Is this a double vapor barrier?
I am renovating an old house in Massachusetts. In the master bathroom the ceiling presents a couple of different situations (both in the same room).
Situation #1: The outermost part of the attic floor above this room (extending in about 7 feet from the edge of the eaves) has been stripped of floorboards and is insulated with 6″ craft-faced fiberglass topped by about 8″ of loose cellulose. This configuration covers about 2/3 of the bathroom ceiling.
Situation #2: The other third of the bathroom ceiling is below the floored part of the attic. That has 6″ of craft-faced fiberglass under the floorboards and 2″ poly foam boards (loosely interlocked) on top of the floorboards.
I am about to have the bathroom ceiling finished with 1/2″ blueboard and veneer plaster. My plan was to staple a 6 mil polyethylene vapor barrier over the whole ceiling first but the drywall contractor is suspicious of that, thinking it would trap moisture within the insulation layer(s).
Any opinions about whether or not this is a problem? Should I use the vapor barrier or omit it?
Thanks.
Bob
Replies
given the erratic methodfology, I doubt it.
What way does the Kraft paper face on the insulation? If down to the room, you are fine. I don't believe that the foam panels will trap moisture, being loosely laid over anot6her layer of materials
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Sorry, I left out that fact - the craft paper is on the heated side of the fiberglass. I've decided, based on comments here, to eliminate the polyethylene vapor barrier.
Now I'm debating the usefulness of the extruded polystyrene insulation board on top of the attic floor. Maybe the best course of action there will be to leave it in place, look for condensation on the bottom surface through the coming winter, spring and summer and decide whether or not to keep it. If there's no condensation problem then I guess the stuff doesn't hurt and maybe it even helps.
If you can get into the small attic area you may be able to do airealing from above. If not, just continue as mentioned.
Yes - I can get into that attic area. How should I seal it? Maybe lay housewrap over the insulation?
If not to much of a hassle, expose the areas where you know you have leaks (lights, fan, stack vent, wiring pentrations and along the beams where the veneer plaster was not seald). Use foam/caulking/plastic to seal these spots. Cut the foam insulation to lay right down next to the ceiling drywall and then add the loose fill/fiberglass.
Yup. Makes sense. I can do at least some of that. What I can't do is get the foam board under the fiberglass without taking up the entire attic floor. So I'll do the best I can I guess. I'll foam around the vents, cracks and penetrations, etc. Hopefully that'll prevent most air movement between the bath and the attic and so I won't have a lot of moisture up there to condense.
Thanks!
The paper faced insulation is installed with the paper to the heated area.....yes?
If so, yes.....this is a double barrier. You won`t trap moisture within the insulation, but you may trap it between any voids in the two barriers.
If the paper face is up, towards the unheated space....its just plain wrong. Remove the paper facing and install the poly barrier from below.
Lemme get this straight....
YOU BANNED REZ?!?!
Holy bagels and lox Batman!
Your problem is not with the polyethylene sheet, its with situation #2.
As JDRHI said, the polyiso is a second vapor barrier. This may or may not be a problem depending on how poorly (or loosely, as you say) the polyiso boards are fitted together. The potential for condensation exists.
The addition of a polyethylene sheet should not be a problem, in my opinion, because it has no insulative value. But I'm no expert.
Drying is to the exterior in massachusetts, so the poly-kraft-fg situation should be okay because you decrese permeability as you exit the structure. I don't like the idea of the polyiso there so I'm not gonna touch that.
BTW, a vapor barrier is considered necessary in only the coldest of climates (i.e. not massachusetts). See
http://www.buildingscience.com/resources/roofs/ceiling_vapor_barrier.htm
Edited 10/25/2005 4:45 pm ET by BobS
BobS,
Wouldn't the poly boards only be a problem if they are extruded poly? From what I remember, expanded poly isn't a vapor barrier.
Justin Fink - FHB Editorial
Edited 10/26/2005 9:36 am ET by JFink
...nevermind, seems like it's been covered by other responses.
Justin Fink - FHB Editorial
I had assumed the worst (I'm an engineer so I always do), that it might be polyiso. EPS is vapor semi-permeable (or semi-impermeable, I suppose). Ray's point about the permeability of a kraft face changing puts this question out of my league. I guess its not apples to apples.Take a look at:http://www.buildingscience.com/resources/walls/insulation_sheathings.pdf
It would be best to skip the polyethylene sheeting. In your climate a vapor barrier is not recommended but a vapor retarder, such as kraft paper is. I presume that you have had no problems with the current setup from a moisture standpoint.
What do you mean by poly foam? There is a big difference between extruded polystyrene, expanded polystyrene, and polyisocyanurate. In any case, the foam board on the cold side of the assembly keeps the temperature of the first surface where condensation could form, warmer. This will make the assembly safer and less likely to form condensation.
There is a widespread lack of understanding of this issue. The potential for moisture problems due to condensation is caused by moisture laden air reaching a surface that is colder than the dewpoint. If the surface that comes in first contact with that air is warm, then no condensation occurs. If the moisture laden air cannot pass through this layer then the assembly is safe.
PERSIST is a system that puts a vapor barrier on the outside of the exterior wall sheathing. The insulation is then but on the outside of that vapor barrier. The exterior sheathing is kept warm by the exterior insulation and is therefore not subject to condensation. In warmer climates, part or all of the insulation can be put in the stud cavities.
The important factor is controlling the temperature of the first condensing surface to be warmer than the dewpoint of the air that is in contact with that surface. It is also important to allow drying to occur towards the greatest drying potential. That will vary with the season. In massachusetts, you air condition in the summer and during those times, the drying potential is to the interior. If you use poly on the interior and there is a great enough wetting potential from the exterior, you can experience a reverse vapor drive where the poly will cause your walls to rot. That is why it is not recommended in your climate.
To put the condensation issue in simple terms, a cold soda will form condensation because it is colder than the dewpoint. A coozie on that same can will not form condensation because the surface of the coozie is warmer than the dewpoint.
The coozie is not a vapor barrier. It is a vapor retarder and an air barrier. The aluminum is the vapor barrier and it is on the cold side of the assembly. No moisture laden air reaches the aluminum sides of the can and diffusion is too slow to allow moisture to build up in the assembly. That is why when you remove the coozie, it is not soaking wet inside. If you put the coozie on a wet can, it will not dry out. That's the same as building with wet lumber and then sealing it up between a vapor barrier and a vapor retarder. If you spill your drink between the can and the coozie, it won't dry out. That's the same as a plumbing or roof leak in a wall with a vapor retarder and a vapor barrier.
Ray,I like the coozie analogy - it helps.But I think if the foam board (whatever it is) is installed loosley as the OP says, then its probably going to allow cold air to reach the warm side of it. So there is potential for condensation unless its installed in an airtight manner.I get what you are saying about the polyethylene in the summer. Makes sense. But I've also been reading a lot of the building science corps books and they routinely put a polyethylene sheet on the under the drywall in their illustrations. How is this reconciled?It seems that if the water vapor condensed on the outside-side of the polyethylene in the summer, it would just as easily condense on the kraft face or outside-side of the sheetrock in the summer without polyethylene (since the drywall has no insulation value). Right?I'm still learning this stuff I guess.
Building Science Corp (BSC) has a series of different books and practices customized for the area you live in. If you're in the humid south and want to air condition, build it airtight with an exterior permeable air barrier and no interior vapour retarder or barrier. The external air barrier is to stop the very humid air getting in the wall system to a cool area or surface. The cool drywall or other interior surface finish may condense water on its outer back with the water having a long way to get back out to where it came from. By leaving the inside barrier out ,at least it only has to diffuse inward (where the air is drier from the A/C) through a relatively drywall and permeable latex paint (I hope its latex).
Side Note: Polyethylene and tar backed kraft paper are both vapour retarders with differing permeances. They both will still let some measureable moisture through. A true vapour barrier would be an unperforated tin foil or sheet of metal.
The most important thing in stopping moisture movement is stopping air leakage since in most normal situations (wet area like pools, indoor zoos, aquariums etc excluded), it is responsible for 98-99% of the moisture movement. Putting a vapour barrier or retarder in to stop the moisture will be useless unless it's sealed. It may be better to leave it out and seal another appropriate layer or parts to make a durable long lasting air barrier system.
In the system we're talking about seal, seal, seal.
Polyethylene sheeting has a perm rating of less that 1/10 of a perm and is therefore a vapor barrier.
Very good point about air leakage. My home tested at an incredible .038 air changes per hour at natural or .76 at negative 50 pascals.
The .76 ach @ 50 pa beats the R2000 standard of 1.5 ach's or less handily. What type of construction?
For eveyones' info: I think Joe Lstiburek helped set the air change rate for R2000 and has since said it may be a bit too tight...not because of "Too tight houses are unhealthy, don't breathe" or crap like that but that the costs/benefits to get them that tight are probably not justified.
As an R2000 site advisor/inspector I had a case to tell a contractor friend of mine that his men were costing him "money". He had 2 building crews that like to compete against each other for building the tighter house. They had them regularly down to just under under 1 ach @ 50 pa and then they started moving down drastically. My firend had a busy life as he had investments in apartment buildings, car washes, etc. and only usually had to show up at sites occasionally unless there were problems. The crews had taken the air leakage competition to extremes by looking for pinprick size holes, etc in the interior air barrier- all this extra time did nothing for him as his houses were already easily passing the required testing without all the extra time he paid for!!!
It depends on how you do it. I use a modified version of the PERSIST system. I use an unvented attic system and use cost reductions like no venting and no IC rated cans to offset the extra costs. My energy consumption of 1/4 KWh per sq. ft per month speaks to it's effectiveness. I achieve this in Austin TX where the climate is rather harsh. We had five days in Sept. of 108 degree afternoon highs. My insulation values are only 14 in the walls and 30 in the rafters. The air tightness is what makes the difference. I use 14 SEER heat pumps.
Joe Lstiburek promotes a drywall air barrier system which is very difficult to get down to those numbers. PERSIST can easily reach those numbers. Joe rightly believes that he can make the biggest overall impact by reaching out to the production builder and therefore, he tailors his recommendations to appeal to that market. New methods are difficult to introduce into the production market. He's taking baby steps and is making a greater cumulative difference while recognizing that he's making big compromises. He loves the way I build my houses but knows it would be a hard sell to Pulte or Weekly. I've talked to those guys about it and it will be a long time before they try it on their houses. Dumping a 4" outside air pipe into their return duct is a big deal for them. Unvented attics is the stratosphere and they are trying it on a percentage of their homes.
Cold air on a warm surface is not a condensation risk, it is an energy loss.
Joe Lstiburek (principal at BSC) does not recommend polyethylene sheeting in your climate. In fact he lives in Westford, Mass. and did not use it in his home. He recommends it only in severe cold climates.
The kraft paper or drywall will allow the water vapor to dry through to the interior where it can be removed by the air conditioning. Kraft paper has a permeance that increases as the moisture level rises making it a smart vapor retarder.
I imagine that since this is above a bathroom, there's a decent amount of vapor being diffused up to the attic in winter. When that hits the cold underside of the poly foam won't it condense? Again, I'm assuming the undeside of the poly is cold because its loosely fitted.I went back and checked references on the polyethelyne sheet from building science corp. The "Moisture Control Handbook" uses it all over the place in cold climates. The "Builder's Guide to Cold Climates" doesn't seem to use it. I guess maybe he's got a bigger idea of what's a cold climate in the first. Either way, I'll take your word for it that it shouldn't be included.
The moisture control handbook is pretty outdated at this point. The builder's guides are updated but are still a little behind the current thinking in some areas.
If cold air is getting under the foam board then the warm moist air is also getting out. The key is warm moist air on a cold surface. If the surface is cooled by air leakage then it no longer has the warm moist air in contact with it. That's why it's not a condensation issue but is an energy loss issue.
Good to know. I don't even have the newest builder's guide (these aren't cheap and the libaries don't have the new ones - not even the westford library next door to BSC!), but I'll look to that before the MCH. Specific examples aside, I really like the moisture control handbook. Too bad its out of date. Its not really that old; makes me a little concerned how quickly the thinking on this is changing.I wish they'd do a book focusing on existing construction, not new construction. But I guess they are going where the money is and it might be a lot harder.As far as the OP's foam board goes, I'll buy what you are saying, but the setup still makes me nervous.
The thinking is not actually changing that much, it's just getting out to the public from the research papers/books of the past. The Division of Building Research (DBR) at the National Research Council here in Canada were talking about and stressing air leakage control as far back as 1953 (when I was 3). A talk in Halifax at the annual meeting of the National Engineering Association stresses the merits of air leakage and its effects on heating/AC, moisture flows, comfort, dust entry, etc
A series of Building Digests (free, 3-4 page articles about various aspects of building science) were produced by DBR at the rate of 1 per month starting January, 1960. The first 8-9 concentrated mainly on moisture and buildings. #9 in Sept/60 has the first reference I know of to keeping holes in the plastic vapour barriers to a minimum and sealing the joints!!!
The author of this particular digest, researcher Gus Handegord was a mentor to Joe Lstiburek when he was becoming a building scientist. The first time I had a chance to seek out and meet Gus was at a huge 1985 building conference titled "Avoiding Failures, Learning from Experience". When I finally found Gus who was with him but Joe who I had heard about 1 year before, so bonus!!
These digests (about 250) are now online for all to view. In 1972 , one states about vapour barriers something that goes like this "if you can't make the vapour barrier airtight, it might be better to leave it out and try to make the drywall airtight" (Joe's idea for the Airtight Drywall Approach or ADA system come from here?) and "an unsealed vapour barrier, even of 0 permeance (like tin foil- my comment), full of holes will be useless"
Wow. Well, first thank you all for giving this question some thought and some pixels. This is a great forum.
What I take from this is that my polyethylene vapor barrier is apparently useless. I know that I'll not be able to make it perfectly leakproof or even very close. It is interrupted by the beams around the perimeter of the ceiling (post & beam construction with beams partially exposed) and cannot be effectively joined to the vapor barrier in the walls (which is itself probably leaky given the remodelling that's been done). From comments here I gather that its principal value would be to block air movement. That ship has already sailed so I guess I'll remove the polystyrene.
What I am still unsure about is the value (positive or negative) of the styrofoam board insulation laid on the attic floor. I'm pretty sure the stuff is extruded polystyrene - it's that blue stuff from Lowe's and I think it may say "Styrofoam" on the face along with the DOW logo.
I'm unsure about it's value for 2 reasons:
1. it covers only half the bathroom ceiling;
2. it is applied loosely.
To put a finer point on it I have these questions about the foam board:
1. Should I glue the edges together or tape over them? Seems unlikely to make a difference since half the room isn't covered anyway...
2. Should I remove it altogether? I put it there because I reasoned that the 6.5" of fiberglass in the ceiling wasn't enough and short of removing the fiberglass and spraying foam or blowing cellulose in the cavity, the only way I knew of to add insulation was to lay it on top of the floorboards. Since I want to walk in the attic, the foam boards (topped with OSB) seemed the only choice. But if it may cause problems by allowing moisture to condense I will remove it. Intuition tells that's not the case - why should moisture choose to condense there when it has the whole (vented) attic to cling to? The bottom of the poly boards ought to be warmer than the underside of the roof sheathing.
3. Did I waste my money on the foam boards? (Just curious. No sense in missing an opportunity for self-flagellation. Isn't that what renovation is all about?)
Again, thank you all for the information.
Bob I