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Lots-o-work
What to use for vapor barrier?

I live in chicago and believe that I need to add a vapor barrier to the walls of a rehabbed sleeping porch. I recently added open cell spray foam to the walls and am trying to figure out what to use to create a vapor barrier. I went to the local Home Depot and they did not know what to use so now I am confused because I thought this was a common thing.

Anyway, what should I use for a vapor barrier? Any specific type of plastic sheeting, calk and tape? Obviously I prefer something that I can pickup at the local hardware store.

I appreciate any advice that you can offer.

canuk
Re: What to use for vapor barrier?

Hmm --- nothing surprises me with HD ----- 4 or 6 mil poly is the common material used for a vapour barrier.
If you can find house wrap tape and acoustic seal caulk pick up those as well.
Run the acoustic seal caulk along the bottom plate and floor junction as well as the top plateand corners. Staple the poly to the top and bottom plates and along the studs. Wherever you have seams overlap them by at least 6 - 12 inches and use the house wrap tape to seal the seams and around window frames.

RJordan
Re: What to use for vapor barrier?

Is the vapor barrier required by code? You want your wall to be able to dry to one side. In most cases the plywood ,OSB and housewrap will be a vapor barrier on the outside. Which is to say you want the wall to be able to dry to the inside. You could spray the foam with a vapor barrier paint or paint the drywall with an oil primer.

canuk
Re: What to use for vapor barrier?
RJordan wrote:

Is the vapor barrier required by code? You want your wall to be able to dry to one side. In most cases the plywood ,OSB and housewrap will be a vapor barrier on the outside. Which is to say you want the wall to be able to dry to the inside. You could spray the foam with a vapor barrier paint or paint the drywall with an oil primer.

Have to disagree ---- they are not vapour barriers but rather retarders -- which means they are vapour permeable.

RJordan
Re: What to use for vapor barrier?

The residential code refers to the need to have a vapor retarder, and that is why I used retarder rather than barrier. Building scientists distinguish between retarder (preferred) from barrier as even very impermeable materials cannot be installed in a way that is a barrier.

However, half inch plywood or OSB have perm ratings of less than 1 and are thus considered vapor barriers by the code. Because of their low permeance, I would not want to put a vapor barrier or rertarder on the interior.

canuk
Re: What to use for vapor barrier?
RJordan wrote:

The residential code refers to the need to have a vapor retarder, and that is why I used retarder rather than barrier. Building scientists distinguish between retarder (preferred) from barrier as even very impermeable materials cannot be installed in a way that is a barrier.

However, half inch plywood or OSB have perm ratings of less than 1 and are thus considered vapor barriers by the code. Because of their low permeance, I would not want to put a vapor barrier or rertarder on the interior.

Actually you did say " vapor barrier " orginally.
The plywood sheathing has a class II rating vapour retarder and can change to class III permeable due to age and/or saturation.
House wrap is a vapour permeable air barrier. At least that's the way it's considered around here.

Just saying and not arguing.:)

RJordan
Re: What to use for vapor barrier?

I hope I'm not getting everyone confused by now as I have gotten myself all twisted up. I agree with everything in your last post. Most building codes call for a vapor barrier when, we both agree, they should be calling for a vapor retarder. Most housewraps, as you state, are not vapor barriers or retarders.

Thanks for the correction in case anyone was about to follow my previous mis-statement.

Lots-o-work
Re: What to use for vapor barrier?

So now I am really confused! Fortunately it;s because you both are more knowledgeable than I so I want to lay out what I know and see what you recommend. The reason I want to apply a vapor barrier is because I watch a lot of Holmes on Holmes and he always uses one. There I go following TV shows again.

Anyway, the sunporch was gutted all the way to an outer layer of sheeting, again this is an old porch, not necessary similar construction as a traditional room. So there is no house wrap. It is hard to remember exactly what the outside wood layers exactly consisted of. From what I remember it was one layer of, maybe 6x1 and then on the other side was the outer wood siding.
The open cell spray foam was applied directly to the inside layer of wood.

So, do I add a vapor barrier? If so, where do I get it and what exactly am I looking for? And when I screw in the drywall, and I pierce the plastic, how will this affect the true barrier?

Thanks again for all of your help!

RJordan
Re: What to use for vapor barrier?

Sorry for our quibbling over semantics. The final arbiter is your building official. My opinion is that you do not need to put anything on the foam. Moisture movement into walls is mostly brought by air movement. Air doesn't move through open cell foam. Vapor will slowly diffuse through it though. The relative humidity wants to be the same in the open cells as it is in the air in the room. So some small amount of moisture will find its way in when the humidity is high and will be released when the humidity is low. Over the course of a year this shouldn't be a problem. If there is a lot of humidity in the room for most of the year, I would put on a vapor retarder and you could spray on a vapor barrier paint. An oil primer should do the trick as well.

canuk
Re: What to use for vapor barrier?

I too apologise for adding to any confusion and hope to not add any more.

In addition to RJordan comments --- this is an answer that should and definately be answered by both the manufacturer ( or the installation company ) and /or the building inspections office for your locale.

For the reasons mentioned certain areas can get away from the need to install an additional vapour barrier with open cell spray foam. Depending on the region covering the foam with primered drywall covered with 2 coats of paint will add to become a vapour retarder. In cold regions --- where I'm from for example --- an additional vapour barrier ( like 6 mil poly ) is required because of the dynamics of extreme temperature , pressure differentials , and vapour drive which are more signifigant variables here than your area.

As you can see it's really a little more complicated to answer with the spray foam than fiber glass batt insulation.

Hopefully this helps.:)

Lots-o-work
Re: What to use for vapor barrier?

I will check out the HD this weekend and see what they have...once more.

Thank you both for the information. I think I can take it from here. Stay tuned!

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