another foam vs cellulose vs batts decision

[Dave-H]

Hi all -

I have a 20 x 20 garage and I'm going to get it finished with a cathedral ceiling, i.e. drywalled all the way up to the peak of of the roof.

Boy it's tough to choose insulation. I am doing some video work so i want it to be quiet, and here in Denver we have pretty hot/cold seasons although nothing too crazy.

I was leaning towards rockwool, since it's fairly inexpensive and easy to install before the drywall goes in, but then a company offered to use closed-cell foam on the 2x6's under the roof, which gives a nice seal.

Then I had someone give me a lecture about how the foam would turn into cyanide and kill the whole neighborhood, and that cellulose is the way to go. Then I learned about the netting they install before blowing cellulose on an unfinished wall and it seems there are some issues blowing in an unfinished cathedral ceiling.

Then I started to wonder if the rockwool batts are just fine after all!! Fiberglass is cheap but I have seen the modest, but real sound deadening benefit that rockwool has so I'm inclined to go with rockwool if we do batting at all.

It's so tough to know what to do. I am tempted to just put rockwool every where, hang the drywall and hope for the best but I keep going back and forth.

Anyone care to chime on this one? There are also two pop vents on the roof that I'd need to seal up, but then the roof it totally unvented. It goes on and on.

What to do?!!!

thanks, Dave

 

[machsnell]

I am in the same boat....

Sent from my SM-G900V using Tapatalk

 

[theoldwizard1]

Then I started to wonder if the rockwool batts are just fine after all!! Fiberglass is cheap but I have seen the modest, but real sound deadening benefit that rockwool has so I'm inclined to go with rockwool if we do batting at all.

Are you planning on making A LOT OF NOISE in the garage and don't want to bother the squirrels ?

It is your money, but I would never use rockwool unless I really need the noise abatement.

If you want a cathedral ceiling either go all spray foam or spray foam the underside of the roof deck and inside walls with 1-2" of foam and then use fiberglass of cellulose.



If you are concerned about cyanide, find out EXACTLY what the product the installer is using and follow up with that company.

 

[e36jon]

I ended up going with the heavy sound / fire mitigating rock-wool batts (Roxul Safe-n-Sound), which don't have a listed R-value, as I was more concerned about fire and noise.

In your climate you really do need some insulation value, so the regular rock-wool batts would be the choice (Roxul Comfortbatt), and they won't do much for you in terms of sound abatement. That said, I would still prefer that over regular fiberglass for their ability to resist mold / not holding water.

The only foams I am interested in are cementatious foam or phenolic foam. They are both hard to come by but have a lot in common with the best aspects of rockwool, so maybe add them to your list?

Just one guys uninformed $0.02...

Jon

 

[Dave-H]

I am actually trying to keep noise out, since I'm doing video production in the garage. With 2x6 under the roof, I could do big rockwool batts, or maybe 3 inches of foam, or blow in cellulose. The batts seem so easy, thoughe, it's so tempting to just go for that.

Doing foam + blown seems like a lot of effort.

 

[Jon_E]

So, sound abatement + climate control sounds like a case for spraying a thin layer of closed-cell foam, followed by rockwool sound-abatement batts.

Years ago I specified Icynene open-cell foam to soundproof a steel office mezzanine in a manufacturing plant. I think I got lucky, as the spray foam systems were still fairly new tech, but it did work well and also provided some climate control in the office space.

 

[DC73]

I like rockwool. I lined my entire garage with it (I used R15 batts in the ceiling against the drywall and then put fiberglass batts on top for the additional necessary r-value). I was going to get the Roxul brand from Lowe's but ran into a guy from 84 Lumber and he priced the Roxul at $4 a bag less than Lowe's. He had to order it but that was okay with me.

Rockwool is sound resistant, fire proof, blocks air flow through the insulation cavity and has a better r-value per inch than does standard fiberglass (R15 vs R13 for batts in a 2x4 sized cavity). Not as costly as spray foam. To do a cathedral ceiling you need to get the details right. One option is to install soffit vents along with a ridge vent and a 1" channel just under the roof deck for air to flow.

Putting fiberglass, cellulose, or rockwool in direct contact with the underside of the roof decking used to be considered a big no-no because of condensation issues. Now, the building scientists have come around to a new way of thinking. If I remember correctly, they now recommend a ridge vent with a vapor permeable membrane which allows moisture to escape. You can read more about this on BuildingScience.com.

DC

 

[Dave-H]

It's amazing how many different opinions I've heard. I we have an unvented roof and an expert with real credentials told me that since I'm going to epoxy the floor (the biggest source of moisture in a garage, evidently), I should just leave the unvented roof sealed and not worry about it!

 

[Ben7203]

Not sure this helps but a friend of mine had a similar project. He took the attic space above his garage and turned it into a "kid zone" The attic had batt insulation under the floor decking and nothing on the ceiling. He used a DIY spray kit and sprayed in directly to the underside of the roof deck, then covered it in plastic then used 6" t&g boards. It looks like you're on the inside of a box when you're up there. A few of us asked about the condensation and he said the mini-split system will take care of it. It's been at least 8 years and I haven't heard of any issues. He has 4 boys and when you add mine in, it's nice to have a place to send them too.

 

[theoldwizard1]

Doing foam + blown seems like a lot of effort.

So, sound abatement + climate control sounds like a case for spraying a thin layer of closed-cell foam, followed by rockwool sound-abatement batts.

The big benefit of the 2 step approach is providing sealed, air and moisture proof barrier with out the cost of full depth spray foam.

 

[Dave-H]

The big benefit of the 2 step approach is providing sealed, air and moisture proof barrier with out the cost of full depth spray foam.

My understanding was that closed cell foam was an air and moisture barrier,

 

[Jackfre]

Another thing that will help is siding choice. When we re-modeled the house we went with Hardi Artisan Plank. It is a 5/8" t&G on the ends panel. Yes, we put in new double glazed windows, but both my wife and I agree that the quiet in the house when the windows are closed is almost disconcerting. Prior to the re-model we felt that the road ran through the place. Now we have no idea what is happening outside. It went in really well, lays flat and looks good.

 

[Dave-H]

Interesting to have lots of opinions here. I am still leaning toward a few inches of foam up top and rockwool batts in the walls. It would be good if I could add something to the attic to boost it even more, but the idea of doing the nets or drilling holes in brand new drywall doesn't really appeal to me.

I wonder if there is such a thing as a 2 inch batt that could be put up before we hang the drywall?

 

[Chris705]

I'll throw you yet another option. Open cell foam..... You then can add a poly vapor barrier if you think you need it. Open cell is way cheaper than closed cell. It's reported (by foam installer) that open cell has a lower perm rating than Kraft faced fiberglass batts when stapled up. My foam install is full depth for less than a 2" closed cell quote. My installer does both but recommended open and said they have hundreds of installs in houses w/o issue. The issue with batts or cellulose is if you have fiberglass/asphalt shingles is they need ventilation or run the risk of cooking the shingles and reducing there longevity. I think the foam would be similar to wool for sound deadening. Especially when installed full depth. Ask your foam installer about there thoughts on using open cell like this.

 

[theoldwizard1]

My understanding was that closed cell foam was an air and moisture barrier,

It is, but is EXPENSIVE ! That is why a lot of people do the 2 steps.

 

[Dave-H]

It is, but is EXPENSIVE ! That is why a lot of people do the 2 steps.

Got it. So the idea is to spray closed cell, then net it up and blow cellulose, then drywall? I guess it would be pretty effective!

 

[Voi]

Got it. So the idea is to spray closed cell, then net it up and blow cellulose, then drywall? I guess it would be pretty effective!

More or less, literally. You want to be sure to get the right ratio of foam for dew point control. There are articles on Green Building Advisor that discuss this ratio for any specific climate zone.

 

[Dave-H]

Does closed cell foam squeak and pop during temperature changes? I'm imagining thst possibility.
Check your wood connections also. Double drywall with a gap, special caulking....
There's a LOT more to silencing that needs considering. Insulation is not the end all to all sound transmission.

I have a finished attic in my house and we blew closed foam into the rafters (6 inch). Not only is there no squeak and pop, it's surprisingly silent now. I am really impressed with how well it works, at least as far as quiet/warmth is concerned.

 

[Dave-H]

I just had another person chime in, a fairly knowledgable scientist who works as a LEED consultant and understands insulation pretty well.

He told me that if I wanted the absolute best possible situation I should go with closed-cell foam. Then, to my surprise, he told me that if it was his garage he'd put rockwool batting around the entire place, skip adding soffit or any vents at all, and leave it as an unvented roof. He even said he's seal up the 2 roof vents that are currently there.

His opinion was that if there is very little air flow, and the roof is reasonably well sealed, it's unusual for enough moisture to build up to really kill the roof. And, since rockwool moves the moisture around nicely it would be unlikely to have a problem.

I was surprised to hear this - so many varying opinions. For a 20x20 roof, it's temping to just fill it with batts and not worry about it - until it's time to worry about it

 

[DC73]

I just had another person chime in, a fairly knowledgable scientist who works as a LEED consultant and understands insulation pretty well.

He told me that if I wanted the absolute best possible situation I should go with closed-cell foam. Then, to my surprise, he told me that if it was his garage he'd put rockwool batting around the entire place, skip adding soffit or any vents at all, and leave it as an unvented roof. He even said he's seal up the 2 roof vents that are currently there.

His opinion was that if there is very little air flow, and the roof is reasonably well sealed, it's unusual for enough moisture to build up to really kill the roof. And, since rockwool moves the moisture around nicely it would be unlikely to have a problem.

I was surprised to hear this - so many varying opinions. For a 20x20 roof, it's temping to just fill it with batts and not worry about it - until it's time to worry about it

I agree with your scientist to an extent. I like unvented roofs and closed cell foam is definitely the best answer as there are no moisture issues associated with it. However, with other insulation materials in contact with the underside of the roof, they've proven that moisture migrates to the ridge and can cause "ridge rot". One solution (as I mentioned above) is to install a ridge vent with a vapor permeable membrane to allow moisture to escape. This is now a recommendation for climate zones 1, 2, & 3 and they are studying other climate zones. You are not in Zone 1, 2 or 3 but you are west of the dry line which could help. You might not have moisture issues and if you do it might be years before you find out but it's worth looking into a little more.

Read this article for sure: http://buildingscience.com/documents/insights/bsi-088-venting-vapor

Here's another related article: http://buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics

One option may be to spray the minimum amount of closed cell foam under the roof deck that will prevent moisture issues and then finish insulating with rockwool.

DC

 

[Dave-H]

I agree with your scientist to an extent. I like unvented roofs and closed cell foam is definitely the best answer as there are no moisture issues associated with it. However, with other insulation materials in contact with the underside of the roof, they've proven that moisture migrates to the ridge and can cause "ridge rot". One solution (as I mentioned above) is to install a ridge vent with a vapor permeable membrane to allow moisture to escape. This is now a recommendation for climate zones 1, 2, & 3 and they are studying other climate zones. You are not in Zone 1, 2 or 3 but you are west of the dry line which could help. You might not have moisture issues and if you do it might be years before you find out but it's worth looking into a little more.

Read this article for sure: http://buildingscience.com/documents/insights/bsi-088-venting-vapor

Here's another related article: http://buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics

One option may be to spray the minimum amount of closed cell foam under the roof deck that will prevent moisture issues and then finish insulating with rockwool.

DC

Boy, it pains me to put any kind of a roof penetration in, even if for good reasons

It's really hard to understand all of this. The close cell foam is easy but that's like $1800 for a 2 inch spray on the roof only - pretty pricey.

Most people say I should put a barrier on the warm side, between the insulation and the drywall, so that the moisture doesn't go in there in the first place. Then there is the ridge-rot thing. Then there is the idea that with a Colorado sun pounding on the roof for 250 days/year, the vapor permeability of the drywall itself should be enough to keep moisture from building up excessively.

It's maddening. The more I learn the less sure I am about what to do!

 

[DC73]

The more I learn the less sure I am about what to do!

Go to GreenBuildingAdvisor.com and visit the Q&A forum. Post your questions and concerns there. They have several building science geeks who respond to questions and may can provide you with some clarity. If a guy named Dana Dorsett responds, pay particular attention to what he has to say. He tends to get it right and is well respected on the forum. Make sure you tell them where you are located so they can tailor their responses to your climate conditions.

DC

 

[Dave-H]

Looks like it's going to be closed cell foam, since the only thing everyone seems to agree on is that closed cell is a must for an unvented roof.

Next decision is how much. I am told that 3 inches under the roof and 2 inches in the walls is 'more than enough' and that adding more is a waste. That will leave a small cavity in the walls, which doesn't bother me but I worry that the 3 inches + 2 inches just wont be enough.

That leaves VERY little space to blow in anything, though. I was told that in a 2x4 cavity, really 3.5 inches, you are looking at just 1.5 inch cavity and with the irregularity of the foam application it would be too narrow to get a decent dense pack. That makes sense to me.

So, most likely if I use foam everywhere, I would just leave that cavity there. It would be nice to be able to fish through wires, I suppose.

So, that's somewhere in the neighborhood of r18 on the ceiling and r12, but with a very nice air seal. Maybe that's enough here in Denver.

 

[milner351]

I've been going through similar mental gymnastics on this subject.

Vaulted ceilings are trick - you need to vent from eve to vent - ideally the underside and the top side of the roof deck will be the same temperature.

It's very hard to argue against fiberglass batts if cost is a factor.

I was pricing out wall insulation for my pole barn addition - and there was nothing even close to the cost per R value of the batts. Rigid foam board insulation which would be "easier to install" was considerably more money.

 

[archy99]

Another thing that will help is siding choice. When we re-modeled the house we went with Hardi Artisan Plank. It is a 5/8" t&G on the ends panel. Yes, we put in new double glazed windows, but both my wife and I agree that the quiet in the house when the windows are closed is almost disconcerting. Prior to the re-model we felt that the road ran through the place. Now we have no idea what is happening outside. It went in really well, lays flat and looks good.

I am using the Artisan siding on my house rebuild...would love to hear of any tip or issues you had with the material/install/contractors that I should look out for. I think we will be in the same boat, since we started with uninsulated 2x4 ship lap-sheathed walls.

 

[Dave-H]

I've been going through similar mental gymnastics on this subject.

Vaulted ceilings are trick - you need to vent from eve to vent - ideally the underside and the top side of the roof deck will be the same temperature.

It's very hard to argue against fiberglass batts if cost is a factor.

I was pricing out wall insulation for my pole barn addition - and there was nothing even close to the cost per R value of the batts. Rigid foam board insulation which would be "easier to install" was considerably more money.

Like I did on my main house, which has a finished attic bedroom, I think I'm going the other way - we're going to seal up the 2 pop-vents that are installed to the roof and seal it up. It being a garage roof, they didn't bother with any ridge or soffit vents because it's unfinished, and that's fine as long as we use closed cell foam and make it into a truly unvented roof.

This worked beautifully on our main house - 8 years and the roof appears to be working great.

next decision: how much foam! I am being told that 3 inches under the roof is more than anyone would ever need, but not sure if that's correct. But probably 3 inches on the ceiling and 2 in the walls will be the plan.