Wrapping walls with plastic before drywall??

[Autorotica]

One other thing, if spray foam is its own vapor barrier, doesn't it make 2 vapor barriers, the outside surface AND the inside surface?

Thinking out loud...
Chris

 

[Krodad]

Forgive my ignorance, but I thought the issue is vapor barrier (not retarder) on both sides of the insulation. That creates a space where IF vapor gets in there it cant get out...

If you have plastic on the inside and a permeable membrane on the outside, the moisture can always move OUT as it has opportunity.

The other point, is there a vacuum sucking the outside air into the building? Fiberglass insulation make a great dirt filter but it doesn't stop air from moving. Where is all this WIND coming from that is placing so much entrained vapor against a surface?

Chris

You hit the nail on the head Chris!

In theory the insulative value of whatever material is in the walls would keep the dewpoint from occuring next to the interior wall surface...which basically never happens with batt insulation. Too many gaps and that batt does indeed, in many cases, just act like a big air filter. If air is moving in that cavity, the insulation is not doing any good.

 

[K588]

After reading eveyones post a few times it is similiar to the advise I was getting verbally and reading on the net. I think I am just going to stick with the Kraft faced insulation and skip the plastic barrier. I appreciate all the responses and views given.

 

[Sims5]

I was going with r19 unfaced wand then poly wrap in the walls and decided on r21 faced without poly wrap and the difference was $600 for my 60x96x16 building. I felt like the r21 was worth it for the years I'll be in my building.

 

[DynoDave]

From the outside in we have: siding, house wrap (tyvek), 1/2" OSB, 2x6 framed walls, r21 insulation, caulk air seal on all framing, Kraft paper, drywall, and paint.

English...sweet!

But still have a question, because I'm just that slow. Where you say "caulk aur seal on all framing", this is to seal the kraft paper edge to the wall studs and plates?

 

[larry4406]

English...sweet!

But still have a question, because I'm just that slow. Where you say "caulk air seal on all framing", this is to seal the kraft paper edge to the wall studs and plates?

Negative.

The description below is what we do on residential homes in MD and northern VA. Your mileage may vary but we are Energy Star and HERS rated.

The kraft paper is simply sandwiched between the studs, plates, and drywall backside.

Prior to insulation, all vertical borings in stud cavities are fire stopped with fire retardant foam/caulk to prevent deck to deck leakage. All multiple ganged vertical studs are caulked where the studs mate to prevent air infiltration between. Same for bottom plates to the deck, double top plate plies, and headers.

After this air seal package, the insulation is applied. On our houses, we spray foam all of the band boards as that is a challenging area to insulate. At the 1st deck band board, we also overlap the sill plate with foam and get the top 2 or 3 inches of the poured concrete foundation as the sill plate is challenging to seal. We the proceed to insulate the bands with fiberglass after the foam seal. Our wall cavities are R21 kraft faced in 2x6 walls. Our ceilings are blown to R49 after hanging drywall.

When installing drywall, we ensure that all of the top and bottom plates and vertical members are continuously glued and screwed; this not only reduces nail pop warranty issues, but also acts as a gasket to reduce air infiltration. On ceilings, all nailing surfaces are continuously glued and screwed for the same reasons.

After drywall is hung, we caulk seal all penetrations to the drywall (registers, grills, outlets, can lights, plumbing, etc) to further reduce the potential for air in leakage.

We also ensure that all components of the hvac systems are 100% inside the thermal envelope (i.e, no attic units with attic ductwork supplying 2nd deck).

Lastly, at the very end, we do a blower door test and check for air in-leakage.

Hope this clarifies/helps.

 

[Sims5]

A friend of mine had sprayed on form directly on the inside of the wall steel. When he cut a hole to run. The exhaust from his boiler he found that the steel was soaking wet between the foam and the steel.


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[ishiboo]

Perfect example of how codes and overall building philosophies can be different even though we are really aren't that far away geographically. I am not one of those "you should build it to code" people in all circumstances but if in doubt I would humbly suggest following code for your area.

Or you're confused.

Tyvek (house wrap) is a moisture barrier. It breathes and does NOT trap vapor, air, etc. House wrap on the outside, vapor barrier inside next to the drywall.

As we go to maximize energy efficiency, we've moved away from paper which tears easier and went to plastic to minimize air infiltration. It's simply easier to put the insulation on and not worry about keeping the paper tight/etc., and then do the plastic at the end and tape it. Paper really isn't bad, though anything organic can develop mold you suppose, but then you go screw a sheet of drywall which has paper on each side to it.

(Interestingly enough, properly installed drywall is basically air tight as well.)

In stick built, it's pretty easy - one vapor barrier inside over the insulation. Tyvek or at least tar paper for moisture would have been nice outside. If this were an air-tight metal building, than the debates could begin...

 

[ishiboo]

This really is a simple matter. The general rule is that plastic visqueen type barriers are to be avoided unless you live in an extremely cold climate like Alaska or Canada or Montana. Warm moist air moves through a wall assemble and will condense on the first cold structure that it finds. The vapor barrier on the inside works in these extemely cold regions because they are heated on the inside predominantly. Mike Holmes is not someone to emulate for most of the US as he is dealing with frigid climate.

If you have AC and you have that vapor barrier on the inside you just created a disaster. The warm moist air from the outside moves right through the tyvek and hits the cold dry inside edge of that plastic barrier on the inside and condenses in the wall.

Unless you live in a frigid climate and have no AC you shouldn't have any vapor barrier at all. Vapor barriers are to be avoided in general and only vapor retarders should be used according to your climactic zone and whether you are going to use AC as well as heat. In Maryland with the plans for AC of the OP he probably should use no more than kraft faced insulation.

Here in SoCal we would use unfaced insulation. The idea is to not trap moisture and if any gets in to allow the wall to breathe to dry out.

Not really. We're in a dual heat/AC climate.

If there were a very cold surface it would be an issue. However, insulation produces a temperature gradient… it is one temperature on one side, another on the other, and gradually changes throughout the dimension.

Without the temperature gradient, it's just as much an issue in a cold climate… turn the furnace on, turn the humidifier on, and watch water condense between the drywall and vapor barrier IF the vapor barrier was exposed to cold.

With proper insulation it is not an issue.

 

[brad900]

Depends where you live. I have no vapor barrier becuase of my humid area.

http://www.naima.org/insulation-kno...nsulation/insulation-and-vapor-retarders.html

 

[DynoDave]

Negative.

The description below is what we do on residential homes in MD and northern VA. Your mileage may vary but we are Energy Star and HERS rated.

Thank you for the clarification.

 

[bullnerd]

A friend of mine had sprayed on form directly on the inside of the wall steel. When he cut a hole to run. The exhaust from his boiler he found that the steel was soaking wet between the foam and the steel.


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WOW, no way I would spray foam against the steel!

 

[Moss]

Nope not that confused I pretty much agree with everything you have typed. Maybe my posts were unclear but I was trying to keep it simple for the OP.