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Last word on vapor barrier in midwest?

rasher's picture

I wish I could understand this issue enough to feel confident:

I live in Kansas City, Missouri. We have freezing cold, dry winters with the humidifier blasting away. Then we have suffocatingly wet hot summers with the AC working away to cut the humidity.

I've got an exterior wall on my uninsulated house house opened up on the interior side and I'm going to install some fiberglass batt insulation. I also have the siding torn off to reside, so I'm going to install 15# asphalt paper under the siding.

Which side of the wall do I install the paper side of the fiberglass batt?

Thanks!

(post #106717, reply #1 of 38)

Paper to the sheetrock.

But also address the reason you need to run a humidifier in the winter with a blower door and duct blaster test. Your low humidity is created by cold dry exterior air being pulled into the house to make up for hot moist air escaping likely through your ceiling plane. When your furnace warms up that exterior air its relative humidity drops and causes the problems you are trying to address with the humidifier. Cut down on bulk air exchange and you'll reduce your need to run the humidifier and improve year-round HVAC efficiency and indoor air quality over-all.

All this starts with an energy audit.

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"You cannot work hard enough to make up for a sloppy estimate."

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"You cannot work hard enough to make up for a sloppy estimate."

(post #106717, reply #2 of 38)

You install the vapor barrier on the warm side.

If you install it on the cold side, the interior moisture will migrate through the drywall and fiberglass insulation. When it hits that cold moisture barrier it will condense into small beads of water. You've seen these beads on your window panes. The water will run down and create a nice wet condition at the sole plate. It will rot everthing down there. Also, the wet insulation will lose some efficiency.

Bob's next test date: 12/10/07

(post #106717, reply #5 of 38)

I know everyone says to "install the vapor barrier on the warm side". However, in the midwest, the "warm side" is to the exterior from June 15 to September 15 and to the interior from November 15 to April 15. So which is it? Vapor barrier to the exterior or interior?

And I understand that need for continuous vapor barrier in the form of plastic stapled to the face of studs over unfaced FG batts. However, many owners of 100 year old homes around here swear by blowing in cellulose insulation into the stud cavities. When you do that, where is the vapor barrier?

In my particular situation, I am looking at eventually tearing off the existing siding, rewrapping the exterior with asphalt paper and residing the whole house with Hardiplank. At most locations, I can't tear out the existing plaster at the interior to install plastic vapor barrier. With these conditions, is there sense in blowing in cellulose while I have the siding torn off? Or will I be introducing the danger of moisture getting trapped in the wall?

(post #106717, reply #6 of 38)

I would blow the cellulose and use some vapor barrier paint.

(post #106717, reply #7 of 38)

Vapor barrier paint? Isn't all latex some sort of vapor barrier? See, that's why I think the conventional logic about having only one "vapor barrier" in a wall assembly is flawed. I every wall, there are several "vapor retarders" and no vapor "barriers".
You've got the paint layer. The paper attached to FG batt insulation. The Tyvek or Asphalt paper. The exterior paint. All serve to "retard" vapor permeation, but none are impervious.

(post #106717, reply #8 of 38)

Tyvek isn't a vapor retarder to any significant degree.


It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #106717, reply #27 of 38)

'See, that's why I think the conventional logic about having only one "vapor barrier" in a wall assembly is flawed. I every wall, there are several "vapor retarders" and no vapor "barriers".'


One vapor barrier or alternative wall assemblies using multiple retardants- what is important is to have a strategy to deal with the moisture that can damage walls.


You can do this in a number of ways. The one that we use here is to have one vapor barrier (6 mil poly) on the inside to stop moist air from migrating through the wall. The outside is left porous, with gaps in the sheathing and breathable building paper, to allow any moisture in the wall to dry to the exterior.


It is the conventional approach which is being challenged by approaches using rigid insulation and exterior sheathing as air barriers, or as Matt suggests allowing the wall to dry to the inside. However, if you are content with the fairly inefficient wall that results from batt insulation, this wall assembly is the one most commonly in residential construction, and properly built has few problems.


Edited 7/26/2008 7:08 pm ET by fingersandtoes

(post #106717, reply #3 of 38)

Vapor barrier goes to the inside. Inside or outside only becomes an issue farther south (eg, in FL), in areas where you'd commonly see condensation (not just morning dew) on the outside of windows during much of the summer.


It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #106717, reply #4 of 38)

Only homeowners use paper or foil faced fiberglass around here. Unfaced batts are the norm with a poly vapor barrier on the inside. Without a continuous and effective vapor barrier, there will be condensation in the wall cavity. Wet insulation looses it's effectiveness and creates the conditions for mold growth or worse. The insulation should fit correctly, without stuffing and make contact with the surface of the drywall.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match

(post #106717, reply #22 of 38)

>> Only homeowners use paper or foil faced fiberglass around here. Unfaced bats are the norm with a poly vapor barrier on the inside.  <<   Careful how you dole out your good intentions...  We used to use poly vapor barriers here but it went out of vogue because a lot of people felt it caused moisture to be trapped in the wall cavity and cause mold.  This is particularly true since most modern wall sheathings have very low perm ratings.  Right now I'm building some to-be Energy Star certified homes and the certification company (building scientists) are having us use NO vapor barrier - just un-faced bats.  I'm sure your method is good for where you live but this is not a one-size-fits-all issue. 

Matt

(post #106717, reply #23 of 38)

Same here, upstate NY>

(post #106717, reply #24 of 38)

same what?

Matt

(post #106717, reply #25 of 38)

   no vapor barriers

(post #106717, reply #26 of 38)

10-4

Matt

(post #106717, reply #9 of 38)

I'm with the wet blown cellulose - it's said to eliminate the need for a vapor barrier.

As someone noted, you need to improve weatherization - cut down on air exchanges.




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http://en.wikipedia.org/wiki/Mary_DyerMay your whole life become a response to the truth that you've always been loved, you are loved and you always will be loved" Rob Bell, Nooma, "Bullhorn"

======================================== "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Niebuhr: 'The Children of Light and the Children of Darkness' http://rjw-progressive.blogspot.com/ ========================================

(post #106717, reply #10 of 38)

Wet or dry blown cellulose is not a vapor retarder. It is quite pourous to diffusion of water vapor. Packed densely, it does fill the spaces in the cavity and retards convective air flow far better than a fiberglass batt.

In the OP's location, I would avoid use of poly or foil vapor barriers (and vinyl wallpaper) and stick with vapor retarders, to allow some drying through it over time when conditions require it. Latex paint on the inside will provide the vapor retarder, as would the paper on a FG batt. However, most water vapor transport typically via convective transport in the form of air leaks from the warm/moist side into the wall cavity. Sealing up the cavity against such leaks should be done before worrying too much about vapor retarders.

Oh, and in the summer, don't run the A/C colder than the dew point of the outside air.

Why do we keep getting posts about vapor retarders and barriers? All this has been fairly well hashed over in this forum and elsewhere.

(post #106717, reply #11 of 38)

> Why do we keep getting posts about vapor retarders and barriers? All this has been fairly well hashed over in this forum and elsewhere.

Because there's no agreement.


It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #106717, reply #13 of 38)

Because there's no agreement. Exactly.

(post #106717, reply #12 of 38)

Because we are vapor retards?

(post #106717, reply #16 of 38)

ick as you know this site is filled with both pros and dyi'ers and that's why it must be rehashed.


That plus many of us have a hard time with this.

(post #106717, reply #28 of 38)

"Dick as you know this site is filled with both pros and dyi'ers and that's why it must be rehashed."

I knew right after I hit "post" I shouldn't have added that last line. And right after that I knew how to answer my own question.

There are many situations, covering many different climates (cold, hot/humid, and everything between), and involving both new construction and refurbishing older structures. The same principles apply to all of them, but the solutions are widely varied.

The nicest thing about this forum and others is that all such issues get hashed over thoroughly, with reasons for and against every "solution." The whole process contributes to furthering the education of all of us in building science issues. Repetition is a good thing, as it leads to more widespread education on the issue. And that is good.

(Stepping down off soap box)

(post #106717, reply #29 of 38)

I knew you know that, I figured you had a bad day!

(post #106717, reply #30 of 38)

An annual (for example) re-hashing can be good.  Things change.  New studies are done.  New materials become available.  We all learn new stuff both by reading and via the hard way.  I agree though that the weekly rehashing is just a waste of bits and bytes.


BTW - I often go back and edit my posts after I look at them "in print".  OTOH, I don't edit them after someone else has commented on my remarks.


Also, some years ago I asked for a FAQ folder that would be controlled by the man behind the curtain.  You know - how to cut crown, best windows, how to hang a door... etc.  Another brilliant idea trashed by the alternate agendas of corporate America.. ;-) 

Matt

(post #106717, reply #31 of 38)

Yeah, hashing is good. From my point of view probably the most important criterion for evaluating new wall assemblies is how forgiving they are. How will they perform of they aren't installed perfectly, or are modified by future work? Does a little water penetration lead to complete failure? I'd select for longevity over optimum initial performance any day. That's why I'm wary of any new approach until it has been used for a while and had a good hashing.

(post #106717, reply #32 of 38)

Well put. There have been alot of "technological" breakthroughs that amount to sh!t, especially in the '70s. I do know that vapor barriers are code in Canada, and codes are enforced to protect people. Believe it or not,your Building Department and inspectors are looking out for you. They are not the enemy. I have learned alot from quite a few of them just by asking questions, and the vast majority are more than excited to answer them for you.


Houses can't be built like they were 100 years ago anymore. We have used up most of the good resources that people mistake nowadays for good craftmenship. Anyone can build a shoddy structure out of good material and it will still last a long time. So new technologies are now used to supplement the fact that you can't build with 100 year old heartwood anymore. That's the reason why mold lawsuits have gone through the roof. 100 year old heartwood is far more resistant to mold and decay than six year old sapwood pine boards. Its just a fact. Knowing that, if I feel like a roll of Tyvek will protect my house if water gets past the siding, or that a layer of 6 mil plastic will protect the framing in my bathroom, I'm all for it.


My mom's house was built 90 years ago and is just about ready to fall off of its foundation. The craftsmen of that time didn't believe in footers, apparently. The house is very drafty, and there is mold somewhere in about every area of the house. I like the interior trim details, alot, and the siding appears to be spruce (too yellow to be cedar, and I'm not sure I would recongnize cypress if I saw it, as its not readily available around here) and is holding up pretty well. Matter of fact, the trim and the siding might be the only things holding the house up! I'm not a purist. First and formost, I have to live in a house and feel comfortable, dry, and safe. Secondly, I'll worry about matching architectural details with those of a particular era.


Its about finding a balance. You want to be energy effecient and prevent water vapor from wreaking havoc at the same time. Its about knowing when to ventilate and when to close up a hole. Don't lose any sleep over it. I think someone said it best when they said "stopping water infiltration is more important than air infiltration". Basically, you just have to go for it. If it doesn't produce the desired result, try it again another way. We are talking about building here, not rocket science!

(post #106717, reply #33 of 38)

"100 year old heartwood is far more resistant to mold and decay than six year old sapwood pine boards"


You should see the fast grown garbage cedar we get now, even though we live on an island of trees. I've used my bare hands to take apart five year old beams - digging through them like I was a superhero! 

(post #106717, reply #36 of 38)

I was thinking more about the OP's situation, refurbishing an old house that had no insulation in the walls. We can figure that likely wind-driven rain has been getting behind the siding and that the windows likely weren't flashed all that well. Fortunately for the house, it also likely leaks air like a sieve. Not so good for energy efficiency, but great for drying out the wood when it gets water inside. Maybe why the house has survived.

Now, stuff some insulation in the walls and instant problem - maybe. If the insulation hold water, or retards the free flow of leaking air, then the leaks lead to mold and rot.

The OP did say that he will be or has been removing siding, so that gives him the opportunity to insulate, then protect the wall cavity from water problems. I have to think that he will come out better than so many cases where an old house gets insulation blown in through holes, without addressing the issue of water getting in from the exterior.

Somewhere else I quoted something from a buildingscience.com article on their site. Summarizing, it said something like don't worry about vapor retarders until you've kept water in liquid form out, and don't worry about insulation until you've addressed air infiltration and controlled water vapor diffusion. (I think I don't have that exactly right, but you get the idea)

Edit: here is what it said:
-----------
Today walls need four principal control layers – especially if we don’t build out of rocks. They are presented in order of importance:

* a rain control layer
* an air control layer
* a vapor control layer
* a thermal control layer

A point to this importance thing here, if you can’t keep the rain out don’t waste your time on the air. If you can’t keep the air out don’t waste your time on the vapor.
-----------

Hopefully the OP will be reinstalling the windows, with sill pans or flashing and side & head flashing, using a good wrap, all properly lapped, and maybe even using a rainscreen of some sort under the new siding.


Edited 7/28/2008 12:41 pm ET by DickRussell

(post #106717, reply #37 of 38)

Thank you for the informative response. Here's where I am right now with a little siding repair job that got me thinking about this:

1. Tore off junky 100 year old wrecked wood siding. Tore off asphalt paper. 1x8 solid wood sheathing looks good, a little spot rot at some portions of the "sub-water-table". The one window I worked around looks good (I completely renovated all of the windows inside and out last year), no rot at the head or sill. All in all, I think the house has been pretty effective at keeping water out.

2. I epoxied the spots at the "sub-water-table" that were showing a bit of rot. Didn't look too bad to me, really.

3. The windows all have a built up cornice at the top. This cornice is basically a 4/4 x 3 board that is sloped at the top edge, nailed from the top through the top edge of the head casing, and has "ears" that extend past the edges of the casing about 3 inches on each side. I know from repainting the house a few years ago that most of these cornices have metal "L" flashing that protects the top face of the board and goes up under the siding.

4. What I did on this one window that I worked on this weekend (which is one of a couple that didn't have the flashing) was cut a peice of 3" x 4" "L" profile galvanized and nailed the 4" leg to the sheathing and the the 3" leg perfectly covered the top face of the cornice.

5. Took a can of low expanding spray foam and filled in at the bottom edges of my walls where the subfloor hit the studs, where the floor framing hit the stone foundation, and any other holes I could get.

6. Took 15# asphalt paper and started at the bottom edge of the sub-water-table and worked my way up. Most of the existing casing and corner trim, window sills, the window cornice, and other trim all had enough of a gap between it and the sheathing that I was able to get the edges of my asphalt paper about an 1" or so underneath the trim. Asphalt paper goes over the top of the metal flashing I installed at the window cornice.

This is where I'm at now.

What I'm planning to do:

7. Cut a new 1x8 water table trim peice with a modified window sill trim board (replicating the existing trim) on top to create a large drip edge. This peice nails through the new asphalt paper to the "sub-water-table".

8. Take a peice of galvanized "L" flashing and flash the top face of this large drip to the face of the new asphalt paper. Then get a strip of "peel-and-stick" and tape over the top edge of the flashing to the asphalt paper.

9. Install fiber-cement siding per mfr. specs. I don't have the room to install this on furring strips, but Hardie specs say installation right over asphalt paper is fine.

-----------------------------------------------

That leaves me with a decent rain control layer on the exterior of the building (rain control). Most of the air infiltration is fixed (air control).

What I'm contemplating now is a DIY foam insulation like Tiger Foam. I'm doing this project a little bit at a time as time and $$$ allows, so I can't afford the setup charges every time an insulation contractor would need to come out. Tiger Foam allows me to buy the foam kits I need and install myself. Now generally, the plan would be to shoot in foam from the exterior prior to asphalt paper, but I didn't have time this time. I do have the plaster removed to the lath on the interior, so I think I'll install the foam from the interior and I'll be able to keep a close eye on how it fills.

Anyway, the foam would then serve as the primary air control and also vapor control and thermal control, right?

Any opinions or suggestions?

(post #106717, reply #38 of 38)

Wew! Thats alot to wrap my little 'noggin around. From what it sounds, you have it WELL under control! Its like the previous poster said, worry about the water first, then the air infiltration, then insulation. I don't have the poly spray in my wall cavaties, but if I could, I probably would. I've heard that it is great stuff(get it?)!


I've got a little advice for the air infiltration issue. Your ceiling. Climb up into the attic and seal every penetration with caulk or foam. You'll see some savings on your bill, and you'll worry a little less about the condensation issue. The attic is hot in the summer, the hot air mixes with the conditioned cooler air and produces vapor. You'll likely see mildew or water stains around light boxes and other ceiling penetrations. Not to mention, you don't really smell that vapor in the air like you would an unsealed house. Other than that, you seem to have the exterior under control and it is way more important, by far.

(post #106717, reply #34 of 38)

I agree, there is nothing like the passage of time to test a technique. What I find most informative is what is revealed when repairs or renovations etc. cause a wall to be opened up and reveal it's secrets.