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insulate HVAC ductwork?

mhamilton on Fri March 28, 2008 3:48 PM User is offlineView users profile

The hvac system for my 2nd floor is all in the attic. The main trunk is galvanized, then has flexible insulated ducts to the registers. I was wondering if I should insulate the metal ducts? The system is only 10 years old, so I'm not sure if they are already insulated... tapping on them doesn't sound hollow.

I ask because last summer we had a few weeks of temperatures near and over 100. The upstairs a/c came on at 9:00 am and ran continuously until past midnight (all the while the temperature at the thermostat continued to climb from 76F to 80F, then back down after dark). Plus, I know the attic is broiling hot, with a black roof and the only ventilation from soffit and ridge vents. The main trunks runs down the length of the main roof on both sides, behind the kneewall.

At the least, I think this summer I am going to install an exhaust fan in the south window, and let it pull air from the north side.



NickD on Fri March 28, 2008 5:11 PM User is offline

Is that your furnace also up in the attic for heat in the winter with an evaporator for cool in the summer?

It should all be insulated, those cold air returns will really be cold in the winter and red hot in the summer and if your evaporator is up there, that is working extra hard as well.

The way it's done up here, is all that stuff is inside the insulated part of the home, only thing outside is the compressor/condenser with the pressure and suction lines attached to it, and those are insulated.

mhamilton on Fri March 28, 2008 6:02 PM User is offlineView users profile

Yes, everything is up there. The 2nd floor unit is a heat pump/air conditioner, with an electric element for aux heating. You can see the return air is the large insulated flex duct in the 1st picture.

You're right about the lines being hot and cold over the year. The room at the end of the line is always hot in summer and cold in winter, no matter what balancing is done. Was going to put in a booster fan, but insulation might fix it. There is sufficient air flow to that room, just not decent air temp.

The only thing is, I don't want to create any kind of condensation problem. The ductwork is all attached to plywood on the floor with L brackets.

Edit: I just pulled one of those small ducts, and all of that galvanized ducting is insulated on the inside. Some kind of black plastic-like insulation.

So, I guess my cooling problem might be just excessive attic temperatures, or extra high heat load?

Edited: Fri March 28, 2008 at 6:39 PM by mhamilton

NickD on Sat March 29, 2008 6:33 AM User is offline

Looks like what you have is polyurethane duct board, seen this stuff on DIY TV where to get into the duct work business all you need is the board, a utility knife, a straight edge, and a roll of duct tape, actually used for taping ductwork. This stuff is only about 3/4" thick, can't imagine the R factor is too great, was attracted to it as it takes quite a bit of equipment to form galvanized. I did look for the stuff around my local building supply stores, haven't found it up here, would be nice for making plenums. Stores are loaded with galvanized take offs, premade registers duct work, but have to go to a HVAC shop for the plenums. Actually not too expensive if you drive in with a trailer and a drawing, but if they come out to your home with a ruler and install it, look for a second mortgage.

Just like in MVAC, you can't have a large enough condenser, in a home, can't ever have too much insulation, they do make a foam spray insulation, but you could also buy aluminum faced 6-8" thick fiberglass bats and a roll of aluminum duct tape and DIY. A thermostatic exhaust fan would be a good investment.

Not sure how thick your floor joist are, typical up here to have at least another 4" of insulation on top of the joists. My gas bill for March was $340.00, fell off my chair when I read that, but we did have a lot of subzero weather and they upped the price to $1.23 per therm, something about demand. But glad I am not on LP gas, costing about a buck more on the therm equivalent. But still a lot cheaper than car gas.

Seems strange, everybody was screaming bloody murder in the 70's when gas jumped from 30 to 50 cents a gallon, today, hear nothing about it. Is the government putting tranquilizers in our drinking water?

mhamilton on Sat March 29, 2008 11:38 AM User is offlineView users profile

The stuff is like a 3M scuffing pad, about 1" thick, black, with a paper facing. The outside of the duct is galvanized, this stuff is just attached on the inside. Funny you mention the fabrication, I noticed each section of duct has a sticker on it with the job, specs, and a small drawing of the piece.

Not saying that company did the best job ever.... they put a humidifier on both units, but then after the house was done and the guy came to charge the systems he said it wouldn't work with the heat pump. Turns out neither works now, after the downstairs humidifier control valve clogged with sediment. And those things just seem to grow over with mold after 1 year anyway. Would probably be better off with an outside air heat exchanger to bring some humidity into the house.

The joists are all 2x12. Was thinking of adding some more insulation to the floor. Our co-op electric company sends a magazine every month with those kind of tips for energy savings.

Have a LP furnace for the 1st floor. But the winters here are never cold enough to really make a big difference in heating costs. The 9 months of a/c add up, though.

Dougflas on Sat March 29, 2008 5:00 PM User is offline

Putting a fan in the attic will create condensation problems in most parts of the country. Things to look for are leaks in the ductwork that pull hot attic air into the system. If you really want to kill the heat load in the attic, install aluminum type barrier on the under side of the roof. Just make sure you have airflow between the barrier and the rafters of the under side of the roof. The sofits need to be vented. The top of the roof should have a ventilating strip. Also, pull your registers loose from the ceilings and seal the joint of the boot and the ceiling. During extreme conditions, the AC should run continuously. The AC will dehumidify the residence. Do NOt run the blower on the continuous mode as it'll pick up moisture from the wet coil and make the residence more humid.

mhamilton on Sat March 29, 2008 5:59 PM User is offlineView users profile

Hmmm... you're right about the fan running continuously. The upstairs unit has one of those variable speed blowers, which goes to low when the compressor is not running. The installer said to keep it on "fan on." The downstairs unit is a one-speed, and fan is set to auto. I will have to set it to auto and see how it does.

It is usually easy to open the unit to clean the fan and evap coil? I'm sure both are dusty after 10 years of blower operation. Will check for any leaks like you said.

I'm surprised to hear that an attic fan will cause problems. Usually the humidity in the attic feels as high as outside, plus the extra temperature.

The a/c running continuously wasn't so much the problem, but that it was running and the temperature was still going up. It was getting up to 79F inside.

NickD on Sun March 30, 2008 8:02 AM User is offline

My home has a two foot overhang with perforated soffits and a full length ridge vent and still gets awfully uncomfortable hot up there. Never measured it, but don't worry about it, nothing up there except 12-14" of insulation.

I was under the impression that adding an exhaust fan if necessary reduces moisture buildup, not increase it, hot stale air can really hold a lot more moisture than cooler air and this guy along with Pansonic seem to be under the same impression. Thing is the cost and maintenance of running a fan more economical than your AC operating cost and better to do it naturally if you can.

In regards to running the blower fan continuously, do that as well at the lowest speed, with constant even small air circulation prevents mold build up in the house. And when the compressor does kick on the evaporator should be well drained as a continuous humidity removal where most of the condensation goes down the drain, that lower air flow does dry off the evaporator so it's not left soaking wet causing it to rot away. I see the DOE is changing there tune about opening windows at night for more economical operation that only loads up your home with pollen and moisture, so the AC has to work five times as hard during the day to bring things back to normal, plus your filter plugs up with debris.

I do not feel putting the evaporator and duct work up in the attic was a good idea. Honestly, can you think of even a hotter space to put in an AC system?

I am not without problems either, a couple more of my argon filled windows have moisture in between the glasses, what a stupid idea, trying to figure out a cheap way to take care of this problem, mostly the south facing windows that receive high thermal cycles with the sun in the day, and cool air at night. Window wood and glass are in good shape, they have some kind of an aluminum frame that is supposed to be bonded to the glass, how do they expect that to last? Insulated single pane glass would last forever and only has a very tiny more heat loss than these stupid things, and won't get moisture in the inside? Did look at many new window designs, if a kid throws a baseball at them, can't even change the glass, have to replace the entire window including the frame that is nailed and glued to the house. What in the hell were they thinking? Selling more windows? Windows ain't cheap.

Dougflas on Sun March 30, 2008 9:17 AM User is offline

The attic fans will suck air from the conditioned space thru leaks in the construction. That is why I suggested sealing the boots at the ceiling joints. So you have this colder air being drawn up into a hot environment. What will happen when cold air meets hot air? Condensation, hense conditions for mold to grow. Natural attic ventilation, the barrier material under the roof, along with ridge venting and sofits will work wonders.

NickD on Sun March 30, 2008 12:42 PM User is offline

I get it! Just like filling a toilet bowl with 55*F water in a warm damp bathroom, that also drips water. Sounds like you also had problems with an attic installed AC system.

Dougflas on Sun March 30, 2008 6:23 PM User is offline

Nick,
I'm an HVAC contractor. I see this quite often in Florida.

NickD on Sun March 30, 2008 7:44 PM User is offline

Guess it's not too easy to dig a basement in Florida, dig down a foot and hit sea water.

2005Equinox on Tue April 01, 2008 1:19 AM User is offline

Do they even try to do basements in Florida? In Tennessee you dont hardly hear of a house having a basement. Here in Wisconsin nearly every house has a basement. I have only heard of a few attic systems here in Wisconsin since most are in the basement.

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2008 Chevrolet Impala LS
1981 Pontiac Bonneville


2007 Sears Craftsman Lawn Tractor


1985 Chevrolet Caprice


1986 John Deere 165 lawn tractor

bohica2xo on Tue April 01, 2008 1:34 PM User is offline

Not many basements here in southern NV, but they are more common in northern NV.

Here many homes (including mine) have the whole HVAC unit on the roof. There is good & bad in all of that. The duct work must run through the hot attic, and we don't have the luxury of all of that attic space in the pics above. A regular inspection requires some crawling...

Condensation & its effects are not an issue here with single digit humidity. Adding the barrier to the bottom side of the roof can help, but it does raise the operating temp of the roofing material. Ridge venting is unheard of around here, so we tell people to add the radiant barrier, but stop short of the ridge by a foot or so.

Power attic ventilation works ok here because of the low humidity, but I recomend a small change... I use a slight positive pressure in the attic. Coupled with good (large) vents near the ridge, I use a gable fan pulling air in on both ends of the attic. I adjust the speed to provide little to no outflow at the soffits. The usual caveat about sealing the ducts & boots applies - but in reverse.

The gable fans shut down when the evaporative cooling is running. Then I run the upblast vents from the second floor into the attic, hence the large roof vents. The exhaust air from the house travels through the attic on the way outside, reducing the heat load there as well. Years of doing this have never produced a condensation or mold issue, but it is a very dry climate. I ran my two 5 ton roof units a total of 27 days last summer, the balance was done on evaporative cooling.

B.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mhamilton on Tue April 01, 2008 4:53 PM User is offlineView users profile

Here in central NC you also don't see basements, only because there is no need. The frostline is not an issue, so builders dig a foot or so down, pour footings, then the block foundation. That's what I have here, which leaves a 3-4" crawlspace under the house. The first floor ductwork and a/c unit are down there (actually, it's a self-contained furnace/ac that just passes the ducting through the foundation wall. Can also be mounted on a roof and ducted down). I do see a lot of condensation from the ducts and water pipes under the house, but with that humidity what can be expected.

The hvac contractor did seem to do a decent job sealing things up. All the ductwork is sealed with either the foil tape or that mastic, and it looks like the register outlets are fairly tight to the drywall. I don't think I will get a large power fan, maybe just a window fan to put on the south side. Then I can open the windows on the north side and let it pull the cooler air in (and from the soffit vents). When it gets warmer out, I'll have to try it and see if there's any noticeable decrease in attic temps. Otherwise, will just leave it all alone.

NickD on Wed April 02, 2008 10:20 AM User is offline

It doesn't cost that much extra to build a basement. At least opposed to having a foundation with floor joist, but nothing cheaper than pouring a concrete slab that you call your ground floor. Ha, when ready mix was nine bucks for a cubic yard it was really cheap to gain an extra 1,500 square feet of usable working space. A nice place for your workshop, I particularly liked the home I build and designed with five steps upward directly to the garage, basement was nice for rebuilding engines in the cold winter months. Also a nice place for a laundry tub where I gave my dogs a bath, if the floor gets wet, doesn't make any difference and can even hose it down toward the floor drain. Safe place to hide if there is tornado warning, that happened a couple of times, plus being nice for the furnace, hot water heater, even a washer and a dryer, but we have that on the main floor now.

If I ever built a home down south, would want a basement and would find some high ground to build on, nice to use natural drainage, still a hole in the ground. My basement only goes down a little more than four feet. They are insulated now, and some contractors use embossed forms so your wall looks like a brick wall. Just put a ping pong table down in our basement, but wife had some ideas of finishing it off. LOL, what for, it's my shop and we have plenty of room upstairs we are not even using. It's easy for me to work on our electrical and plumbing down there.

mhamilton on Wed April 02, 2008 12:28 PM User is offlineView users profile

A basement is nice for some things, but it would have been a considerable cost on this house. Excavation might not have been too bad, but the materials and labor would be a killer. 1st floor is just over 2,000 sq.ft. (rough estimate of perimeter is ~180ft) so that footprint 8' deep would have required enormous amounts of block, plus the labor, and the concrete floor. Then have to put a staircase down there, would loose my pantry space. Would probably just end up being another space for junk to collect... unless I wanted to put in a bowling alley or something.

Actually have the garage detached from the house. Didn't really want all that noise attached to the living space. But, winters here are not as extreme or long as yours, so only loose project space for 2 months.

My neighbor does have a full basement in his house. Even had wood stove put in down there (under the living room fireplace). He had planned to finish it when he built the house, but 40 years later it's just a spot for his forced air unit.


Edited: Wed April 02, 2008 at 12:33 PM by mhamilton

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