Vapor Barrier 101 requested

[jake26]

Hi Gang,

Whenever I post stupid questions, I always feel the need to explain that I am a poor Noob who has no experience in construction. Since I have found this site, it has given me courage to try some home improvement on my house that went untouched for 10 years.

Anyhow, I am remodeling my dream garage based on a thread I found here and I am done with the attic storage (which was a learning experience in itself), the electrical and now I am starting the insulation but I have found some snags that I did not foresee.

1- How do you seal the vapor barrier to the wood? Tyvek Tape does not stick to anything but plastic it seams
2- Do you have seal the seams or is overlapping good enough?
3- How do you seal around the electrical boxes?

There is a part of me that thinks, "Who cares, it is only a garage" and another part that thinks, "You should do it right" ... or as right as I can

Any help is appreciated. Thanks!!!

 

[Zeke]

There are tutorials on the Net. Use staples to hang the stuff and do accurate cuts and the seal will be "gutenuf." The overlap instructions are right on the sheet, no?

 

[1991Syclone]

Staple the vapor barrier to the studs/trusses and cut out around any outlet/light box. Make sure to tape any overlaps or cuts that you may have made. I believe you can tape around the sides of the electrical boxes as well. Here's some visual on the electrical boxes

 

[jake26]

Thanks for the replies!

I did a search on the web and found no information on the gang boxes except those gang boxes (such as Syclone showed) with the flange for taping. This confuses me because my 10 year old house was built without such gang boxes and I didn't use those to do my electric. Being a garage, I do not need it air tight but I do want to protect the insulation from condensation.

If staples are "gutenuf", then that is fine for me because I know my house went through inspection and I thought the barrier was only stapled with seams overlapped, I thought I would check first.

 

[bigdav160]

No location for the OP.

Are you talking about an exterior house wrap or a vapor barrier as shown Syclone's pics?

If you want to do it "right", the vapor barrier should be on the warm side of the insulation. Inside for northern states, outside for southern states.

 

[jake26]

Hi Big Dave,

I am a proud Wisconsinite and I am taking about an attached garage and all the walls I am insulating are exterior walls. At least I have the vapor barrier on the right side of the wall.

 

[mypov]

make sure that you don't use regular "caulking" because after time it gets solid - the proper adhesive for vapor barrier is acoustical seal it's horribly messy stuff - it's black and if you're not careful you can track that stuff everywhere...But the good thing about this stuff is it maintains its flexibility, will continue to hold the vapour barrier to itself and if applied in the correct amount will keep it's air tight seal almost forever...
good luck!

The other thing to mention as well is you should Tuck Tape (the red really sticky tape) the seams...You can get poly hats for any electrical gang boxes you may have, these are tuck taped as well to the vapour barrier and the electrical wires are poked through the hats and into their perspective boxes and any gaps are siliconed or tuck taped... but that's if you want it really air tight. Doesn't sound like you're going taaat extreme.

If you are heating your garage though any additional effort you put forth now will save you piles of money in the future for heating and cooling. Might as well do it right at this stage, but that;s just me.

 

[mypov]

X2 on the bigger tubes!!

 

[dirttracker18]

Yep, X 3 on the sealant.

Tuck tape all other joints, garage or not. Do it right or don't do it at all

 

[Zengineer]

Handy tip, don't try to use your teeth to start a rip of Tuck Tape off the roll... carry a knife for that. Otherwise you'll end up with some lost skin around your mouth.

 

[rsa]

Easy reads:
http://www.greenbuildingadvisor.com/blogs/dept/musings/forget-vapor-diffusion-stop-air-leaks
http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers

Still pretty easy, but longer: http://www.buildingscience.com/resources/vapor_barrier_code_changes

Short version: Building scientists today say that in Wisconsin you should omit the vapor barrier entirely, but make sure you air seal the walls. You want the walls to be able to dry both to the outside and inside.

 

[jake26]

Easy reads:
http://www.greenbuildingadvisor.com/blogs/dept/musings/forget-vapor-diffusion-stop-air-leaks
http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers

Still pretty easy, but longer: http://www.buildingscience.com/resources/vapor_barrier_code_changes

Short version: Building scientists today say that in Wisconsin you should omit the vapor barrier entirely, but make sure you air seal the walls. You want the walls to be able to dry both to the outside and inside.

Thank you very much for the post. However, it just added to my confusion. Do I need to apply plastic to my insulation and seal it or not? As this gentleman writes, there is confusion in defining "vapor barrier" and "air barrier".

"Thanks for the informative blog. I just wanted to point out that in many places, especially northern climates, that the poly barriers are detailed and sealed because they are not only doing the job of "vapor barrier" but as the primary "air barrier" as well. It is by far the most common method used around here. Almost no one actually calls it an "air barrier" though...almost always referred to as " vapor barrier" which I believe adds to the confusion..."


I assume adding the plastic will create an air barrier that is just as important. Am I not understanding? A little help please.

 

[rsa]

I assume adding the plastic will create an air barrier that is just as important. Am I not understanding? A little help please.

The air barrier is very important, but doesn't justify putting a condensing surface like polyethylene sheeting behind the drywall in your Wisconsin climate. Here's an eye-opening (for me at least) translation of a building science podcast discussing air and vapor barriers.

So how do you achieve an air barrier if you don't use polyethylene sheeting? One possibility is to use the "Airtight Drywall Approach" developed by Dr. Joe Lstiburek. If you want a plastic sheet attached to the interior side of the studs solution, there's MemBrain.

Some more reading that might help later: The first part of Understanding Vapor Barriers covers the main points quickly. When Sunshine Drives Moisture Into Walls is instructive and describes a worst case scenario, when a large home builder went bankrupt as a result of uses of polyethylene sheeting behind the drywall. Vapor Barriers Are a Good Thing, Right? has a lengthy discussion in the comments section.

Not every misuse of polyethylene sheeting is going to ruin a house. I made the vapor-barrier-on-the-inside mistake when I insulated my 12' x 16' shed/workshop in 2000. I haven't been punished by the mistake since it's not air conditioned. In the next couple of years, I'll probably add A/C and you can bet I'll pull the poly out. Luckily, I used screwed plywood and OSB instead of gypsum board to sheath the interior so it's not a big deal to pull the polyethylene.

I'm neither a contractor or building scientist. I'm like you. When I build my garage, I want to do it as right as I can, so I've been doing a lot or reading.

 

[camarotoolman]

I have
tyvek on the outside, plastic on the inside, 6" pink in the walls, sun barrier and12" in ceiling, double pain windows, good doors, used smaller windows, planted shade trees, built a smaller house and have the cheapest electric bill in my hood.

 

[bgarrett]

If you want to do it "right", the vapor barrier should be on the warm side of the insulation. Inside for northern states, outside for southern states.

Where is the line between north and South? Where do you rate southern Missouri?

EDIT: Nevermind, I found it
Thanks to the posters of this excellent info

 

[Steevo]

I have
tyvek on the outside, plastic on the inside, 6" pink in the walls, sun barrier and12" in ceiling, double pain windows . . . . .

That's good. No Pain, no Gain!

 

[Red Leader]

Jake,

Like you, I have wanted to know more about this stuff, but the more I have read, the more confused I've become! It seems that most of the information is extremely dependent on location and climate. It also seems that a lot of this information is very contextually based (historically and culturally). What was practiced religiously 20 years ago isn't fine now and what is practiced religiously now won't be fine 20 years from now, or so it has seemed to me based on what I've read. With that in mind, I'd hate to follow everything to a 'T' just to hear in 10 or 15 years that everything we knew back then was wrong and has created more problems.

Also, most of the comments I've read (take it all with a grain of salt) have indicated that older, more drafty houses haven't had the same moisture issues that some of the newer, more modern structures with tighter moisture tolerances have. It seems that every time I think about doing something with a moisture barrier, I read something about how a moisture barrier ended up creating problems.

Again, this is all from my limited understanding. I am a fellow student in trying to learn the best I can about these things, but one thing I have learned for sure is to do your homework and be wise regarding the latest and greatest building practices.

I still have a lot of reading to do. I put kraft-backed paper insulation in when I finished my garage and decided that should be just fine.