Author Topic: flat roof soundproofing  (Read 7967 times)

uglyokie

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flat roof soundproofing
« on: April 25, 2006, 02:51:52 AM »
I live in a rural area infested with boom cars. I'm building a shell to completely enclose my mobile home. The walls will be about 4' away and the bottom of the rafters about 1' away from the mobile home. I have no building codes to worry about. The roof will be flat (1/4" slope) and waterproofed with APP modified bitumen. The rafters are 2X10's on 16" centers.
What do you think about this:
I plan to use homasote sheets to completely enclose the underside of the rafters and install natural cotton fiber insulation in the rafter voids. With over 9" of void that should leave a space between the decking and the insulation laying on the homasote. If I understand correctly from your web site empty space is good. Or would it be better to layer the insulation thickly so it touches the bottom of the roof deck?
I've never used homasote before so I don't know if it can be cut like this:
To separate the plywood decking to rafter board vibration route I was going to try sawing the homasote into furring strips about 2" wide and nailing or liquid nailing them to the tops of the rafters before applying the plywood decking. That would save the expense of using entire homasote sheets as a sub deck under the plywood.
Or is there a better way?

johnbergstromslc

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Re: flat roof soundproofing
« Reply #1 on: April 26, 2006, 03:36:23 AM »
Couldn't you just pour a foundation or lay block and build a real house?  Just save up a little more money...

uglyokie

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Re: flat roof soundproofing
« Reply #2 on: April 28, 2006, 03:00:40 AM »
I didn't know I needed to explain all my reasons for this project in order to get tech advice. I weighed time and cost of this project close to the suburbs against how much time and money to divert from building a house on the mini ranch far away. I only have a few more years to live close to the city before retiring full time to the mini ranch. Considering my plans for the suburban acreage after I move to the ranch, a house will be in the way, but I'll need a small barn structure out here. Pull out the mobile, get rid of it and I have my little barn. You see?

johnbergstromslc

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Re: flat roof soundproofing
« Reply #3 on: April 28, 2006, 03:42:19 AM »
Gotcha.

I wouldn't worry too much about insulation, homasote, etc..  The physical separation from the mobile home will be the thing that kills the most noise.  Just make sure you seal up all the seams, gaps and holes that can transmit sound.  Besides, if you're just going to use it as an unheated barn later, insulation is hardly necessary.  

Of course, you'll need windows and a door - they will leak a lot of sound.  And the large door needed to remove the mobile will leak like a sieve.  (Or are you planning to cut that out later?)

Bottom line is the OSB/plywood you're going to use for sheathing isn't a very good soundproofing material;  It's  not dense enough.  If you could drywall the interior of this shell (with 5/8" type X, mud and tape), that would add more mass to the structure and help a lot more than any insulation.  Probably cost less, too.      

Of course, those boom cars, with their powerful bass down to 20 Hz (sometimes way lower) can sometimes be heard through any structure.  

uglyokie

  • Guest
Re: flat roof soundproofing
« Reply #4 on: April 30, 2006, 03:09:59 AM »
Thank you for the help.

I was amiss by not describing the scope of the entire project and it's future use. I see now you needed more info to tailor your advice, especially for such a weird project.

Winter heating is not a problem where I live. The AC runs almost all year. I will be working in the barn from time to time but probably not long enough to suffer much.

You're right, pulling out the mobile home is a concern. I'm constructing the pull out wall with screws. When its time I can unscrew everything. I'll have to cut a post but I can repair it and have everything back in place sealed tight in a few days.

I'm going to use your web site's advice about window plugs and sealing doors. I see several product that I can use to do that.

Dang, I was afraid you were going to say drywall. I can handle most types of construction with passable ability but I hate to look at my own mud and taping job. It goes against my grain to admit defeat and have someone more skilled do it for me. I guess it will be good practice for the ranch house.

Thanks again.

johnbergstromslc

  • Guest
Re: flat roof soundproofing
« Reply #5 on: May 01, 2006, 03:53:04 AM »
Yeah, the drywall doesn't have to be perfect.  The important part is to get some more mass up on the wall and seal the seams and edges - those small cracks will transmit a lot of sound.  And you will definitely get better with practice.  

One trick I use is to shine a flashlight at extreme angles (right up against the board, parallel to the drywall) on the mud joints when I am sanding.  It's easy to see the ridges or low spots you have to take care of.  You can get a perfect finish a lot easier that way.  

If you want to go a little better, you could throw some insulation in the walls, too.  R-13 is only 45 cents/sq.ft. and you'd get a bit quieter structure and not suffer so much in the winter.  

joel

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Re: flat roof soundproofing
« Reply #6 on: November 30, 2006, 12:42:58 AM »
Hello uglyokie,
If you plan to use any insulation inside the assembly then use the bonded cotton insulation.  This class 1 insulation has an NRC of 1.15 (that's 115% of sound absorbed - as measured in Sabins) and adds over 20+ STC.  No fiberglass insulation comes close to that performance.  For isolation of the homasote use isolation tape - it's much better than ripping strips of homasote.  The isolation tape shown on the following link is a closed cell PVC pressure sensitive adhesive backed foam tape - http://www.soundproofing.org/infopages/Tape.htm
Joel