Bob's Guides

Cutting Down the Noise of your Air Conditioner(s)

Noisy Air Conditioners

One of the few things I really hate about living in an RV, even temporarily, is the amount of noise put out by my two roof air conditioners.

On a hot day, it's tough to carry on a conversation without shouting and I have to crank up the television volume (usually with the closed-captioning on) in order to watch TV.

I came up with the idea of adding noise-deadening boxes to cover the air conditioners on the ceiling of the coach.

The Boxes

I built 2' X 2' boxes about three inches deep to cover the ceiling air conditioners. They're made from 1/4" oak plywood, which is surprisingly light, and a combination of 1" X 1" and 3/4" by 3/4" oak outside corner trim. The 1" X 1" goes all the way around the bottom and provides the track for the sliding bottom panels. The 3/4" by 3/4" goes everywhere else. (the larger trim is actually 1 and 1/8" wide, but I was too lazy to type that in over and over).

The first picture shows the front unit, looking toward the front of the coach. If you look closely, you can see the small brass screw that holds the panel in place for travel. It's in the center of the bottom trim on the near side. The screws were left over from the brass angle brackets (I had to buy twelve and only used ten brackets).

Home Depot sells the trim by the linear foot. Because of the waste, it takes about 9 feet to go completely around the box if you make no mistakes. I used four 9' sections and a few feet extra to make the corner braces. Try to pick the straightest pieces you can find. I used a single piece of pine 1" X 2" (ripped in half for the corner uprights) for the cross-bar and the inside trim for attaching the ceiling brackets.

If you make no mistakes, both boxes can be made with a single 4' X 4' plywood panel (I ended up using two).

You can buy the 4' X 4' X 1/4" oak plywood panels at Home Depot, Menards, or (I think) Lowes. Home Depot calls them "Project Panels". Menards calls them "Handi-Panels ." Home Depot also has 4' x 4' "Underlayment" panels, but they're heavier and made of cheap Luan mahogany, which doesn't look very good when varnished.

I ran a pine 1 X 2 across the middle (held in place with two brass angle brackets) to provide a little more stability and some scraps of pine in the corners and where the angle bracket holding it to the ceiling are mounted. Be careful not to block the track for the sliding panel with the pine pieces.

I lined the boxes with Noico Sound Deadening Mat.

It's too late to weigh them, but the boxes are surprisingly light even though the Noico mat has a fair amount of lead in it.

One end of each box is left open for air flow, and the bottom panels slide out so I can service the filters. They're held to the ceiling with 1.5" angle brackets and bulbex blind rivets.

Pop-riveter

You'll want a good rivet gun if you don't already have one. You can get a very good one for less then $25 and you'll find a lot of uses for one if you own an RV. I use this one and it has worked well for me for years, though Amazon has some other pop-riveters that have better reviews.

This riveter would probably be easier for putting rivets into a ceiling and has excellent reviews, but there are many situations where you wouldn't have room for it, so I prefer a right-angle model. Stanley makes a rotating head riveter, but it has terrible reviews so I can't recommend it.

If you haven't used a pop-riveter before, practice on some scrap wood before using it for real. The Stanley takes several passes and two hands to do long rivets like the ones you'll need to go through the ceiling. Push the head as hard as you can and hold it before squeezing and open it up and push and hold again between each squeeze.

In your placement of the angle brackets, try to avoid riveting into the air ducts. It's not the end of the world if you hit one, but you'll get some very small styrofoam pieces blowing out of the ducts for a little while if you do.

Building the Boxes

The boxes are quite a challenge to build because there are a whole lot of ways to go wrong. I built them upside down and it's easy to get confused. I glued them up with wood glue and J-B Weld Quick-setting epoxy (for the stress points) and I used a brad nailer with 3/4" nails to hold them together while the glue dried.

The J-B Weld epoxy sets in 5 minutes and cures in an hour. I also used some Pony wood clamps to pull them together when I had to re-glue some of the corners (with epoxy) that had come loose (18 gauge brad nails don't hold all that well unless they're at an angle or you bend them over inside).

The first try, I built the box with the bad side of the plywood showing and didn't notice it until the glue dried. I also ended up with some of the nails blocking the slot where the bottom panel slides in. I had to cut them off with a side-cutter after the glue dried -- some I couldn't reach with the side-cutters and had to cut off with an orbiting multi tool. (does not include Milwaukee battery) with a carbide blade (if you do that, be aware that the nails get *very* hot when you cut them).

It's also tough to get the boxes square and the mitered corners on the trim correct because everything shifts around as you build it.

They look great now that they're varnished. I was worried that they'd look out of place or too obtrusive, but once in place, you don't actually see them unless you look up, and when you do, they look really attractive (imho). They don't look out of place even though my rig has dark woodwork because they're on a white ceiling.

Details

This picture shows an inside corner. I used epoxy for the cross braces. They only need to be held in place for 5 minutes, so I didn't need to use clamps, and the glue sets up in an hour. Be careful not to block the track for the sliding panel when putting them in.

Here's a view of the inside of a box. You can see the 1" X 2" pine cross bar. Part of it is routed out to reduce the weight, but I don't think it's necessary (it was a scrap from another project). Notice the short piece of 1 X 2 between the end and the cross-bar. I nailed and glued it to the inside of the side plywood and let the glue cure before installing the cross-bar. You can see the angle bracket that attaches the box to the ceiling at the top left. Be sure that the top of the angle bracket is flush with the top outside-corner trim so there's no space between the bracket and the ceiling when you mount the box.

Be careful where you put the cross bar. Make sure it doesn't prevent you from removing the panel to clean or replace the filter. Also make sure it doesn't push out the sides of the box.

This Silicone glue brush is really handy for applying the wood glue (I wouldn't use it with epoxy). It lets you create a thin, even layer of glue, and the glue peels off it once it's dry.

You can't see the angle bracket that holds the cross-bar in place, but it's on the top of the cross-bar. Be sure to leave room for it when placing the cross-bar.

This picture shows an outside corner. These went on last. The trim above and below them are mitered at 45 degrees, but these are cut straight on each end. I used glue and one nail in the middle to hold them in place while the glue cured. Don't nail near the ends or the oak may split.

Here's a picture of a box with the sliding panel half-way open. It does come all the way out (otherwise, you couldn't mount the box to the ceiling).

Here's a picture of the sliding panel removed from the box. Notice that the sound-deadening mat doesn't go all the way to the edges except at the near edge. If it did, it would jam up in the slot. At the near edge, it goes right up to the 1 X 1 outside corner trim (except at the right and left edges). The corner trim also serves as the handle for removing the panel. The ends are mitered to match the mitered ends of the bottom side trim.

This picture is a view of the opening in the front unit with the sliding panel removed. Notice that the trim goes all the way around the opening.

The picture below shows the slot that the panel slides into. You can see the 1 X 1 outside corner trim at the lower left, and just inside it, the gap between that trim and the plywood side panel.

I formed it with the panel in place. I used pieces of toothpick between the sliding panel and the plywood side panel to make sure the groove was big enough that the sliding panel wouldn't bind up in high humidity. Be careful not to glue or nail the toothpicks or the sliding panel in the process.

Here's a picture of one of the angle brackets holding the box to the ceiling.

The Results

The result is a three to four db reduction in the noise according to the Sound Meter app on my phone. Because DB is a log scale, that means the sound is cut by more than half. Subjectively, it's still pretty noisy, because a lot of the noise is coming from the air flowing through the louvered vents, but in a quick test, it's easier to carry on a conversation, partly because more high-frequency noise was eliminated.

I suspect that in a rig without ducted AC, the improvement would be much more dramatic.

I aimed the opening forward on the front box and toward the rear on the back one to help keep things quiet in the living room. I find things are quieter when I close the front shade and curtains so the sound coming out of the opening in the front box doesn't bounce off the windshield. This led to some complications in the design of the rear box, since it overlaps the bathroom wall. That meant that both boxes have the bottom panel slide forward even though the openings point in different directions. I used a small single brass screw (countersunk) to hold the slides in place when traveling.

Mounting the Boxes

There are several points in the process where an extra set of hands would have been helpful, though I did everything but the mounting by myself. To mount the boxes to the ceiling, though, you really need a helper and a short step ladder (This one is perfect and has lots of other uses in an RV). Some one has to hold the box in place while you mark the holes to drill for the rivets and again when you do the riveting. I used blue painters tape to mark two edges of the box and made sure it was square to the rest of the rig (though I discovered that the original AC cover was not square at all).

I used three angle brackets to hold each box to the ceiling — one on the closed end and two on the sides about two-thirds of the way toward the other end.

The process of drilling and riveting went very fast, so you don't need your helper for long.

Do not nail the mitered corners. It will split the oak. I used epoxy on them, but you could drill them and put in small screws.

Compromises

Because the rear box is partly blocked by the bathroom wall, I had to mount it further forward than I would have liked. I left a half-inch gap between the box and the wall to prevent sound conduction to the wall. As a result, the plastic cover of the rear AC unit is more visible than I'd like, though once I got it mounted, I find that you don't notice this at all.

Requirements

I bought more wood than I needed and ended up returning some of it, so I'm a little hazy on how much trim you need. I wanted the grain to match, more or less at the edges, so I oriented the grain of the bottom panel from front to back, horizontally, on the two sides, and vertically on the end. That also makes more efficient use of the wood. I ripped the panel in half with the grain, and cut one side in half for the bottom panels (using the best wood for the bottom panels). Then I then cross-cut (across the grain) the four side pieces (3" wide). Then I ripped the two end pieces with the grain.

I think this is everything I bought originally (some of it went back):

Wood

• Two oak plywood panels 4' X 4' X 1/4"
• Three 9' outside corner trim pieces 1 and 1.8" X 1 and 1.8"
• Three 9' outside corner trim pieces 3/4" X 3/4"
• Two pine 1 X 2s

Tools and Hardware

• A miter saw or miter box and hand saw
• High-quality pop rivet tool
• Saw to cut the plywood
• 12 Bulbex rivets. I used these, but I think these would be a slightly better choice.
• Three packages of 4 brass angle brackets 1 and 1/2" with screws
• Drill and 3/16 bit for the rivet holes
• 5/16 drill bit for the angle bracket holes
• #4 countersink bit for the two screws to hold the bottom panels in place (you can use a metal-cutting bit the size of the screw head, but it won't look as nice)
• Brad Nailer (optional -- requires a compressor)
• orbiting multi tool. (does not include Milwaukee battery)
• carbide blade
• 3/4" Brad nails (optional)
• Phillips Screwdriver or drill bit
• Tape measure
• Carpenter's square
• Silicone glue brush

Sound Deadening Mat

• 1 package Noico 80 mil 18 sq foot Sound Deadening Mat
• Roller for installing the mat (see below)

Miscellaneous

• Wood glue
• Epoxy glue
• Blue painters tape
• Varnish
• Sandpaper
• Wood filler for mitered corners and minor imperfections (optional)

Notes

You need a very good roller to install the Noico mat. You need to put as much pressure as you can on the mat to get it to bond. Do not get the one recommended by Amazon for use with the Noico mat, it doesn't work well and doesn't hold up long. I used an old wooden roller designed for installing Eternabond RV sealing tape, but it's no longer sold. Eternabond now sells a metal version, but it's fairly expensive. This one looks like it would be a good choice.

You can cut the Noico mat with a good pair of scissors, though it probably won't be a good pair if scissors when you're done. Be careful of the adhesive on the mats. It's black and almost impossible to remove from shoes and carpets. It's easy to drop a scrap of it, step on it, and spread it around as you walk.

You have to buy 100 rivets and these were the only ones I found with a large enough grip range to get through the angle brackets, the ceiling and the plywood above the ceiling. I've found plenty of uses for the rivets and my 100 are mostly gone. I used them to hold up my rig's sagging entertainment center, to repair my chrome wheel simulators, and to install some nice wood trim over my peeling wallpaper strips. Putting some wood glue on them is another idea, but it would make them fairly difficult to remove. The rivets hold much better, and since they're aluminum, they can be easily removed by drilling them out with a 3/16 drill bit.

You could use wood screws instead of rivets to attach the boxes to the ceiling, but the plywood above the ceiling is very thin and not the greatest quality, so I'm not sure they would hold over time and you'd probably have to re-tighten them every so often. If you do use wood screws, I'd suggest using four or five brackets instead of the three I used.

The edges of the plywood are all covered by trim, so if you don't have a table saw, you could use a rotary saw, a jig saw, or even a hand saw to cut the plywood, though the construction would be easier if the edges were straight. If you don't have room to transport a 4' X 4' panel, the place you buy it might be willing to cut it in half or even in quarters for you.

Be careful not to get glue on the visible parts of the trim and panels. It will show when you apply varnish.

Remember that although the epoxy sets in five minutes, it's not very strong until it cures. That takes an hour (more when it's cold), so don't put any stress on the joints until it's done curing.

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Bob Ray