Rockwool questions

[Rabbit]

I am in the process of constructing a small recording room on my back porch. I am using the double wall soundproofing idea primarily to keep highway sound OUT. I am planning on putting 2" rockwool (mineral wool) inside the outside wall studs (the rock wool that I have came in 2'x4' panels) . I also plan to put the rockwool between the rafters in the ceiling. Here are my questions:

1 Will this mineral wool support itself or do I need to attach it somehow to the outside wallboard?

2. How do I hold the rock wool between the rafters if I don't install a ceiling (I have a low roof with exposed rafters in that area).

3. An insulation salesperson told me that I need to put plastic sheeting inside the outside wall before I put the rock wool in to prevent molding. Will this plastic have an effect on soundproofing?

 

[Tonio]

1. you need something to hold up the wool. some people use 1x's strips then cover with fabric. there's many ways to do it.

2.same as 1.

3. Good idea-vapor barrier. I don't think plastic will effect sound isolation(soundproof).

Your biggest problem will be the ceiling, rock wool won't really soundproof it. Your idea of double wall-ing may help depending on how its constructed. But thats just the walls, the ceiling will be the weak point. Which in turn defeats the whole purpose-unless of course you just want to tame the hwy noise. You may not get enough, however. I leave the rest to the more knowledgable folks.

Just me 2 pennies.
anyone correct me if I'm wrong.

 

[Rabbit]

Thanks Tonio. I would like to put in a drop ceiling or mount one on resilient channels, but my rafters are already about 6 1/2 ft on the lower end. I guess I'll just wait and see how it sounds with just rockwool and if need be I'll make a low ceiling.

 

[Rod Gervais]

1 Will this mineral wool support itself or do I need to attach it somehow to the outside wallboard?

You can buy 1" wide nylon strapping at Home Despot....... install this 2" above the bottom - then mid span and then 2" from the top - just staple (or nail it) to the studs. It works like a charm.

2. How do I hold the rock wool between the rafters if I don't install a ceiling (I have a low roof with exposed rafters in that area).

You would use the same tecnique as above.

HOWEVER - you just defeated yourself - if you aren't going to finish the ceiling with mass and decoupling (RC-1 Channel ) then building the inner walls is just a waste of your money.

For all intent and purposes - you will have done nothing without finishing the top.

3. An insulation salesperson told me that I need to put plastic sheeting inside the outside wall before I put the rock wool in to prevent molding. Will this plastic have an effect on soundproofing?

This should actually go on the inside face of your inner most wall - and no - it will not effect soundproofing at all. But the salesman is right - this is very VERY important - otherwise you will rot out your exterior framing when the warm inside moist air reaches dew point on the colder outside sheathing. (Or just the reverse if you are in the deep south).

Rod

 

[fitZ2]

Hello Rabbit. You said your rafters are exposed. What is the nominal width of these rafters, and length of the span? Depending on the roof sheithing, this is what I would do. I would INCREASE the mass of the roof leaf, by installing TWO layers of drywall BETEEN the joists, up against the roof sheithing, as tight as possible, using construction adhesive on the first layer, and 1x1 cleats to hold the second layer in place. Caulk every joint. Then insulate with rockwool or fiberglass, then suspend the second leaf(also 2 layers of drywall) of the ceiling via RC. Of course, if your rafters are only 2x4 over a porch, then it is probably not strong enough to hold this additional weight(depending on span and spacing). If they are 2x6, it will probably work, although span and spacing are still the deciding factors. You could possibly sister(double up) the rafters at every other one also, to increase the strength. Although it may be a chore. Not knowing the exact construction makes it hard to determine these things when offering solutions though. So ignor my suggestions if it is not an option.
fitZ

 

[Rod Gervais]

Hello Rabbit. You said your rafters are exposed. What is the nominal width of these rafters, and length of the span? Depending on the roof sheithing, this is what I would do. I would INCREASE the mass of the roof leaf, by installing TWO layers of drywall BETEEN the joists, up against the roof sheithing, as tight as possible, using construction adhesive on the first layer, and 1x1 cleats to hold the second layer in place.

FitZ,

The typical design requirements for roof framing are as follows:

Dead Load: 10 psf.
Live Load: dependent on local condions, for example - in the state of Connecticut - 40 psf snow load.

The problem (with a roof) using your approach would be this,

When you consider the dead load - you include rafters, sheathing and shingles - plus any other load hanging from the rafters (in your case the additional drywall).

Shingle weights vary - typical 3 tab shingles run from about anwhere around 170 # per square (100 sf) to 300 # ps. Laminated shingle run from around 300 to 400 # ps.

That mean shingle weights of roughly 1.7 to 4 psf - add (1/2)plywood at 1.5psf, 2x6 (DF) rafters are 4.08 psf.

So right off the bat - with an inexpensive shingle - you are 7.28 psf. and withthe best laminated architectural grade shingles - 9.58 psf dead load.

Now - consider that the residential building codes allow the installation of a 2nd layer of shingles before requiring the roof be stripped - and you have from (once again best case to worse case) between 8.98 to 13.58 psf on a roof designed for a 10# dead load.

This isn't generally a problem because the worse case snow loading is usually a short term load rather than long duration - but that isn't always the case.

Now you want to add to this a couple layers of drywall (we'll forget the weights of the furring and caulk for this exercise - but we wouldn't in the real world for engineering purposes) that weighs (5/8") 2.5 psf - so another 5psf to the rafter loading -

Now we are up there - (best to worse case) 13.98 to 18.58 psf of total dead load on this roof.

You are beginning to make me nervous here. That is unless we double up EVERY rafter in the roof to accomodate the additional load.

I also have another problem with this approach.

In order for it to work (from an isolation point of view) you have to completely seal the roof - and this is not a good situation from a structural point of view.

1 - the roof has to breathe...... drywall is not a vapor barrier - and vapor passing through it to the underside of cold roof sheathing is going to condense and cause serious structural (as well as mold) problems.

thus - you would have to do this the way we construct cathederal ceilings - with a path for air to flow above the insulation (we generally use proper vent for this) then insulation - then a real good vapor barrier - and then mass......

Which brings us back to my comment regarding RC-1 and mass to the UNDERSIDE of the Rafters........... after beefing them up of course.

Food for thought..........

Rod

you would need an air space between the sheathing and the

 

[Rabbit]

Thanks to both Rod and Fitz. The advice is well appreciated. I'll probably be back with more questions.

 

[Innovations]

Good advice here. For the best isolation you need to think of a box-in-a-box construction on all sides, even the floor.

 

[Rabbit]

"This should actually go on the inside face of your inner most wall"

Rod, I am planning on putting insulation only in the outside wall. Don't I need to do something to keep the outide moisture from getting to the Rockwool?

 

[Rod Gervais]

"This should actually go on the inside face of your inner most wall"

Rod, I am planning on putting insulation only in the outside wall. Don't I need to do something to keep the outide moisture from getting to the Rockwool?

Rabbit,

Outside moisture isn't the issue - the wall and outside air will attain the same relative humidity....... the issue here is inside moisture getting to the outside....... that's the problem - and what you need to solve for.

Rod

 

[Rabbit]

Rod,

Thanks for the help. However, it looks like plans have changed. We are now planning on doing a single wall and using resilent channel on the inside. Do I still need to put the plastic on the inside face of the studs? Thanks again.

 

[Rod Gervais]

Rod,

Thanks for the help. However, it looks like plans have changed. We are now planning on doing a single wall and using resilent channel on the inside. Do I still need to put the plastic on the inside face of the studs? Thanks again.

You want to put the vapor barrier on the inside face of the resilient channel.

Rod

 

[fitZ2]

Food for thought..........

Yes, and I agree. No more brain farts from me. Now, lets see, I've given up on acoustical stuff, electrical stuff, HVAC stuff, vocal booths, isolation......mmmmmm, and now structural.......yep. That about covers it for me. And I thought I knew how to build. Well guys, good luck with homerecording. I'm better at playing geetar than this stuff, so I guess thats what I'll get back to. You know, I pictured this as a typical ole porch, with typical homerecording goals, typical skills, budget etc. Two years ago, I re-roofed my 1958 tract house. No trusses, no supports, and only a 1x6 tie at every other rafter. The span to the ridge was 18 ft, with 2x6 rafters at
16" oc, and there was FIVE old layers of comp that I had to take off, but had no time before delivery of the new shingles. When they delivered the high profile comp shingles, they had a conveyer that brought em right to the ridge of the roof. But I had to move them down to the center of the span to remove the top portion first. Because of a time frame for delivery, I had the NEW shingles delivered ON top of the old. Now, I may not know codes, or span tables, or load calculations, but I DO have some common sense. I walked on that roof, JUMPED UP and DOWN, and it deflected less than an inch. Now that tells me something. Reading about an old porch, and BECAUSE there were no dimensions of span, and I pictured(my downfall) a typical old porch, with a 2x4 rafters, in the heart of California with NO snowfall, no budget, no inspection, etc. Because thats the way I've lived my WHOLE LIFE. I've helped build 3 homes, lived in homes that you would swear were about to fall, but didn't, and wouldn't even if you jumped up and down, hit em with a car, or dumped a 6" load of snow on em, cause its happened. Like my dad says, you don't know a thing about living in structures that you THINK are going to fall down, untill you live in a one car garage with 5 sisters and brothers, with 1x12 siding and NO BATTENS, branches for rafters, packing crate roof sheithing and a dirt floor. When you begin worrying about a 2x4 roof caving in cause its rotted from a leaky roof, then just sister a few in there and stop worrying. If you think your wrong, then if you have the money to HIRE someone to do it correctly, DO IT!! but if not, WHAT ARE YOU GOING TO DO? Thats where I was coming from Rod. To me, its the same as the guy at John Sayers, who was bitchin about people not doing calculations for compression of rubber pucks, under a floated floor. He must have spent a month on his, calculating this and that, not to mention he had a CONCRETE floating floor installed, walls with SEVEN LAYERS of DRYWALL, the most expensive set of monitors that money can buy, and lord knows what else. And for what. Cause his wife would rip his balls off if he woke her up at 4 in the morning, because he thought he needed to practice and record at 140 fucking DB. He had a bigger problem than calculations. The point being, HOW MUCH DOUGH DO YOU HAVE? period. Recording has been happening for over 60 years withOUT all this stuff, this is HR, and I've reached my limit with the technical stuff. To all you guys who DO have the money, time, and resourses, good luck, but I don't anymore. I've tried to help for 3 years here with what I know, but now I see this has become more than I thought it was, simply because I never had the resources in the first place to think like a lot of people here. So, that being said, I'm going back to my sagging ceiling and floor, dig out my geetar, and hit the RECORD button. Its been fun. Adios.
fitZ

 

[Rabbit]

Fitz,

Your a trip man! Let me say this. Don't think that your advice is not appreciated and needed. One thing for sure is that most of us don't have a budget or the resources to do everything the "A" way. So ideas about cutting corners is valuble. I sincerely appreciate the info you gave.

Rabbit

 

[Rod Gervais]

Two years ago, I re-roofed my 1958 tract house. No trusses, no supports, and only a 1x6 tie at every other rafter. The span to the ridge was 18 ft, with 2x6 rafters at 16" oc, and there was FIVE old layers of comp that I had to take off, but had no time before delivery of the new shingles.

FitZ2,

Please, 1st off - I like you and think you've given some good advice around here....... I just didn't agree with this particular situation - for a couple of reasons I noted - both of which I consider valid.

I'm in this business for my living........ work on big projects - and never EVER recomend anyone do anything either outside of the codes - or without pulling permits.

One of my reasons for this happen to be - if Joe does something that ends up damaging his house - in any manner - and permits wern't pulled - codes wern't adhered to - the insurance companies may very well say "Sorry - but this isn't covered - we drop our insurance due to the illegal work performed."

This can not only put someone in the position of not having the money to fix something right - but also possibly tearing it all apart to bring it back to code - fines might be involved - maybe even being in technical default of their mortgage.

All of which ends up with this work costing a whole lot more than just doing it "right" the 1st time.

I do understand exactly what you're talking about. I came up through the business working on old houses built before codes even existed, with joists and rafters carrying loads that would never be allowed by today's codes.

But then again - these were full sized members - with wood that exceeded (by far) the structural capacity of today's lumber.

I have seen ( in the past 30 plus years I've been doing this for a living) the span tables for lumber downgraded twice due to the degradation of lumber along the way.

So I assume nothing - and would not consider advising anyone to do anything outside of the codes and structural standards of today - especially over the internet - where I cannot personally inspect the premises - where I can not personally see the exact conditions that exist.

If I err - I err on the side of conservatisim - this due to the fact that I would never want anyone to suffer due to my possibly being wrong.

I apologize if my post offended you - it was not meant to - however - this is an area where I consider myself "expert" - and would never give advice that I would not use in my own home.

By the way - I do not have unlimited funds available for my home projects - which means I save my money until I can afford to do it right. Doing it right means a lot to me......... advising people to do it right means just as much.

Rod

 

[fitZ2]

Hello Rod, perhaps my nature came through a little too strong I apologize if it sounded as if I was offended. I wasn't. In fact, YOU are absolutely correct. Thats why I think I'll refrain from posting anymore. I look at things in an entirely different light lately. I think its from detailing to long Anyway, I seem to have bogged down in things that irritate me (septic tanks buried under concrete slabs will do that to you ), and solutions that I'd have laughed about 2 years ago, now seem to become routine as time and money become shorter in supply the older I get.
So take my reply with a grain a salt. It really has nothing to do with you. You give fantastic replys, that are DEAD ON, and people can trust your word. I on the otherhand have become an opinionated and grouchy old fart with a crappy attitude and very little sense of humor lately. Especially after jackhammering this slab that the idiot previous owner poured OVER the septic tank. I dug up my entire backyard trying to find the SOB, as lines ran to nowwhere, out the hillside, and other wierd stuff this guy did. All just to have it pumped. I moved in a year ago, and the guy that pumped it said it looked like it hadn't been pumped in 20 yrs.
Boils down to another no no, which is having your house inspected before purchasing. But NOOOOOOOOOOOOOOO. I went ahead and bought it out of shear panic, as I was running out of money and time. I spent 3 months on the road, traveling through Oregon a year ago, and came across this place
after 4 deals fell through, I just plain paniced, since I was living in a motel, and money was pouring through my fingers like water ,while looking for a home. Well, enough of my crap. Keep up the great posts Rod.
And my apologys for the rant Rabbit. AND,DON"T take my advice.
I'd hate for the roof to cave in on you and your studio cause of my knucklheaded attitude. Sometimes it gets the best of me. Good luck with your project.
fitZ