Joist Bay as HVAC "duct"?

[Chris Padilla]

I'm planning on having spray foam insulation put into the joist bays under my house in the crawl space.

I have one particular set of ductwork that over the years, folks crawling over it have crushed it pretty good and have thus worn holes in it here and there and I'm sure it could use a replacing but how does one get a ~10-12' piece of rectangular ductwork under a house??

One idea is to cut the ductwork and fit a rectangular to round transition and connect "duct hose" (not sure what diameter to use...8" min I think) and tuck it up into the joist bay out of the way and so it could be encased in spray foam.

The other idea was to somehow use the joist bay to direct the heat/ac to the floor vent. I might be able to feed pieces of sheetmetal through the small access door into the crawl space and make myself a duct by tacking the sheetmetal into the joist bay. Or something like that.

Either way, I feel I need to get this ductwork fixed before the spray foam come along. I think my joists are 2x8s but I cannot recall exactly.

Any other ideas?

[Al Willits]

Older homes here in Minn used to take sheet metal and close off the floor joist sections to use as ductwork, will it work, sure.
Is it code or any of that other garp? don't know.

I think that's what you asked?? :)

Al

[Chris Padilla]

Al, I have no clue what I asked...I just string random letters together and hope that it generates random letters from other people that I can understand in hope of solving my issue. ;)

So the joist bay itself would NOT be lined...only the bottom would be closed off with sheetmetal? For heat, I think that would be fine as it might warm up that joist bay and warm up the floor above it slightly as it reaches the floor vent. I worry about condensation from running the a/c, however.

[Al Willits]

:D:D:D

lmao.....

Well, it sounded good, whatever it was you posted...:D

I think you were wondering about using the floor joists as ductwork...??

edit...they do have flexible insulated round ductwork that might work too.
Al

[Jim Becker]

It's somewhat normal to use joist and stud bays for returns, but I've not seen them used for the heat/cooling side of the system without metal or flex duct. IMHO, even if you "can", replacing the damaged duct with an insulated flex duct is probably the best practice. It also means that the duct will be properly sized...closing in a joist bay may likely result in improper sizing.

[Chris Padilla]

Yeah, I was wondering about the sizing issue. I also don't recall the size of the rectangular ducting I have under there right now.

[Jim O'Dell]

I'd think the problem, other than condensation problems with just covering the bottom of the bay as mentioned above, would be rough wood affecting the air flow. Low volume of a HVAC system might not be a problem, or it could catch any lint or dust to the point of chunks letting go from time to time. I'd opt for the insulated flex hose. Much easier to get under the house and into position as well. Jim.

[Chris Padilla]

Insulated flex is sounding like the way to go to my ears as well.

Survey says: Insulated Flex!!

[Jim Becker]

'Kinda like using the right kind of wire to connect a phone to the network...something different may work, but it's not an "optimal" connection... ;)

[Greg Cole]

Al, I have no clue what I asked...I just string random letters together and hope that it generates random letters from other people that I can understand in hope of solving my issue. ;)

Here's my $0.02...... gjoa fjfsae0fa fasdfas fvaesdopig a gpoiealv nmhnxazesrdy sd fewrpo ed vfgp9ui csaerc .

Makes sense to me.....:D Sorry but couldn't resist.

Al's suggestion for boxing in the joist bay is acceptable here in the midwest & we get plenty of humidity in the summer to make some condensation a sure thing when using the AC, but there's a drain for said condensate.

Greg

[Lee Schierer]

It's somewhat normal to use joist and stud bays for returns, but I've not seen them used for the heat/cooling side of the system without metal or flex duct. IMHO, even if you "can", replacing the damaged duct with an insulated flex duct is probably the best practice. It also means that the duct will be properly sized...closing in a joist bay may likely result in improper sizing.

I agree with using joist bays for air returns, but generally they are not tight enough to be leak free for the "pressurized" air being delivered to a room. They make a 1" thick fibergalss duct board that can be cut to form insulated rectangular ducts. I have it in my house. It comes in 4 foot lengths and the sections are taped together with aluminum tape. They have a special tool to miter the conrners so the board folds up into a rectangle.

[David G Baker]

I live in an semi-upgraded old farm house. My air returns are floor joists covered with plywood. They do leak and are a freeway for critters to move around my house. I am considering replacing the air returns with rectangular boxed sheet metal. The round flex will not fit with out invading my already limited basement ceiling height. The ceiling in my basement is approximately 7 feet. Wonder if the make rectangular insulated flex pipe that would fit between the floor joists, I will have to check that out. The fiberglass duct board may work for me as well. I am in the "slow" process of removing and replacing the ceiling in my basement so while I am doing that the air returns are high on the to do list.

[Chris Padilla]

7' is a lot more room than my barely 2' crawl space with dirt, bugs, various shapes and forms of concrete and rocks digging into my hands, back, face, etc. Also, it is quite dark.... :eek:

[Chris Padilla]

I agree with using joist bays for air returns, but generally they are not tight enough to be leak free for the "pressurized" air being delivered to a room. They make a 1" thick fibergalss duct board that can be cut to form insulated rectangular ducts. I have it in my house. It comes in 4 foot lengths and the sections are taped together with aluminum tape. They have a special tool to miter the conrners so the board folds up into a rectangle.


One thought I had for the joist bay as an HVAC "pressurized" deliverer of air--the joist bay runs the length of my floor and the regsiter is at the end. Assuming I seal it well and insulated it from the crawl space point of view, it still coud serve to warm up the floor above it a little bit I think before the hot air comes out the register at the end.

However, odds are excellent I will go with the insulated flex because it sounds TONS easier. I just need to figure out what size now.

[Jim Becker]

7' is a lot more room than my barely 2' crawl space with dirt, bugs, various shapes and forms of concrete and rocks digging into my hands, back, face, etc. Also, it is quite dark....

Maybe you could train that cute little blond "technician" to squeeze in there for you if you agree to hold the light... :p

[Jason Beam]

However, odds are excellent I will go with the insulated flex because it sounds TONS easier. I just need to figure out what size now.

You prolly already know how to do this, but for those following along who don't: Match the area of the duct. You'll have to do math, i'm afraid, but that's what you want. Take the size of the rectangular duct - say 12x8 - that's 96 square inches and find the diameter that comes as close to that as possible. An 11" round duct has about 95 square inches, so that comes real close, for example.

[Al Willits]

"""""""""
7' is a lot more room than my barely 2' crawl space with dirt, bugs, various shapes and forms of concrete and rocks digging into my hands, back, face, etc. Also, it is quite dark.... :eek:
"""""""

Ah.................er...never mind........:D

I should have said, almost all the joist style duct work was for returns, sorry to have kept ya in the dark on that....:)

Your best bet would be the flexible duct and the cute blonde to install it for ya.

Al...who's fast running out of smily faces

[Randy Denby]

Years ago, when I was just learning the business, one of the senior techs at the company I worked for, built a home. He designed the crawl space to be the supply plenum, by insulating and sealing it. He also built his fireplace to have a massive heat sink down into the crawl space to help heat his home. All he had to do was cut in floor grills where needed for supply air. That was about 28 years ago, and it is still a very energy efficient house. Oh yeah, His hot water heater was a heat pump and he would vent the absorbtion coil (evaporator) into the crawl space in the summer to help in cooling the home.
With all that said, back on topic....for a DIY 'er, I'd stick to flex duct . If you'll give us the size of the existing duct , I'll help you size the replacement. The only problem with flex duct is, make sure your crawl space is screened off from any animal intrusion....they love to tear into it and cool themselves off in the summer time. Good luck !

[David G Baker]

Chris,
The small California crawl spaces taught me a very important lesson, I will never have another house with out a basement until I go into a rest home.
Do you have any black widow spider problems?

Al,
I used up all of my smiley faces, can I borrow a few from you?

[Randy Denby]

:) :D...there you go David. I had a few left. Your post reminded me of what I meant to add in my above post....I will not go under a house again to install a unit...ever again. After being bitten by a scorpion, and seeing my ductman blowing a hole in his own floor from underneath trying to kill a few snakes...uh-uh, call someone else :)

[David G Baker]

Thanks Randy, I will add the smileys later.
Snakes and creepy crawly things, enough to keep one out of a crawl space.
I removed some paneling from my basement wall. Under the panel there was some 1" blue-board insulation. I removed the insulation and locked onto the concrete wall was a hibernating bat. No idea how it got there. I love the fact that bats eat their weight a day in insects I caught him and transported it to one of my barns. Another reason to close in my air returns. Don't think the LOML would appreciate a bat flying around the house.

[Chris Padilla]

I originally hail from Colorado where EVERYONE has a basement but I was in for a shock (besides the real estate prices) that basements are a rarity in California. Thankfully, my crawl space is very critter-free every time I've gone done there so it isn't too big a deal. At least the crawl to the ductwork needing repaird is only about 10' from the access point. Trust me, it could be worse and on the other side of the house.

[Chris Padilla]

Maybe you could train that cute little blond "technician" to squeeze in there for you if you agree to hold the light... :p

My daughter actually came under the house with me once and I think that was plenty for her but she might be braver this time. She does love to "help daddy" fix things around the house. :)

[Chris Padilla]

Chris,
The small California crawl spaces taught me a very important lesson, I will never have another house with out a basement until I go into a rest home.
Do you have any black widow spider problems?

If I ever build my home, it WILL have a basement...it is tough growing up with one and then living in a house without one. I do not have any black widow problems that I am aware of. Squirrels...yes...few mice...yes...that is about it near as I can tell. I did see a rat under there once...only once.

[Frank Hagan]

I originally hail from Colorado where EVERYONE has a basement but I was in for a shock (besides the real estate prices) that basements are a rarity in California. Thankfully, my crawl space is very critter-free every time I've gone done there so it isn't too big a deal. At least the crawl to the ductwork needing repaird is only about 10' from the access point. Trust me, it could be worse and on the other side of the house.

The worst area is right around any vent or the crawl space opening ... that's where you'll find the bugs and spiders. Get farther in, and they don't want to go there either.

The fiberboard sounds like the best solution to me. HVAC systems are supposed to be designed to balance the air flow from room to room, so a rectangular duct might end up too big ... you'll get air to that register, but the rest of the house will be cold. The insulated flex duct works well too, and would probably be easier to use.

[Randy Moore]

and warm themselves in the winter[/color]. Good luck !


Don't use the flex round pipe!! Tell us what size the square duct is and we will help you get the right round size. The round pipe comes in 2' (two) foot lengths and 5' (five) foot lengths.
Flex, thats what they call the flex round, restricts the air flow so much that you might as well not put any duct in the room. you are asking for trouble to use the joist space.

Thats my .02 from a sheet metal worker with 36 years of experience.

[Jim Becker]

Randy, the flex used in my addition is very smooth inside and with proper positioning will deliver the intended results. It's not like the corrugated flex we use for dust collection. I went over that with my HVAC contractor in detail as I also had concerns initially. The actual installation is the key.

[Randy Denby]

Jim, I realize your talking to the other Randy.....just wanted to pipe in. While I try and limit the flex length to a minimum, the flex (good commercial grade) made today is not the same as years past. It is definitely smoother walled and alot more durable. I like to design the last few feet of a duct run with flex, even when using hard pipe. This will quieten a system considerably. ....
Now the flex of years past was a different story. There was a period of a few years that the flex made was of terrible quality. I've seen some that just rotted away from attic heat after only a couple of years . This gave flex a really bad rep., and it was overly restrictive
All that said, I wouldnt want to go over 20 ft with a run without upsizing unless the unit can handle the static pressure

[Jim Becker]

Randy D...thanks for that. That was the same impression I got from my contractor who is a pretty straight shooting individual.

[Chris Padilla]

Well, I was at the borg last night and they have a good selection of diameters to choose from (6, 8, 10, 12, and 14 inch) so tonight I need to get under the house, measure some things up, and then figure out what I need.

The run of flex will be from 10-12'.

I guess I need to take a closer look at the flex the borg has. They are about my only option if I want to get this completed this weekend.

Oh, I think I can handle the calculation of the cross-sectional area to match the rectangular duct to the round flex...thanks. ;)

I'll even take some pics once I get under there and post later for y'all to drool over. :D

[Al Willits]

Wondering with flex vent if there's a difference in air flow?
Maybe if size is in question go with the next larger?

Anybody have a air flow chart for ductwork?? :D

Al

[Larry James]

I'm planning on having spray foam insulation put into the joist bays under my house in the crawl space.

I have one particular set of ductwork that over the years, folks crawling over it have crushed it pretty good and have thus worn holes in it here and there and I'm sure it could use a replacing but how does one get a ~10-12' piece of rectangular ductwork under a house??

One idea is to cut the ductwork and fit a rectangular to round transition and connect "duct hose" (not sure what diameter to use...8" min I think) and tuck it up into the joist bay out of the way and so it could be encased in spray foam.

The other idea was to somehow use the joist bay to direct the heat/ac to the floor vent. I might be able to feed pieces of sheetmetal through the small access door into the crawl space and make myself a duct by tacking the sheetmetal into the joist bay. Or something like that.

Either way, I feel I need to get this ductwork fixed before the spray foam come along. I think my joists are 2x8s but I cannot recall exactly.

Any other ideas?


... "DESIGN, FABRICATION AND INSTALLATION

General Issues
Ducts, plenums, and fittings should be constructed of galvanized metal, duct board, or flexible duct. Building cavities may not be used as a duct or plenum without a sealed duct board or metal liner". ...

SOURCE: http://www.energy.ca.gov/efficiency/qualityhomes/procedures.html

Never use a joist space for supply (warm/cool) air. A Panned (bottom of joist space covered with sheet metal) joist space may be used for return air if the "jurisdiction having authority" approves - read that as local code official. Flex duct may be allowed, but the rough surface tends to collect dust in return air applications, and offers high resistance to air flow compared to rigid duct of the same size. A properly sized metal round or rectangular duct should be used. Be sure whatever you decide to use is sealed to prevent any toxic spray foam gasses from entering the duct system.

Larry

[Chris Padilla]

What I did with my weekend.... **whew**

So my duct was actually 8" round so that made replacing it with 8" round pretty easy. You can see the duct being crushed over the years with folks (including moi) climbing over it. The original duct cut across the space at an angle. I decided to tuck it up into the joist bay and so made it an "L" shape. You can see me laying out some 2' sections of duct on the floor above.

[Chris Padilla]

I thought I could make life easier by making 2 4' sections "above ground". All seams were taped/sealed shut. Below you can see the finished product after covering everything in Reflectex insulation. I used a total of 4 2' sections, 1 3' section, and 4 90 degree fittings (a pair at the register and a pair to make the turn).

[Chris Padilla]

One thing that made life much, much easier on me was a small 8" diameter piece of flex that I had leftover from my dust collector install. Thank goodness I still had it and thank goodness I was able to locate it pretty quickly. You can see below that I had to stuff it over the girder and while I know this will be a restriction in the airflow, it will just have to suffice. Figure the joist bay is 14.5" x 5.5" for a cross-sectional area of 40 sq. ft. while the 8" circle has an area of 50 sq. ft. so I'm willing to live with that. It did save me, perhaps, several hours of figuring out how to get the duct thorugh this bottleneck.

Well, my back, shoulders, and legs are red, scratched up, and sore from crawling around down there for the weekend but it is all done, looks good, and should be trouble-free for many years. It is now ready for the spray-foam insulation to be installed!

Man, I'm tired....

[Paul Girouard]

I'll say one thing that's not the worst wiring job I've ever seen , but it is close .:eek:

I hope you didn't do it , and suspect you didn't. So my comments are just of a general nature.

Duct work looks OK. Nice dry crawl space, maybe a vapor barrier and some insulation in your future , maybe not your down southern Ca. arn't you.

[Jim Becker]

Here I was expecting a really nasty looking crawl space...that's quite clean compared to many I've seen over the years!

[Chris Padilla]

It is as dry as the Mohave under my house...the dirt is actually quite powdery/dusty so a dustmask is quite essential. I also pulled out an old lineset my installer left under there when he put in a new one for my a/c.

Yeah, there are wires for an old defunct alarm system under there as well as for the lawn sprinklers and they are laying on the ground, twisted all over. One of these days, I gotta rip 'em out and tack up the 'laying around' wires. I've been wanting to put lighting under here for a while now but you know, it is just hard to do it. Who wants to crawl around under their house if they don't really need to? ;)

Now I have that old ductwork sitting in a corner under there. I suppose I should cut it up and haul it out as well. At least it is only a few feet from the opening so I might actually do it...but it could be a while! :o

[David G Baker]

I think you did good with the duct work. What I did in the crawl space in my home in California was to dig a trench under the very low spaces so damage to things above could be avoided. I had to keep lowering the trenches due to weight gain.
Your crawl space is very similar to the one I had with the exception of the adobe clumps.

[Chris Padilla]

David,

Thanks for the kudos...I'm happy with it...just so much work but it is good for the soul, I think! :D There are low spots dug out in various locations so that one of "reasonable size" can crawl under. I'm fairly lean so no need to have, uh, improved the "dug-outs" over time! :D This one spot, however, was more easy to go over the duct rather than crawl to the dug-out and hence my problem.

I really do miss a basement...wish I had one in this house be we decided to save our money in having a basement installed and put it towards some new house in the future which will have a basement come heck or high water!!!