pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 4:15 p.m.

I went looking for a thread on this and couldn't find one so I'm starting one now.  I'm planning out the insulation & wall covering aspects of my garage.  I'm using rolls of pink R-13 meant for use in 2x4 stud walls, kraft paper faced, stapling as I go.  And then I remembered the words "vapor barrier".  I do some reading and it seems like 6 mil plastic sheeting should work?  Give it some overlap over the insulation, staple at the studs, but then there's a new wrinkle; how much effort should I put into sealing this stuff off?  I see tape, I read something about using caulking to form a seal, what makes sense here?  For reference I'm in KS, I do have a heater in the garage that I use, and I may add A/C at some point.

For wall coverings I'm this close to pulling the trigger and buying a bunch of 4x8 whiteboard sheets and using that besides the area around the heater which I'll cover with drywall due to the greater heat that area will be exposed to.  So the stack-up from the outside in is insulation between 2x4 studs, then a sheet of plastic, then a 4x8 sheet with whiteboard material facing out towards the room.  

Where am I screwed up?  What should I do differently, if anything, with that vapor barrier plan?  Can I just staple it down to the studs or do I need special tape (and if I do need tape, please tell me what I need here) to make it seal better between sheets and around stuff like doors and the breaker box etc).

Thanks!

Jesse Ransom
Jesse Ransom GRM+ Memberand UltimaDork
11/17/22 4:42 p.m.

I'm... The opposite of an expert, but this differs so much from my limited experience I thought I'd post.

The only membrane/barrier I'm aware of is the "house wrap" over the sheathing and under the siding on the exterior. My impression is that it should be waterproof but vapor permeable. In the new shop I used Benjamin Obdyke Hydro Gap, which has a bunch of little bumps to make sure there's drainage and air between the wrap and the siding.

On the interior, it was non-kraft-backed fiberglass batts, and then drywall. No further barrier.

This was all to local code (Oregon), as specified by an architect, with both insulation and drywall implemented by the same established contractor (i.e. no opportunity for dropping a step handing off between those phases).

This combo jibes with my understanding that the rain and wind need to stay outside the structure, but the whole thing does need to "breathe" a little to dry out when appropriate. It sounds odd to me to put a non-breathable barrier between the interior and the insulation, or in fact to have any totally non-breathable membrane.

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 5:06 p.m.

In reply to Jesse Ransom :

I really appriciate your chiming in with your experiences and sharing what you were told by an expert.  I have little experience in this area (where little means zero) and I'm glad I can ask here for help.

Here's the site that has me thinking about putting up a vapor barrier on the inside, facing the room;

https://www.cielowigle.com/blog/how-to-insulate-a-garage/

See the image where everything is under a sheet of plastic?  It's described that way as well.  I just started on this project so it is easy for me to stop now and try to make sure I'm doing the most correct thing so here I am.  Agreed that letting vapor/moisture out seems smartest... but here we are with the above reference.  

03Panther
03Panther PowerDork
11/17/22 5:13 p.m.

My 40 years ago info. remembers that the Kraft paper (facing the "in use by people" direction) is the vapor barrier to protect the pink stuff from "our" moisture laden breath!

there is probably more modern interpretation available 

93gsxturbo
93gsxturbo UltraDork
11/17/22 5:58 p.m.

My stick built garage has tyvek housewrap on the outside between the sheeting and the insulation without facing, then a clear plastic vapor barrier, then 5/8 drywall, taped and mudded like a house.  

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 6:06 p.m.

Maybe that's the situation, where if the insulation is faced with kraft paper, I don't need to also have a layer of vapor barrier aka plastic over it and under the drywall/whiteboard/etc?

jimbbski
jimbbski SuperDork
11/17/22 6:08 p.m.

Like someone mention the interior vapor barrier is to keep moisture generated on the inside of an insulated room away from the exterior wall surface.

This moisture can travel through the insulation and then freeze on the inside of the outside wall. Not good.

This is also why the attic of a house is vented so that moisture that does get into your attic has someplace to go rather then  condense and freeze to the underside of the roof.

It's not just to keep the roof cool in the summer. 

If your garage is free standing and is not going to be heated constantly then I wouldn't worry, but it's not a bad thing to do.

If the garage is attached to your house then I would cover the insulation with the 6 mil plastic and tape it at the edges. 

03Panther
03Panther PowerDork
11/17/22 6:52 p.m.
pres589 (djronnebaum) said:

Maybe that's the situation, where if the insulation is faced with kraft paper, I don't need to also have a layer of vapor barrier aka plastic over it and under the drywall/whiteboard/etc?

Completely true 30 plus yr' ago. That was before high tech Dino oil was used to replace renewables like Kraft facings!

Seriously, though, I'm sure there are better ways, today; I just don't know the most correct one!

03Panther
03Panther PowerDork
11/17/22 6:58 p.m.

In reply to jimbbski :

Very good explanation!

never doing any building far enough north to worry about long term extreme freezing, I would not have thought of that... only the damage to moisture itself does. But great point!

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 7:37 p.m.

In reply to jimbbski :

The garage is free-standing and won't be heated constantly.  But it also, as you mentioned, seems like not a bad thing to do.  Probably won't be perfectly done but I can try without going insane.  The plastic I've spotted; what tape am I looking for?  This stuff always has some special name...

Did you ever tell us what the existing walls are built of, and what state you live in?

The plastic in the inside over the kraft paper is probably overkill, especialy if you put up plastic panels.

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 8:07 p.m.

Existing walls are open 2x4's.  I live in Kansas.

There must be some sort of exterior siding/brick/logs/etc on the other side of those 2x4s?

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/17/22 8:21 p.m.

Oh.  Yeah.  Currently there's garbage Masonite on the outside but that's getting swapped by a crew this spring.  Some kind of black sheathing under the Masonite.

Honsch
Honsch Reader
11/18/22 1:36 a.m.

If the space will only be occasionally heated then there woun't be a "warm moist" side enough to care about.

I assume the garage is somewhat drafty?  Shortly after you turn off the heat the humidity will equalize and things can dry out.

STM317
STM317 PowerDork
11/18/22 8:37 a.m.

In Kansas, you'll probably want a semi permeable moisture barrier (tyvek, etc) between the framing and exterior siding, and an impermeable vapor barrier on the inside of the insulation under the wall sheathing. The moisture barrier will keep bulk water from entering the wall cavity, but still be permeable to allow any moisture vapor that enters the wall cavity to dry to the exterior. The vapor barrier is impermeable and prevents moisture from passing between the interior of the building envelope and the wall cavity.

Now, technically, Kraft facing can be a vapor barrier. You'd need to seal off the seams and edges. When you're sealing a seam along every stud, that's a lot of effort and material cost. So most people would use unfaced batts and sheet plastic to have fewer seams to worry about.

Rigid foam panels (or spray foam) of proper thickness can be a vapor barrier as well, while also cutting down on convective currents within the stud bays that reduce effective R value. (They cut down on drafts more than batts do, which makes the building more comfortable and easier to heat/cool). Foam is an awesome insulator, but it's going to be more expensive than batts.

It sounds like the exterior siding might be replaced too, which also opens the possibility of using rigid foam panels on the exterior, under the siding. The will insulate and air seal really well, but would potentially move the vapor barrier to the exterior of the wall which would mean you shouldn't have any barrier on the interior of the wall.

The key that needs to be highlighted is to avoid having more than one impermeable vapor barrier. If you've got a true vapor barrier on the outside of your wall construction, then you don't want to add any on the inside as well.

pres589 (djronnebaum)
pres589 (djronnebaum) UltimaDork
11/18/22 9:30 a.m.

Exterior siding is definitely getting replaced; not this minute but hopefully in the next six or so months. 

Sounds like I'm picking up some 6mil plastic.  There's "housewrap tape" at the big box stores that I'll grab at the same time unless I get told otherwise.  I'll make sure the contractor knows what I've done on the inside so that we don't end up with two impermeable vapor barriers on both sides of the walls. 

MadScientistMatt
MadScientistMatt UltimaDork
11/18/22 12:54 p.m.

Good read on the subject here: https://www.energyvanguard.com/blog/you-don-t-need-a-vapor-barrier-probably/

VolvoHeretic
VolvoHeretic GRM+ Memberand HalfDork
11/18/22 2:04 p.m.
03Panther said:

My 40 years ago info. remembers that the Kraft paper (facing the "in use by people" direction) is the vapor barrier to protect the pink stuff from "our" moisture laden breath!

there is probably more modern interpretation available 

Craft faced insulation is in noway a vapor barrier. Cover it with 6 mil polyvinyl sheeting.

03Panther
03Panther PowerDork
11/18/22 7:31 p.m.
VolvoHeretic said:
03Panther said:

My 40 years ago info. remembers that the Kraft paper (facing the "in use by people" direction) is the vapor barrier to protect the pink stuff from "our" moisture laden breath!

there is probably more modern interpretation available 

Craft faced insulation is in noway a vapor barrier. Cover it with 6 mil polyvinyl sheeting.

40 years ago, the Kraft paper on the batts, designed to go between 2 x lumber set on 16" centers with the paper side facing the "occupied" side, was , indeed, considered the vapor barrier. I can also stipulate that the facing called Kraft at that time looked and felt exactly the same as the Kraft facing of today. 
I did mention, if you read it, that my 40 year old information has probably been superseded my more modern materials and methods. 
So ease up on the absolutes, if it's not too much to ask. I feel I was quite clear in my info. 

03Panther
03Panther PowerDork
11/18/22 7:51 p.m.
pres589 (djronnebaum) said:

Oh.  Yeah.  Currently there's garbage Masonite on the outside but that's getting swapped by a crew this spring.  Some kind of black sheathing under the Masonite.

The kinda black sheathing, if approx. 1/2" thick, 4' x 8' sheets, used to be used a bunch under siding. Called Celotex by brand name. 
Was supposed to be used on the bulk of outside walls, with plywood in the corners. 
My house in VA, built in '50, had Celotex on all exterior (including corners) with 1" lap strake wood siding. No insulation in the 2 x 4 walls, since oil heat was so cheap. 

VolvoHeretic
VolvoHeretic GRM+ Memberand HalfDork
11/18/22 11:59 p.m.

In reply to 03Panther :

I apologize, I never intended what I said as a personal attack. Craft faced F.G. insulation has a useful purpose as an easy way to attach insulation in a horizontal direction. I just wanted to emphasize that it is not a vapor barrier. The way that a water and air infiltration barrier works is by having the spaces between the fibers in Tyvek or the little holes punched in the other plastic house wraps being too small for water to penetrate through because of water's surface tension being to great to go through the holes, but lets water vapor pass through. It's the same way that plastic dock floats have a brass plug in there tops with a tiny hole drilled in them that lets air move in and out of the float as they heat up and cool down, but doesn't allow water to enter. Anyway, sorry.

03Panther
03Panther PowerDork
11/19/22 12:54 a.m.

In reply to VolvoHeretic :

No worries! I'm super sensitive atm, with thing circling the drain for my ... well anyway! Trying to always be polite though, and hope I was!

And the modern tyvec theory is 100% correct!
But before hi tech Dino products took over everything, the old timers I knew  - most have died by now - considered the Kraft facing to be the vapor barrier, that needed to face the occupied space. 
I agree there are better materials today, but it WAS the norm

All the rolls of faced "pink" insulation for 2 x 4 or 2 x 6 studs, that I've used, was designed to be installed vertically between studs set on 16" centers. The flaps on the edges fold out, to staple to the interior part of the stud, so that the Kraft facing will face the living space. Never seen it run horizontally, on a wall. 
in the attic, the facing is designed to go against the ceiling, even though, after the fact, it would be easier to put the facing up. The reason it used to be done that way, was cause the old timers had been taught the vapor barrier faced the living area. 
same for crawl space. Would be easier to (after house was built) put the facing down, and staple the flaps to bottom of floor joists. There is a reason the old timers did not do that. 
Again, the disclaimer that there are modern materials and methods that are better. There are probably younger construction guys, that don't know why it's installed the way it is, since I faced makes more sense today!

Sorry, OP, I know this ain't answering your question, but the thought exercise may help think of what vapor from breathing and condensation might do laugh

You'll need to log in to post.

Our Preferred Partners
IwzC76UV3NClS7VTXZQGsZRqmz33SxKSe9Omn7WuWHUBN0MGeR49f3gkFulxSLV7