Backer box size for MC80

dragonsam

Hi, I purchased two pairs of MC80 for an Atmos setup though still waiting for their delivery so do not yet know their actual physical size.

I am a bit concerned about the 4 holes of maybe 9-10" size in the ceiling / attic, might be a cause for an air draft, warm air leaking, vapor barrier (plastic sheeting) cut and no longer working well, sound might be leaking through attic etc ..

So I was considering building some MDF boxes for them, maybe line them with polyfill etc ..
Once in place, I would maybe use duct tape to seal the MDF box with the plastic sheet / vapor barrier I have on top of the sheetrock.

The speakers are probably designed for open space like the attic and the sound signature might change, but if it is not terrible I would still rather have backer boxes for the benefits they bring.

That said, I do not know what box size would be best, is there a recommended cubic size for a box that might work well for these speakers ?

Or if someone did something similar, what did they use ?

Thanks

Viking64

Putting speakers in an exterior ceiling is ill-advised from an energy/heating/cooling standpoint. As is a vapor barrier in any flat exterior ceiling.

If you build the MDF boxes, probably your best bet would be to add 2" rigid foam board to the outside of the box. Use screws (not nails or tape) to secure it to the MDF. And most importantly of all: Use expanding foam to seal all of the joints in the foam board.

Most foam board you can buy at Home Depot, Lowe's, etc. is NOT adequate (thickness and density-wise). Since you only will need small pieces, you might be able to find plenty of large scraps in a dumpster at any residential or commercial construction site.

Not treating these holes properly will be an energy nightmare and cost you more money than you would imagine.

Viking64

Here is a link that explains a bit and shows a good example: http://structuretech1.com/recessed-lights-are-evil/

I would avoid any 1/2" foam and if you must use 1" foam, double it up.

dragonsam

Is it an option to maybe not have them flush mounted in the ceiling, but instead build like a dropped box attached to the ceiling ..

I could maybe build it off some 14x14 plywood (top and bottom) with some 1x1x6 pieces for the corners, (or maybe some round dowels for the corners), and screw the top plywood into the attic joist through the ceiling, cut an opening in the bottom plywood for the speaker and mount the speaker inside ?

I could live the sides of the box open for some goofy look where you can see the speakers, so the open space that would normally have been the attic will just be my room now ... or maybe just wrap some white cardboard around the perimeter to visually hide the speaker, .. or could fully put in a box by also attaching some plywood to the sides which would fully encompass the speaker and paint it white all around.

This option would obviously not look like an in ceiling speaker anymore, but like a speaker box attached to the ceiling. Primary reason for people buying these speakers is normally to have flush mounted in ceiling speakers .. but I might be fine with this too as long as I do not need too much of a depth that would bring the box and speakers way too low.

What would be the recommended depth for that box I was to go that route, how much depth do the MC80 need ?

I am trying to gauge how hideous that might look.

Viking64

I am not familiar with those speakers, so I can't comment on the sound dynamics of different installation methods. With the application you described above, even having it professionally done by a carpenter and/or sheetrocker/drywaller, it has the potential to look....not so great. It all depends on the room, though too.

Is there a way to access the attic space above or were you planning to install the box (in your original post) from the room below?

dragonsam

I am not sure what you mean by "access to the attic space from the room below".

If I cut the hole in the ceiling from below, yeah then I will a big hole looking into the attic .. I do have access to the attic and can walk in there to right above where I would mounted the MC80 originally, though I may need to really lean and probably get my head poked by nails for some of the speakers which are closer to the edge of the room

Viking64

That was one or the other. Access to the attic space, meaning being able to walk up there OR install those boxes from below, meaning standing on a ladder in your listening space.

I just took a look at the MC80. Those seem like they are designed to be installed in a ceiling space that has a floor/second story above it. I wouldn't install those in any type of insulation.

That's an important question. What kind of insulation is there above the ceiling of your listening space?

dragonsam

It is covered with batts of what looks like pink fiberglas insulation, below which I believe there are tons of tiny little pieces of some pink stuff spread all over which I guess is blown in insulation .. sorry not an expert in these things, so not sure how to better explain it .. maybe the later can be described as some pink fluffy snow.

Viking64

It sounds like blown-in fiberglass with fiberglass batts laid on top. What is your plan for running the speaker wires? When you cut the holes for your speakers, that blown-in fiberglass is going to come pouring down into your living space and make a nasty, dusty, itchy mess.

dragonsam

I was going to first go up and clear up around the space, push the insulation to the sides, and after making the hole, installing the speakers, and putting backer boxes, put the insulation back all around.

I was planning on running the speakers wires up through a wall into the attic space, and then in the backer boxes (drilling a small hole in the boxes for the wire to go through)

Viking64

That all sounds fine. You won't need a very large box, either. If you replace the insulation over the box, you could get away with skipping the foam that I suggested. I was picturing much larger boxes based on your original post, so I thought that they would be sticking out above the fiberglass level.

I strongly recommend that you buy a few cans of spray foam and seal all of the corners of the box, particularly the whole bottom edge where it sits on the sheetrock. And also the hole where the speaker wire goes in.

If you do this the way you planned, you will be able to remove the speakers at any time without having to deal with the nasty mess.

dragonsam

Yeah that was the plan at least .. the thing is I do not really know how high (deep) I should build the boxes to have enough clearance for the speaker, and whether that may be too high and make it stick above the insulation.

Usually if I was to build the box, I would have used some expanding glue when putting it together, which should seal the edges, but would you say it is better to just screw it together, and use spray foam instead ?

Viking64

Believe me, the more sealing you can do BEFORE you get the boxes up in the attic, the better. It would be much easier in your garage or wherever to make the box tight.

I just read the Q & A for these. They recommend a one-cubic-foot box. So, I would add pieces of foam board to the box and seal it with spray foam everywhere except the wire hole (and the bottom, of course). Let it dry for an hour or so, and then bring them up into the attic.

Put them in place over the speaker holes and run the wires into the boxes. I would then go back to your listening space and hook the wires to the speaker before doing the final spray foam. Once they are wired and the speakers are fully installed, go back up and do your final foaming, taking care to make a good seal where the box meets the ceiling. And don't forget to do the wire hole. This will seriously minimize any airflow/heat-loss issues.

Then just spread the blown-in fiberglass back across the exposed sheetrock and up against the box. Then lay the fiberglass batts back over the blown-in and take extra care to cover the top of the box with batts. It is OK for the batts to not be perfectly flat. Just try to make sure that they butt up against each other tightly.

spade1966

Viking64 wrote: »

Believe me, the more sealing you can do BEFORE you get the boxes up in the attic, the better. It would be much easier in your garage or wherever to make the box tight.

I just read the Q & A for these. They recommend a one-cubic-foot box. So, I would add pieces of foam board to the box and seal it with spray foam everywhere except the wire hole (and the bottom, of course). Let it dry for an hour or so, and then bring them up into the attic.

Put them in place over the speaker holes and run the wires into the boxes. I would then go back to your listening space and hook the wires to the speaker before doing the final spray foam. Once they are wired and the speakers are fully installed, go back up and do your final foaming, taking care to make a good seal where the box meets the ceiling. And don't forget to do the wire hole. This will seriously minimize any airflow/heat-loss issues.

Then just spread the blown-in fiberglass back across the exposed sheetrock and up against the box. Then lay the fiberglass batts back over the blown-in and take extra care to cover the top of the box with batts. It is OK for the batts to not be perfectly flat. Just try to make sure that they butt up against each other tightly.

You say that a one cubic foot box is specified for the MC 80's. I could not find the Q&A information you referenced; do they specify optimum dimensions? LxWxH? There are many ways to achieve the desired one cubic foot and I do not want to configure a box that is going to diminish the performance.

txcoastal1

http://www.dynamat.com/architectural-home/architectural-home-dynabox/