# Drop ceiling with fiberglass batts on top.



## Guardian (May 17, 2018)

Hi, 
I'm installing my drop ceiling right now and i'm stressing about the drop height. I want to put R20 6inch batts on top, so I decided to put the drop ceiling 7 inch down from the JOISTS not from the roof. There is 6 inch of joist which I thought I would leave to fresh air, it's vented at the sides where the over hang is. See photo. The new piece of wood is where i'm ceiling will be. 

I really really don't want to put up the whole ceiling and find out it's not low enough or that I should be putting the batts between the joists. 

The way is at 7inch will mean that the batts should touch the bottom of the joists but not really get squished. the ceiling is 1 inch thick.

What do you think?


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## Bud9051 (Nov 11, 2015)

Sounds like no drywall, thus no air barrier, a very important layer. In your cold climate warm air will be leaking through the ceiling assembly and meet a cold roof, thus condensation and ice.

Where did you get this design? You need a rigid ceiling layer in contact with a good air barrier and the insulation. Insulation levels in Quebec I suspect need to exceed the r-20 mentioned.

What am I missing?

Bud


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## Guardian (May 17, 2018)

R20 apparently is good for flat roof, it's r30 for attic type. I don't know why the diff. This comes from the Gov. website. 

No i'm not putting drywall. This is a garage not a living space. My plan was to put the fiberglass then vaporbarrier and no drywall on top of it. 

The roof itself is membrain and then the wood you see.


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## Bud9051 (Nov 11, 2015)

Not trying to bust your chops Guardian, just help with the proper design of your roof assembly. I believe Canada requires a vapor barrier, but the VB is not your air barrier. An air barrier is rigid and installed in contact with the insulation. The vapor barrier would go on the warm side of the insulation.

Even though this is not intended to be living space, as a garage it can encounter moisture from vehicles, that moisture will go somewhere. If this is a shop and no snow covered or wet vehicles then less of a concern but codes may not differentiate. Best I could find was R-41 for flat or vaulted ceilings.

In addition to the soffit area venting that air needs a place to go, usually a higher vent. Height is a necessary component of natural air flow. 

Not sure what guidance I can provide for what you have described. Probably best to make sure local inspectors approve, they have the final say.

Bud


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## Guardian (May 17, 2018)

"Not trying to bust your chops" But you so are trying.. and its not coming off well. and on top of that you didn't even bother to answer the question. "Not sure what guidance I can provide" I'm pretty sure you can't offer any other then telling me I need a vapor barrier after I just told you i plan to put up a vapor barrier and it doesn't mater if it's a living space blah blah.. not at all what i was asking. If you want to come on here and just tell people you don't know that they are doing everything wrong before you even know what they are doing, just do that so they don't waste time reading your message and trying to get useful info out of it, when none exists. 

Now if anyone else knows if my planed ceiling is low enough for the 6 inch of insulation, I'd love to hear from you.


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## Nealtw (Jun 22, 2017)

Guardian said:


> "Not trying to bust your chops" But you so are trying.. and its not coming off well. and on top of that you didn't even bother to answer the question. "Not sure what guidance I can provide" I'm pretty sure you can't offer any other then telling me I need a vapor barrier after I just told you i plan to put up a vapor barrier and it doesn't mater if it's a living space blah blah.. not at all what i was asking. If you want to come on here and just tell people you don't know that they are doing everything wrong before you even know what they are doing, just do that so they don't waste time reading your message and trying to get useful info out of it, when none exists.
> 
> Now if anyone else knows if my planed ceiling is low enough for the 6 inch of insulation, I'd love to hear from you.


I think bud just wants to understand the system.
Is the garage attached to the house, that would bring code in, in a more important way. To his point about vapour barrier, any warm moist air getting to a cold surface will add condensation and that is why you want it on the warm side of the insulation.


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## Bud9051 (Nov 11, 2015)

I didn't mean to light your candle but I don't appreciate or deserve your flame. I'm not going to try to offer any more assistance so calm down and listen to others here.

Enjoy your project,
I'm out.
Bud


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## Guardian (May 17, 2018)

Awesome, yes and totally understand how vb's work. That's why i'm putting one in. maybe i'm mistaken but does that effect the needed height.. like i just don't get the whole conversation about the vb. i wasn't asking about a vb and when it was brought up, i said i'm putting in a vb. I know i need a vb. I love vb's. no one here ever considered insulating without one. Yes on the warm side. i know. 

What i don't know is the height of my drop ceiling needs to be below the joists?

Do i put insulation in between the joists or leave is over for air like i originally planed. 

my question is one of do i need air flow completely on top of the insulation of if is touches the joists does it matter? or do i want to fill the joists making no room for air, like walls?


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## Guardian (May 17, 2018)

Yeah, Bud. I did think you were being a jerk on purpose. If that is not the case, then you are absolute right that you don't deserve the flame. Sometime in written form things can get vastly misinterpreted.


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## Nealtw (Jun 22, 2017)

Guardian said:


> Awesome, yes and totally understand how vb's work. That's why i'm putting one in. maybe i'm mistaken but does that effect the needed height.. like i just don't get the whole conversation about the vb. i wasn't asking about a vb and when it was brought up, i said i'm putting in a vb. I know i need a vb. I love vb's. no one here ever considered insulating without one. Yes on the warm side. i know.
> 
> What i don't know is the height of my drop ceiling needs to be below the joists?
> 
> ...


 In new construction the roof would have been strapped with 2x4 on flat under the sheeting to provide air flow above joists and insulation.


There all kinds of discussions about unvented, some call for spray foam but then you want drywall over that or for most of them for fire protection.
If you vent it like you are thinking, you would want at least 1 1/2" between joists and insulation and then you have to look at vents to allow air flow thru the system.
I have not considered some of the stuff you are considering.
Here is a discussion on the subject. Good luck.
http://www.greenbuildingadvisor.com...insulating-flat-roof-ventilated-or-unventilat


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## Guardian (May 17, 2018)

Thanks Neal. Interesting read. I think I agree with it that injection or spray could be a nightmare if you got a leak. I also have suffits which I think is not common, (not sure) so making it complete unvented would mean replacing them with something more solid. 

your suggestion of 1 1/2 inch between joists and insulation, i currently did not plan for that. But given what I read, I think i'll go for a few inches of space. I'm reading allot of difficulty air flow. Air doesn't like to move horizontally. 

it's too bad it would be so easy to spray. From past horror stories i'm totally against spray, but i'm going to explore the idea.


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## Nealtw (Jun 22, 2017)

Guardian said:


> Thanks Neal. Interesting read. I think I agree with it that injection or spray could be a nightmare if you got a leak. I also have suffits which I think is not common, (not sure) so making it complete unvented would mean replacing them with something more solid.
> 
> your suggestion of 1 1/2 inch between joists and insulation, i currently did not plan for that. But given what I read, I think i'll go for a few inches of space. I'm reading allot of difficulty air flow. Air doesn't like to move horizontally.
> 
> it's too bad it would be so easy to spray. From past horror stories i'm totally against spray, but i'm going to explore the idea.


 If you spray or go unvented you would just solid block the open spaces above the walls.


I guess you could make Swiss cheese out of the joists and drill holes in them.


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## APA (Jul 13, 2018)

FYI- When it comes to Green Building Advisor, the answer will always be closed cell spray foam covered by an avalanche dollars of cellulose covered by a taped layer of spray foam. Then when you have invested $100,000 and chokes off any air flow in your space, they will advise you to install an air exchanger and use up any utility savings running that thing...lol.


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## jhayKnaufAU (Jul 5, 2018)

Don't you think 6inch of insulation if way too thick for a very low ceiling? Anyway I wouldn't really mind since it is not a living area, but I'm thinking if your garage is detached from your home


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## Guardian (May 17, 2018)

One year later.... and I can't remember why but unfortunately I did go with spray foam. Never again will that stuff be used in my house. thankfully it was just a garage. I now have to pull up the roof and put on a new roof with proper ventilation on top of the sprayfoam. This time not a flatroof which honestly has no business being in a cold climate. I wish wish wish i could go back in time and do the rockwool under a drop down like i originally planned. This is a warning to anyone considering spray foam. You don't come back from that decision so make is very carefully.

Ice accumulated above my sprayfoam by tiny cracks that let condensation in. I know this because IT RAINED inside when it melted in spring!!


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## Dave Sal (Dec 20, 2012)

Guardian said:


> Ice accumulated above my sprayfoam by tiny cracks that let condensation in. I know this because IT RAINED inside when it melted in spring!!





Bud9051 said:


> Sounds like no drywall, thus no air barrier, a very important layer. In your cold climate warm air will be leaking through the ceiling assembly and meet a cold roof, thus condensation and ice.
> 
> Bud



Looks like Bud was right after all.


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## Guardian (May 17, 2018)

Dave Sal said:


> Looks like Bud was right after all.


Not only is there drywall.. but I also caulked every crack i could find hoping that like people like Bud had said that an air barrier was possible. it's not. Hot air inside - cold air outside only needs a millimetre hole like from an old screw or like the giant gap between the wall and foundation shelf along the wall. a thousand and one places for air to get sucked up and impossible and silly to try to plug them all. And I didn't even want dry wall i had to put it in because of the fire code. So not only did have an ugly drywalled garage, but now it's crumbly wet plaster sandwiched between 2 pieces of paper. Huge mistake. Now if i have to use foam, and i don't like to because its so flammable, i use rigid foam and then just a can of spray foam to seal it. That way i can rip it out if need be. There are so many foam contractors out there pushing the virtues of spray foam. and lobby groups like green building advisor appears to be. beware.


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