Furr-UP instead of furr-down to include ducts in the envelope?
Hi all,
I’m building a new home in south Louisiana and I’ve been lurking here for a few week soaking up some energy efficiency building techniques.
One of the biggest points I see on here is to include the HVAC system in the insulation envelope. But, that isn’t standard practice in Louisiana for new construction ( as in, NO ONE does it in my area). I get the deer-in-the headlights when I even mention a closet unit with ducts installed in furr downs. This also goes for Framers and Home Designers
So, I was wondering if anyone has ever seen a Furr-UPS (into the attic) instead of a furr down?
I’m a fairly handy guy, So I was thinking I coudl have the HVAC installed the typical Louisiana way, in the attic 🙁 Then, frame up an insulated mechanical room myself, lay the ducts out in straight-ish “raceways” and frame furr-ups branching off my mechanical room. These furr-ups would not ducts buried in insulation, but instead an insulated box around the duct.
I figured I could use rigid foam with sealed seams on the interior face of my furr-ups and then install batts or rockwool on the back side of the rigid foam.
Thanks in advance for tips, opinions and sanity checks.
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Replies
Jiggernaut,
In theory it's possible. In practice, it's likely to be more expensive than easier options. If this is a new home, why not design it right from the start, rather than designing it wrong and spending time fixing the mistake?
For the idea to work, you'll need to consider these issues:
1. The mechanical room has to be big enough to enclose all the equipment, with room for a maintenance worker and enough room to change filters and open all access panels. You'll need lighting and a switch, as well as a weatherstripped insulated door.
2. You need an easy way to get to the attic (and to be able to bring tools to the attic) for future maintenance.
3. You can't install any combustion appliances in this kind of mechanical room unless they are sealed-combustion appliances with ducted combustion air.
4. Building airtight insulated raceways with a high R-value is expensive, and the raceways will make maintenance difficult.
P.S. for readers who don't live in Louisiana: "Furr-down" is a noun. Definition: an interior soffit. It's what you get when you furr down (verb) from the ceiling.
Jiggernaut,
One possible approach to your dilemma is to consider building a very tight envelope with a high R-value, and then heating and cooling the house with one or two ductless minisplits. That solves the duct problem.
Hey, I spent a fair amount of time considering the same thing :) Somewhat hard to find synonyms: Furr-up, Reverse bulkhead, Plenum truss. There's a useful article about it on this site, which seems particularly actionable for building a new home:
https://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic
In new construction you also have the option of doing the roof with a plenum truss to manage the mechanical system ducts & air handlers, keeping it all inside the pressure & thermal boundary of the house:
https://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic
That's effectively a furr-up, but it's designed into the truss rather than added on, and is easier to make air tight.
If it's all in the truss plenum you'll have to figure out the service hatch scheme & location. If it's just the ducts in the truss plenum with the air handler in a service closet it's easier to deal with.
It's pretty common for both heating and cooling system to be ridiculously oversized for the actual loads, in part to overcome the parasitic loads of ducts and air handlers outside of conditioned space. When designing a new house it's possible to keep track of the load numbers as the design evolves to avoid the oversizing that occurs when the HVAC is an afterthought, "someone else's problem" rather than the builder's. According to (GA resident) Allison Bialles' collection of careful Manual-Js, the typical floor area to tonnage ratio for smaller houses is a ton per 1400', whereas most installers are going for a ton per 750' or less. In new construction you have control over this.
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
https://www.greenbuildingadvisor.com/articles/dept/building-science/manual-j-load-calculations-vs-rules-thumb
Even though LA is a gas producing state with cheap retail natural gas, it's greener and simpler to do both heating and cooling with a heat pump, preferably 2-stage or modulating, and sized correctly for the cooling load, which will probably be a bit oversized for the heating load, but not disasterously so in most cases. The typical alternative is a 100,000 BTU/hr or larger gas furnace married to the cooling coil in the air handler, which is grotesque oversizing for the heating load of a new code-min house in your area. With gas furnaces that's not an efficiency problem, but it's definitely a COMFORT problem. You'll almost always do better comfort-wise during the heating season with a (merely modestly oversized) 2-stage or modulating heat pump right-sized for the cooling load.
Jiggernaut and James,
For a description of all the ways you can keep ducts indoors, see this article: Keeping Ducts Indoors.
James: Wow! Great minds (or even idiots like us :-) ) think alike?
Yes, plenum truss (aka "raised soffit truss" or "tray ceiling truss") is a good option. If you go this route, make sure the raised area fully encloses the duct layout, unlike the example in the GBA article.
Another option, and this one is my favorite because it makes for a wonderful air barrier as well: if you're willing to build the roof with dimensional lumber...frame the ceiling joists with open web trusses, sheath the top side of the open web trusses with plywood and seal seams (for your air barrier) with air sealing tape or sealant. Ridge beam and rafters are installed over the top of the plywood. Air handler goes in a mechanical closet, ductwork runs in the open web trusses, blown-in insulation goes over the plywood.
I'm attaching a few images...
John: Is that another 1.5 ton Fujitsu slim-duct cassette? (I'm guessing "yes", from both the form-factor of the air handler and the use of long-throw Hart & Cooley registers.)
A good example of a slim-ducted Fujitsu with a "furr down" soffited duct approach showed up in a GTM blog piece a couple of weeks ago (that both John Semmelhack & I commented on):
https://www.greentechmedia.com/articles/read/what-does-it-take-to-electrify-everything-in-your-home#gs.52jjh6E
The before-soffits picture is here:
https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg
The after soffits and super-slim mechanicals closet picture lives here:
https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg
The 1.5 Fujitsu was probably a bit overkill for the 1200' house in CA, but might not be for a 2000' house in Louisiana.
3/4-ton, Dana...this one just serves the 2nd floor. But yes, Fujitsu (no one else makes one of these with optional vertical supply air discharge).
There's another 3/4 ton unit in the conditioned crawl to serve the 1st floor + crawl.
Side note on the Hart + Cooley registers - they're not particularly long throw...but they get the job done.
Thanks!
The form factor and proportions are about the same across the series, making it hard to tell from a picture without a tape-measure. It's nice how easily they can be tucked into a shallow closet type of "mechanical room" when mounted vertically!
Hopefully Jiggernaut is following this and taking it in (?). I cringe when I read or see what "typical" HVAC installations look like in the gulf coast states (which are every bit as bad as the worst-cases I find near me.) For instance...
Earlier this year on another forum a guy in Irving TX was replacing his AC + electric furnace systems with a AC + propane furnace in a 2400' circa 2002 two story slab on grade, and was concerned about how to provide sufficient make up air for TWO propane furnaces, one 60,000 BTU/hr, the other 100,000 BTU/hr. His real problem clearly wasn't a make-up air issue, but rather an out-of-control oversizing issue.
The original equipment was two AC+ heat strip units, something like a 2-tonner for the downstairs zone, and a separate 3 tonner for upstairs. The proposal for the replacement apparently kept the same ridiculous AC oversizing but included the even more ridiculously oversized 2 stage propane burners. Based on his wintertime electric bills it was obvious that the whole house heat load was well-under 40,000 BTU/hr, even including the parasitic losses of the ducts & air handler in the attic. With ducts indoors and some air sealing I suspect he'd be in the 25K range for heating.
I don't know for sure, without the Manual-J that this house could have been heated and cooled with a pair of 1 ton Fujitsu slim-ducts with the duct units or a zoned single 3 ton old school ducted heat pump with tight insulated ducts still in the attic. Not sure what he eventually settled on, but I'm pretty sure he went looking for other proposals. A competent AC contractor in Orlando FL had chimed in with billing data and specs on similar vintage type & size house that had single 3.5 ton AC + heat strip unit cooling and heating his house, with the ducts & air handler in the attic.
The house wasn't luxury housing, but it wasn't a bottom-of-the-barrel tract house development either. (I hate to think what happens in developments where there is serious corner-cutting.)
So Jiggernaut, PLEASE run the load numbers ahead of time, and specify the equipment accordingly!
Dana,
Yes, I am soaking up everyone's answers! Those slim Fujitsu cassettes and the Attic "Furr-up" that John Semmelhack posted may be just the ticket !! I have a ceiling height change in my attic that would work beautifully as a soffit if I framed it out in that manner. Then, the attic floor would just be a large flat platform.
I'm currently searching for an online company that does Manual J , S and D. I've spoken with a few contractors in my area and they don't really do load calculations or their out right dismissive of them.
One popular contractor told my buddy the Manual J isn't worth the paper it's printed on. He then installed 6 tons in a 2300 sq/ft. He has multiple homes with mold problems but, blames it on "over-venting the attic" and goes on to say that those homes have too many soffit vents. Same contractor, put 2 x 3 ton single stage units in my cousin's 2400' foam house, No dehumidifier. When a cold front moves in condensation runs down the walls :o
I hope to get a plan with proper sizing and duct work. Then, specify equipment when I get quotes.