spaces, especially in the summertime, can get really hot. It’s not unusual. I’ve been in many attics where it’s 130° up in the attic and you might see humidities in the 70% or maybe even higher, which means that those ducts are running through a very inhospitable place.

If we do the math real quick, let’s uh, let’s do the math in black right here. If my air that’s inside my system is at 55° and that attic’s at 130°, so 130° minus 30° minus 55°, that’s a 75° delta. 75° delta in T for temperature.

That means between the duct work and the outside of the duct work there is a 75° differential. That is a huge differential. And typically ducts and attics are something like R6 or R8 whereas the fluffy stuff down here at the base of the attic could be, you know, R38 is for me here in Austin Texas, it might be even higher for some other folks.

That ductwork is real real thin and this is really dumb from an energy efficiency perspective but even more importantly it’s dumb from a condensation perspective. Everybody’s had that glass of cold water that condenses. Even though my studio today is fairly cold, it’s not particularly a huge delta between the glass of ice at 32° I’ve got huge condensation happening on this glass and that’s exactly what happens in our ductwork.

The other thing that happens when we put our ductwork outside of the conditioned envelope, outside of the air barrier, is we have some loss from that ductwork, the cabinet itself, the fire, the furnace box, the AC box is a leaky device and the ductwork where it connects also has some amount of leakage. Even new construction is typically around 5% leakage.

Existing houses, older houses you might see 20, 25 even 30% duct leakage as standard, which means that if I’ve got a thousand CFM running through this system and I have 5% leakage, that’s 50 CFM lost into my hot attic and that’s what these squiggly blue lines are representing right here.

That’s cold air squiggling out of the house and how’s that air going to be made up? It’s going to be made up with these red lines. We might have some air leakage underneath the bottom plate, we might have some air leakage through our windows and doors, and even worse, we might have some air leakage around those ceiling registers in our house.

Let’s take a look at that from a real world example. Let’s go up into an attic on a typical house in the South that has fluffy stuff at the ceiling line and ductwork up there. This is what we might see: standard atmospherically vented, you know, 80% gas furnace, all this ductwork snaking around this hot attic space, around the furnace itself you’re going to see a lot of this kind of blackness happening.

What is that? Microbial growth. You’re going to see it around the condensate line certainly but even worse you’re going to find that inside of our ductwork for this exact reason right here: cold condensing surfaces that are in contact with air that has some humidity in it are going to develop condensation and that condensation can lead to microbial growth.

A bunch of these next photos are from my friend Sean Harris at IAQ Texas who regularly goes into Texas houses and checks these systems out and none of this is good guys. I mean look at these inside of duct photos. Remember this is plastic duct and yet you’re seeing this forming on the inside of the plastic duct and that is not good.

That is some type of microbial growth. Same is true in plastic ducts, flex ducts. Duct board is probably almost worse in some respects. This is that kind of fiberglassy type material. You’re going to see it on plenums, you’re going to see it on supplies, you’re going to see it on returns. It’s nasty.

The other nasty place you’re going to see this a lot is on ceiling registers. This is a ceiling register, insulation is above this. The drywall was getting cold in that space, had condensation form on it which you’re seeing right here, is leading to all kinds of nasty stuff. We do not want that in our airstream. We don’t want that in our houses.

Inside of boots can be another place that you’re going to see that. This is probably a duct board boot that we’re seeing here. I mean this is just gross stuff. We don’t want that inside of our airstream and we don’t want that in our houses. So ducts outside of conditioned space: dumb from an IAQ and health perspective and dumb from an efficiency perspective.

The other thing that happens though is as we depressurize, the other problems that we made, the other mistakes we’ve made in our attic, are also contributing to that air infiltrating back into our houses. This is looking down on a top plate, probably an interior partition. The electrician drilled some wires in that, that’s a source of an air leak, that’s a path for an air leak.

Let’s talk about solutions. If we’re if we’re remodeling or we’re building new, one sort of easy thing to do that we’ve been doing, that I’ve been doing for you know more than a decade now, is to come into the attic and spray some type of either open or closed cell foam right at the roof line. I would caution you, we don’t want any roof leaks with this.

We need to make sure that we’ve got a good watertight roof on the house. We often don’t do this on remodels without changing the roof at the same time. We really want to make sure that we’ve got a good peel and stick underlayment, that we’re not going to have problems because if that water does leak into there it’s not going to have the ability to dry like a vented roof would. So we need to be cautious about this but this can be a way to retrofit.

And these are both remodel situations where we’ve gone in and sprayed. In this case these are both my houses that I’ve done in the past. We sprayed open cell foam. You can see on this new construction job that’s also a relatively good way to do it: put 10 inches of open cell foam in that attic and encapsulate that attic, bring that insulation, that air barrier all the way up to the roof line and now this ductwork here is running through pretty much the same air condition space as the rest of the house.

Be cautious though, if you’re remodeling you need to make sure that you’ve got not an atmospheric vented furnace. You need a 90% internal combustion, or pardon me, sealed combustion furnace, like you’re seeing here. What you’re seeing in this photo is that this furnace is drawing in outside air to combust and then the flu gases, including all the nasty stuff from those flu gases, is going out that second pipe.

Because it’s so efficient, it’s a 90% efficient plus, it can actually use PVC as a vent.  It doesn’t need metal anymore which means we can spray foam right up to that PVC without any problems. So if you’re remodeling remember you’re probably going to have to change out your furnace if you have a metal B vent going through your house. You don’t have to use spray foam though.

Here’s my personal house. You can see there, there’s my attic hole to get up to to my house and all of my ductwork is below the rock wool insulation that I used. Now go back and check out my other videos from Oppol framing. I monopoly framed this.

There’s no vents into this space. I also purposely did the architecture on the house in a pretty easy to insulate fashion with rock wool bats and I did some exterior insulation. But here’s another house. This happens to be Trey Farmer’s house that I saw years ago that gave me the idea.

Look, it’s pretty easy to use rockwool. If we’ve got a standard joist bay, let’s say, or rafter bay to insulate with, and now all this HVAC equipment is within the normal HVAC condition space of the house. And oh by the way he’s also got a dehumidifier up there to make sure that we’re really dropping the humidity in that house and we’re not going to lead to any condensation issues.

And here’s a simple illustration of this as well. The fluffy stuff now is on the outside so we’ve insulated on the exterior of the building. The insulation is here. That’s also the air barrier. So now if my ductwork leaks some of that cold air out of the ducts in any way, shape or form, there are no pressure differentials.

You know, we’re at the same pressure in this space as we are in here and we’re not worried about depressurizing the house and forcing air to leak in. We also have very few, not zero, but very few issues with condensation in this type of system as well.

Well, insulating at the roof line can be a great way to go. On a side note, I have moved to recommending closed cell foam even in the South. This is definitely best practice. It forms a much better air barrier.

There’s a lot of other benefits. So I do like closed cell foam. This is a house I did a few years ago. You can check out the videos on this. This happens to be Ultra Pure foam which is now an Owens Corning product.

But let’s go to crawl spaces as well. Crawl spaces, if you’re going to have ductwork down in the crawl space, you should treat your crawl space like a short basement. Notice this crawl space that I built a few years ago does not have any vents on the outside.

There’s none of those telltale like blocks that you see or blockouts on this because I built this like a short basement. In this photo before, you can see we did a full vapor barrier all the way through that crawl space. We taped to the concrete then we poured a slab down there.

That is a little bit optional if you’ve got a crawl space. You don’t necessarily have to do that if you good have a good let’s say vapor barrier down there. But what we did then was we went in later with closed cell foam and insulated the entire perimeter walls of this crawl space so that now when we’ve got ductwork down in that space we’re not worried about that. It’s inside the conditioned envelope now.

If you have a crawlspace though, like we built on this house, and in this case we had zero ductwork. About the only thing we had down there was a couple of wires that then it’s no big deal, you know, we can use closed cell foam on the underside of that crawl space, seal it up pretty darn well and now we’re not worried about ducts outside of the condition space.

Here’s an open cell version we did a few years ago as well. Not my favorite but it does work. So now that insulation is on the bottom line of the house, right? And then we’ve got our insulation in the walls and our air barrier there and we could also have our insulation up here as long as all of our ductwork and our HVAC system is within the air conditioned envelope of the house.

Here’s just a couple final photos but let’s finish off the video here. The point of the entire story here is it’s really dumb to take ducts outside. You don’t want any ducts to go outside of your house. If this is a remodel situation, consider figuring out how to re-duct your house, bring those ductworks inside. Check out the video that I made recently with my build friend Jake Burton who regularly does vented attics but has all of his ductwork inside the condition space or consider a retrofit where now we can get those ducts that were formerly outside inside the house.

Guys, thanks for joining me a little building science, a little rant, a little bit of therapy for me but trust me this is something you must do in your houses to build healthy, to build with good air quality, and to build with efficiency.

New content here every Tuesday and every Friday. Oh by the way, new content, 15 videos a week over on BuildOnNetwork.com. Follow us on Facebook or Instagram, otherwise we’ll see you next time on the Build Show.

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