AC duct. Seal or not seal ??

[JoeDS]

While checking on a AC duct flow problem I removed the inside ceiling AC cover to be sure there was no obstructions in the "ducts".


I say "ducts" because there aren't really any ducts, just open channels in the Styrofoam ceiling insulation (now I know while some brands advertise they have real insulated sealed duct in the ceiling). While there I noticed their was no caulking or sealer to prevent the forced air to blow in the roof cavity.


Your opinions on this:
Might it be good to pressurize the cavity between the outer roof and the inner ceiling with air to help it to be less prone to moisture entrapment?

Or is it likely to allow moisture to "get" trapped in this area.


I can kinda see arguments for either approach. Maybe good in summer but bad in winter?!?!?!

[johnpaul]

I have read other threads and seen other videos on poor AC performance. This thread is the first match I have seen on the same issue I am working on. Like you, I realized there is NO AC duct and started to use the word “Channel”. I am not an AC person but I do believe if this channel was sealed, I would not have an AC problem. Your concern is the same one I have, what if this creates a second problem. I decided if a second problem does show up, I will find a solution for that problem.

Here is my story, it is work in progress, I have been thinking about it for the last 2 days. It might be a month or two before I implement my plan. You are not alone.


Thor 2019 24BL Chateau Sprinter
AC Coleman Mach 3

AC Duct/Channel layout on my RV. It is not a seal duct but a channel formed by styrofoam between the inside ceiling and the roof.


Problem: Low air flow from AC vents.

105 degree days in an RV park with NO trees, from noon until 7:30PM it felt like an oven in the RV. We were there for 2 days. It was cooler outside in the shade created by the RV. At night, after 9PM, it was pleasant in the RV.

When we came home I removed all vents and AC covers to inspect and map out this AC Channel layout. More than one vent, I can see daylight. In others I can see holes in the styrofoam for DC power cables and in some places no styrofoam at all. It is obvious my low air flow from AC vents is due to the AC channel NOT seal. Most of the cold air from the AC is going to places unknown.

Propose Solution:
I plan on blocking channels that go off to places unknown and any breaks or holes in the channel.

Currently I am working out the logistics on what material to use, create a template I can use to cut the material and how to install the blocks through the vent holes. Initially it will be temporary, if it works as planned then glue the block in place.

[JoeDS]

As best as I could tell mine were Styrofoam on all 4 sides where 2 layers of foam were sandwiched together. No evidence of any wires coming thru them.


Good luck. Keep us posted.

[Cwray]

Had the same problem in my Hurricane 34J parked in gravel “drive through sites” ( really was just a gravel parking lot with sewer/electric/water poles spaced out every 20 feet) on a hot July. Rig couldn’t keep cool. I would bring it down below 70 overnight, but it would warm up to mid-80’s by late afternoon...

Here is how I solved that problem.

1) Bought an el-cheapo “as seen in TV” snake camera at Menards for $20 and verified the same problem you noticed with styrofoam channels in the ceiling having large gaps between sections and holes cut into them for whatever reason.

2) Used the latex version of Great Stuff expansion foam (about 3 cans if I recall correctly) to seal those holes. The process was very messy. I stuck together a bunch of plastic straws to be able to make the injection straw long enough to go to arm’s length, squirted in a bunch of foam, then used my hand to smush it into the holes/gaps and smooth it out. Used lots of paper towels and kept the trash can nearby to wipe off my hand/arm and discard without making too much of a mess. The latex foam is very soft, even when dry, cleans up easily, and has almost no smell.

3) waited a bit for the foam to settle and dry, then rescoped to see if I covered the holes and see if I needed to flatten the foam to keep the channel smooth for best air flow.

4) I did this at every vent in the rig, also check out your vent fan & bathroom skylight, as those also likely cut into those channels and have large gaps.

5) reviewed a bunch of YouTube videos on RV air conditioning to learn how to check the main vent for gaps, cleaning, and using aluminum tape to seal/smooth out the intake and output channels.

6) went on top of the rig and put Dynomat around the cowel (don’t know what that’s called). I put the .25” tar mat all around the aluminum box, and the .5” foam padding on the inside of the plastic surround. My thoughts are the mat helps dampen sound/vibration and keeps the ducting cool from the inside, and the foam padding with aluminum foil layer keeps the sun radiation from heating it from the outside. It’s a snug fit getting the plastic cowling back on, but it works. And I made certain I wasn’t blocking any airflow at the back of the unit. I did this for both AC’s.

I now have no issues keeping the rig cool to whatever temp I want. This year, we had a mid-travel stop at Holland, MI and parked for 4 hours in a large parking lot. Outside temp on the lot registered 114 degrees. I had no issues easily maintaining 70 inside the rig for my dog while we were away.

I joke that we sublet our rig to meat butchers to hang meat in because we keep it so cold now.

Hope some of these ideas help...

Cheers,

Curtis

[JoeDS]

wow curtis, thats quite a job you did.
Good info

[twovillagers]

Originally Posted by JoeDS

While checking on a AC duct flow problem I removed the inside ceiling AC cover to be sure there was no obstructions in the "ducts".


I say "ducts" because there aren't really any ducts, just open channels in the Styrofoam ceiling insulation (now I know while some brands advertise they have real insulated sealed duct in the ceiling). While there I noticed their was no caulking or sealer to prevent the forced air to blow in the roof cavity.


Your opinions on this:
Might it be good to pressurize the cavity between the outer roof and the inner ceiling with air to help it to be less prone to moisture entrapment?

Or is it likely to allow moisture to "get" trapped in this area.


I can kinda see arguments for either approach. Maybe good in summer but bad in winter?!?!?!

Another forum member pointed out that the front A/C channels (forward of the front vents) continue to nowhere. I installed a baffle to block the air and that seemed to make a difference.
I have a "snake camera" and plan to scope my vents as another post suggested. With the A/C air under pressure, it would not take much of a hole or crack to lose cool air.

[Pete'sMH]

My coach (SOB) has actual ducts and still can’t keep up on 95+ degree days. I have been using a portable AC powered through the 20A outlet on the pedestal and that worked fine. Like the coach enough that I finally decided to cut a 14 inch hole in the ceiling and roof to hard wire a second, unducted, rooftop unit just for the bedroom. Blocking off the original ducts to the back half of the unit forces more air out the front. Not finished yet but expecting a good outcome. I look to be into it for about $700 for a permanent solution to an ongoing concern.

[JoeDS]

Originally Posted by Pete'sMH

My coach (SOB) has actual ducts and still can’t keep up on 95+ degree days. I have been using a portable AC powered through the 20A outlet on the pedestal and that worked fine. Like the coach enough that I finally decided to cut a 14 inch hole in the ceiling and roof to hard wire a second, unducted, rooftop unit just for the bedroom. Blocking off the original ducts to the back half of the unit forces more air out the front. Not finished yet but expecting a good outcome. I look to be into it for about $700 for a permanent solution to an ongoing concern.

We put curtains between the cab area and coach area and that made a world of difference. Of course our coach is much smaller than yours being only 25 feet long. Too bad you dont have a vent back there to put an ac in.

[Cwray]

Originally Posted by twovillagers

Another forum member pointed out that the front A/C channels (forward of the front vents) continue to nowhere. I installed a baffle to block the air and that seemed to make a difference.

I can confirm that was the case for my 34J. The farthest back two vents (over the master bed, of course) had about an extra foot of vent, the front most two had about 3 feet extra on each side. Both areas on each side had significant gaps from a loose styrofoam plug that was just thrown in there (one was missing). I used the foam spray to block it off right at vent. Even the wifey noticed a difference—which, of course, made the effort (And a bit of a chuckle at my expense taking this quest to extremes) worthwhile. The nice thing about the latex foam is that after it dries, you can easily just pull it out (or apply more or shape it).

Between the “keep it cold” and “keep it quiet” projects, I’ve been very happy finding a new hobby of RV modding for all the spare time I didn’t realize I had. Lots of little things have been done here and there over the last couple of years.