vapor barrier

[jonboy]

Question concerning vapor barrier on exterior walls and ceilings. I live in zone 4 climate zone (table N1101.2 IRC). The code states that all framed walls, floors and roof/ceilings shall have a vapor retarder installed on the warm-in-winter side of the insulation. Exception 3: In counties identified in climate zones 1-4.

My question is this....all builders in my area use a vapor barrier on the walls between the insulation and drywall, but I never see it installed on the ceilings. Why not? What would be the benefit of having it in the wall system but not the ceiling, why use any at all on the walls if you are going to omit the ceiling. And does it state in the IRC that all joints have to be taped and caulked to top and bottom plates, good idea to do it anyways. I never ever see this being done.

Another question is when a vapor barrier is installed in the crawl space on the ground does this meet code for the floor? Do you have to have it on the ground in an VENTED crawl space, I know its a must for unvented.

[Martin Holladay]

Jon,
There are probably more job-site myths about vapor retarders than about any other building component. Although plenty of builders will volunteer theories about why a vapor retarder should be installed in a wall but not in a ceiling, you can safely ignore them. Your hunch is correct: when it comes to controlling vapor diffusion, there is no logical reason to treat ceilings differently from walls.
Remember, a vapor retarder does not have to be polyethylene. Kraft facing or two layers of paint are adequate vapor retarders in your climate zone.
Finally, it is always a good practice to install a heavy polyethylene vapor retarder on the floor of a crawlspace, whether the crawlspace is vented or unvented. Whether or not your local building inspector will require one is an entirely different question; if you are unsure, call the building department and ask.

[Sweep8]

The code requires all framed walls, floors and roof/ceilings to have a vapor retarder on the warm-in-winter side of the insualtion except where the framed cavity or space is ventilated to allow moisture escape.

Before thinking about vapor diffusion through a ceiling it is necessary to describe how the attic is designed especially with regard to insulation and ventilation. There are many more possibilities for design configurations in an attic/roof than in a wall and it is more common to ventilate the attic or roof above the insulation therefore vapor retarders are rarely required by code in ceilings. But I would never suggest that a building be designed only using the standards of the building code.

The only thing that is true for all design conditions is that moisture laden air is heavier than dry air and therefore it is more of a problem for the lower part of walls than for ceilings but it can certainly be a ceiling issue for bathrooms.

Moat alkyd paints or special vapor retarder latex primers can act as an adequate vapor retarder for ceilings but not typical latex paints.

[CB Rob]

[QUOTE=Sweep8;328740]
The only thing that is true for all design conditions is that moisture laden air is heavier than dry air QUOTE]

Sweep, I think you have this one backwards. moist air is less dense than dry air.

[frenchie]

Really?

I can see where warm air is lighter than cool air, and warmer air has the capacity to hold more moisture... but...


Heck, no, you're right:

The density of moist air, rm , is lower than the density that dry air would have if it were at the same pressure, p, and the same temperature, T, as the moist air. Thus, water vapor is a source of positive buoyancy in the atmosphere.

http://www.engr.colostate.edu/~ramir...apor_CE322.htm


Or again:

Although pure water is not a gas at room temperature and sea level pressure, water in the vapor phase mixes readily with dry air, as do any two gases, until the partial pressure of the water vapor reaches the saturation water vapor pressure at the current temperature. Such moist air is lighter than dry air at the same temperature, because the molecular mass of water is lower than the average molecular mass of dry air.

http://en.wikipedia.org/wiki/Lighter_than_air



...I did not know that.

.

[BillHartmann]

That is a common misconception because. Most people think that moist air is heavier because of how warm/hot moist air affects humans.

But for the affects of air on things like aircraft lift Denseity Altitude is calculated.

It is altitude adjusted for the affects of tempature and humidity on air density.

And moisture does reduce the density.

However, IIRC the affect on density would be smaller than the other things affect air in a home so you won't get any signficiant air move just from differences in humidity

[Sweep8]

Well, I stand corrected. I once observed frost behind insulation in a wall cavity in a cold climate with no vapor barrier and it was at the lower half of the wall. But it occurs to me that I was once told by an HVAC engineer that vapor pressure was like air pressure but it moved very slowly. I suspect the frost was located low because the source of moisture was from a kitchen sink so it might have condensed there before it could disperse throughout the house.

[frenchie]

Well, the other factr is that once it's condensed, water is heavier than air - so it runs down...

[jeffrsapp]

I was told by an "expert" that according to the latest International Building Code, for my climate (Climate Region 4), a vapor barrier/retarder "is not required".

So, I purchased the 2006 IBC and I can't find it anywhere! Can someone please direct me on this? Was he wrong? Is the 2006 not the latest? WHAT?

I really need to know so I can talk to my architect.

Thanks!

[Roger P]

jeff:
Don't know where you are located and what type of building you're talking about so this may be a moot point. What code(s) has your AHJ adopted?
2006 IRC SECTION R318
MOISTURE VAPOR RETARDERS
R318.1 Moisture control. Exceptions: 3. In counties identified as in climate zones 1 through 4 in Table N1101.2.
Same exception 2003 IRC 3. In counties identified with footnote a in Table
N1101.2.
You can view Table N1101.2 here http://ecodes.iccsafe.org/iccf/gatew...3.0$vid=icc:sc

2000 IRC R322.1 Moisture control.Exceptions: 3. In counties identified with footnote a in Table
N1101.2.

[jeffrsapp]

I live in Missouri--I believe that is Region 4. I don't have to worry about any local codes--there aren't any. Instead, my architect wants to go by the national code.

According to an expert that I emailed, "The International Building Code has been changed and does not require an interior vapor retarder in climate zones 4 or lower (less than 5,400 heating degree days)."

Anyway, since you have the IRC, could you please quote me a little bit more detail from that section?

[Roger P]

IRC 2003 R318.1 is unchanged in 2006 except for exception 3 clarification. Sorry, but copyrights are involved. You can view the sections using the link provided above. Click on the page icons to expand the chapters/sections. You might have to register for free to access the code.
R318
MOISTURE VAPOR RETARDERS

318.1 Moisture control.
In all framed walls, floors and roof/ceilings comprising elements of the building thermal envelope, a vapor retarder shall be installed on the warm-in winter side of the insulation.
Exceptions:
1. In construction where moisture or freezing will not damage the materials.
2. Where the framed cavity or space is ventilated to allow moisture to escape.
3. In counties identified with footnote a in Table N1101.2 .
You can see a zone map here http://www.energycodes.gov/rescheck/pdfs/03mo.pdf
ZONE HEATING DEGREE DAYS

9 4,000 - 4,499
10 4,500 - 4,999
11 5,000 - 5,499
12 5,500 - 5,999
13 6,000 - 6,499
2006 Zone 4C 3600 to 5399, Zone 5 5400 to 7199
Here's a current zone map (see page 9) http://www.iccsafe.org/cs/codes/2003...a/FAA-IECC.pdf

[Sweep8]

If there is no code requirement and you have an architect why would you or your architect care what any code said?

[jeffrsapp]

Roger P, it looks like I am going to have to buy that code book because I'm not exactly sure what you are saying. Here's my situation: I am building in Callaway County, Missouri, which is in Zone 11 according to that webpage. Do you see where the code book says that a vapor BARRIER is not required? A vapor retarder might be required, but drywall with latex paint is a vapor retarder and would suffice IN MY CLIMATE.

Sweep8, I wish it was that easy! I MUST work with this architect, I have no choice. He is old-fashioned and from Alaska. He is using old techniques and Alaskan techniques--did I mention I was from Missouri?!? If I can quote from the code book, either him or his bosses might actually listen to me. But, I WILL NOT MAKE ANY PROGRESS UNLESS I QUOTE FROM THE CODE BOOK because that's what he follows, period.

I would appreciate any help I can get!

[NW Architect]

OK. Get the code book. International Residential Code. Also get the EEBA Builder's Guide for your climate. Everything you need to know.