Insulating round ductwork in cold crawls
rexwoodman
| Posted in Energy, Heating & Insulation on
I’ve got approximately 27′ of 6″ round FHA ductwork (in 2 sections) in an unheated crawlspace that is in dire need of insulation. What’s the best thing to use? Right now there is some type of 7″ slide on insulation (with some mesh for support) that is on 80% of it, but it looks pretty minimal. The ductwork is 3″-4″ below the floor joists and hanging loosely. The boots are totally uninsulated. The outside temp right now is 28 degrees F. Would it be worth trying to box the ductwork in with 2×2, fiberglass, styrofoam and top with plywood? I also have 5′ of cold air return. Yeah, cold air for sure.
Thanks.
Replies
No good info on insulating these ducts well (or if that's even possible - if it is, I've never seen it), but -- have you considered simply properly insulating the crawlspace? Makes for warmer floors and eliminates the need to insulate the ducts. Kill two birds, etc.
Mike Hennessy
Pittsburgh, PA
Mike,
Yes, I have thought of 100 ways to fix this area. Unfortunately, the former owner and contractor failed to take the foundation below the frost line, so at only 24" to the footing, I have frost heaving that causes the block foundation to move, the sill plate to ride off the blocks a bit in winter, it's a mess. However, with fuel oil costs up, I thought maybe I better tackle the ductwork pronto. It will also make the 12' X 21' room a whole lot warmer.
When I have lots of money, I am planning on jacking up this room and doing a proper foundation, but not now.
You can buy duct insulation ... it's a glorified fiberglass batt w/ e.g. a vinyl backing. made in lengths/widths conducive to wrapping the duct.
Also ... if you expose your ductwork ... prior to doing your work ... seal all the joints ... but don't use classic 'duct tape'. Just like a house ... much of your inefficiency can be because of air leakage ... which can still be there w/ insulation on it.
You can buy rolls of 1 inch glass insulation and wrap the ducts with a thicker layer of material. If you decide to go that route, buy a couple cans of spray adhesive, Dow 77 works good. Another thing is to make sure you don't have hot "conditioned" air leaking from the duct works, usually at the boots or where it branches off, etc. HVAC places have material that can be applied by hand to the outside of these connection areas to seal any potential leaks. Don't use ordinary duct tape, that won't last more then a few months. Foil tape works but fairly expensive and not as good as the white/gray goop that HVAC guys use.
Easiest thing to use is flex duct the same size as the metal. The flex duct is just a tad larger, so it pulls around most bends somewhat easily.
An insulated sleeve is also available, but it is easier to use the flex duct, and flex duct can be purchased with R6 and R8 jackets.
Do not try to pull the inner liner over the metal duct, and do not pull it out first, either. The inner liner will act as a support so the insulation does not snag on the pipe opening.
It will be easier to pull the metal duct down to sleeve it. If you have the room and the help, you could insulate the whole thing at one time on the floor with a flex duct sleeve.
Wrapping the duct with roll insulation is also an option, but that material is quite a bit more $$ than flex duct in small quantities. It is also easy for novices to pull it too tight, rendering it useless.
As mentioned by others, seal the pipe joints.
Plain old foil tape works well- the stuff with the UL listing on it is a bit thicker and stickier than the tape without the UL certs. Condensation on the pipe from your breath in cold weather will keep it from sticking.
Brush on duct mastic also works, but you must let it dry first. Water based mastic must be applied in temperatures above freezing... solvent based is more forgiving but the vapors will knock you out in poorly ventilated spaces.
A final option is mastic tape- that is an aluminum foil tape with a butyl rubber backing... really sticky... not inexpensive... but it works great even in cold weather. Word of warning: it *does not* come off the metal.
Not much you can do about the boots other than stuffing insulation around them.
home depot here has a insulating material thats appears to be a foam sandwitched between aluminum foil comes in rolls various widths and lengths about 1/4 " thick
first use high temp tape on all the joints cold and hot
cut material with scissors to suit and form around pipe as needed then tape
the above works best if you have a small helper & second person cuts material and peels off tape as needed , did my pipes in crawl space last year and found it helped my heating costs
>> I've got approximately 27' of 6" round FHA ductwork << OK - but what is "FHA ductwork"? I'm assuming it is some kind of hard duct. You may want to consider replacing it with 8" R-8 flex duct. Hard duct can result in superior air flow but unless installed in a very meticulous manner it is inherently leaky. The air flow issue is offset by upsizing one size. When installing the flex duct use duct mastic on the inner sleeve and secure in place with a minimum of UL181b duct tape and mastic and cable tie the outer sleeve, puling them well up onto the boots. Then use duct wrap to insulate your boots. The flex duct will reduce your leakage and will be insulated better than you will be able to do wrapping the old resulting in more efficient system performance. I don't really know what your present configuration is but you may likely need to get a few junction boxes. You will end up with less than $200 in materials.
In an Energy Star seminar I attended some years ago we were told that the average duct installation done during the 90s or prior had between 15 and 20% leakage. A carefully installed duct system, as outlined above can easily result in a 3% leakage number.
If you must keep the old duct work, remove the old insulation, seal all joints with duct mastic, and insulate. Still, you will have trouble getting the R-8 insulation value that the flex comes with.
Hi Matt- FHA stands for forced hot air...meaning the ductwork is for that purpose.
Do you have air conditioning?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Nope. Someday maybe central air, but not for a long way off.
How about having the ductwork spray foamed like this.
View Image
Air seal and insulate at the same time. Has anyone here had it done?