I have this question I can’t get answered to my satisfaction. I’m a beginning to middling home remodeler working on my own 100 yr-old house. My pattern so far has been to go room by room; take down all the lath & plaster; do whatever structural or elecrical or whatever work needs to be done in that area; then insulate with fiberglass bats and hang sheetrock. Between the insulation and the sheetrock I staple 6 mil plastic; I was taught this a few years ago, and it seemed to make sense (as I recall, the reasoning was the the plastic forms a vapor barrier, which keeps interior moisture from getting “trapped” in the insulated walls). But then another friend told me I that I am in fact trapping moist air inside my house, destroying its breathability, etc.
So, plastic or no?
All help appreciated.
Replies
Installing a vapour barrier on the warm side of a wall in cold climates is standard practice and code, at least in Canada. As is often the case there are all kinds of silly theories about this and why it shouldn't be done, but its needed to protect from vapour condensing in the insulation and other stuff, and ruining it. Basically, water vapour travels through till it hits a cold enough temperature (the dew point) and becomes water.
Drywall isn't that permeable to moisture, but with a big enough area and enough time the vapour will soak the inside of the wall. This is not a good thing. The plastic also helps keeps drafts down, if done properly. Joints should be taped and/or sealed with something like an acoustic sealant.
There is some excitement about super tight homes developing mold inside the walls due to the lack of circulation. Some contractors are putting little vent holes in walls to provide circulation, but this should not be an issue with an older house.
Beware of uniformed friends and neighbors. I had a halfwit neighbor who diligently sealed all - I mean all - the ventilation in his roof. No doubt this degreed engineer thought all them holes were just there due to poor workmanship .....
Not much moisture will go through the drywall, but everywhere you have punctured the drywall (and plastic) for outlets, switches or junction boxes air with interior moisture will stream through, rendering the plastic moot.
It really depends where you live. Check with your inspection dept. Where vapor retarders are required, 4mil will be ok.
Every insulating contractor I've talked to tells me they would use nothing but kraft faced insulation in their own home. Plastic has a history of causing moisture problems. Just ask remodlers, they will tell you about wet walls opened up during heating seasons when plastic vapor retarders are used. Maybe it's an installation issue, but I have heard of high rates of problems. Fine Homebuilding had a great article on this subject many years ago.
I remember the days (about 20 years ago now) when I was hanging drywall in these high dollar track homes in Pennsylvania. One builder wrapped the entire inside of his homes with poly and caulked everything. Most of the time he would have the heating system working. Before we would start hanging, we would have to poke holes in all of this plastic (mostly in the ceilings) to let the collected condensation out.
Well, one can always find individual experiences to show how some code requirement is silly. Its true, you make a pile of holes through the plastic, but is the total area of those holes is around 1/2 a square inch (assuming 32 1/8 diameter holes) on a 4 by 8 sheet, thats a lot less than the 4,608 square inches of permiable surface on drywall. In fact, its about 1/12,000th as permiable. Yes, drywall is permeable to water vapour, as is concrete and most other building materials.
Frankly, if I saw water on the inside of the plastic before I drywalled, I sure would want to know where it came from. It shouldn't have been there. If it was there, then poking holes in the plastic just meant that wherever the water came from, instead of being locked up in that plastic, its now being absorbed by the drywall, which doesn't sound good. The plastic didn't make the water, it just showed you the water.
The thing is, code requirements have a lot of thought to them, I am not sure that insulation contractors have studied the issue in as much detail as the engineers and architects who wrote the code. I read a fairly lengthy treatise on dew points, condensation, and the permiability of drywal and other materials before I believed they had a good point.
Look at it this way: check out the recommended practices: http://www.cmhc.ca/en/imquaf/himu/codemo_013.cfm .
At least if the code is wrong, you have somebody to blame it on.
Houses NEED to breathe.All materials expand and contract [steel,wood,plastic] a building is affected by the weather its practically a living object.The plastic over the insulation is overkill, what is the paper face on the insulation doing?Skip the plastic it's a waste of time and money and you might do more harm to your house than good in the long run.
Thanks Piccioni,
The water was condensation and we saw it in the homes that the heat was turned on and the outside temperature was below freezing.
I suppose my point is, wrapping the entire inside of a home does trap condensation inside walls and ceilings, damaging the materials. (CABO 321.1, exception #1) Both CABO and BOCA code books require a "Moisture Vapor Retarder" not to exceed a perm rating of 1.0. It is my understanding Kraft faced fiberglass insulation meets this requirement. I couldn't find any mention of the use of polyethylene.
I am a commercial general contractor, not a seasoned home builder. I am starting construction of a new home (for myself) in the next few months. I do not want the inside walls and ceilings of my home covered in poly. If the facing of the fiberglass insulation does not meet the local code requirement, what other option do I have?
To open another can of worms, keeping that perm rating in mind, would you staple the insulation to the face or side of the stud?
Thanks
Chris,
I insulate all of our new homes as does Frugalarch. Unfaced batt insulation{friction fit} between studs in walls, with poly vapor barrier over walls and ceilings, Blown in attics with minimum R-40 insulation, attention to detail. If properly done, this should not cause a condensation problem in the wall cavity unless you are leaking interior air thru gaps/etc. in the vapor barrier. From my experience, when there has been a moisture problem in a wall, it has been either a bad vapor barrier installation or exterior leakage entering the wall. With kraft faced insulation, you have a minimal vapor barrier at best. Now air leakage from inside carrying water vapor, moves into and thru the insulation, condensing as soon as it hits cool enough temperatures. Next thing you know you've got ice on the inside of your wall. Remember, if your insulation is done right, your drywall temperature should be warm enough so water vapor does'nt condense. Only if you have poor insulation install, would you see any drywall damage due to condensation. Use raised heel trusses at ext wall plates so you can maintain full insul thickness out to exterior side of walls. This is the most common spot I've seen drywall problems show up due to moisture.
The main concern I have is with air quality in this tight of a home. It's worth looking into air to air heat exchangers, especialy if anyone is prone to allergies, as these will bring a constant stream of fresh air while not wasting much energy. Proper venting of baths and kitchens is essential also.
Bish
I agree pretty much with what Bish said. I believe the major problems people have had are with poor insulation, air leaks, and houses which are essentially sealed on both cold and warm side, which is probably something you have to work to do. Up here this was done for 'R2000' homes, to cut drafts but not any more because fungus & stuff grows in there.
We were shown to friction fit batts, staple the poly (code here is 6 mill), tape poly seams with a special tape and/or lap joints and seal with acoustic sealant. Outlet boxes, and the like are supposed to have these sort of plastic booty boxes around them (including ceiling boxes). Ditto with recessed lighting fixtures, although you can find sealed fixtures.
Of course this is only done for exterior surfaces. You don't do this on interior walls.
I believe that if you are using kraft backed insulation, you are supposed to slash the kraft paper because even though its a lousy vapour barriers, its enough to lead to condensation. There is some insulation sold in a kind of plast 'sock' which is already perforated.
By the way, the link in my prior post shows CMHC 'best practices' for vapour barriers. The CMHC website is useful for finding out the proper way of doing things. Its almost an online residential building code, piece by piece.
I have been using poly as a vapor barrier for years over kraft faced insulation. I have never had a call back concerning this practice. I did this in my own home over eight years ago and I have no complaints. So long as its kraft and not foil faced and you staple insulation to the face of the stud not inside edge. I`ve heard tell that the channel created between facing and vapor barrier when stapling insulation to inside of stud is an ideal locale for moisture to condense.
J. D. Reynolds
Home Improvements
"DO IT RIGHT, DO IT ONCE"
Vapour retarders provide a line of defence in your wall assembly that prevents the passage of water vapour laden air through the wall assembly to a temperature point in the assembly at which it condenses, usually soaking the insulation (as the insulation provides the wall's most significant temperature drop) reducing the thermal resistance of the assembly and introducing the possibility of rot, mildew and/or corrosion.
This line of defence can be provided in a number of ways. There are rigid or foamed insulations that are (largely) impervious to water vapour. There are batt insulations faced with vapour retarding sheets. And there are assemblies that cover unfaced batt insulation with vapour retarding polyethylene sheet.
In all such assemblies, however, the key is to provide a single line of vapour retarder. This allows the assembly to breathe on either side of the line of the retarder at the different relative humidities and temperatures that exist on the opposite sides of the assembly.
If one provides two separate lines of vapour retarder in an assemby, and a construction error in the assembly leads to the accumulation of moisture between the retarders, this moisture cannot disperse, but will continue to accumulate, encouraging rot, etc.
Because the vapour retarder is employed to restrict the migration of moisture laden air to cooler temperatures within the wall assembly, the retarder must be installed on the warm side of the insulation- ie the inside in cold climates, the outside in tropical climates.
Our assembly detailing (our office is in southern Ontario) for stick framed buildings typically calls for unfaced batt insulation between wood studs with 6 mil poly on the inside. The poly has to be caulked, taped, and outlets/switches wrapped and sealed.
Our office has been detailing buildings and supervising their construction for more than thirty years, and we have never had a field review problem with a building so detailed that was correctly constructed. Southern Ontario is littered with successful long standing examples of this approach. If you adopt this approach and moisture appears on the inside of your poly before you put up your gypsum board, you have a problem that should be addressed before you cover it, and that will not be solved by punching many holes through the poly.
I would suggest that you find out what your local building practice is, and, in light of the above explanation, try to understand why it is the way it is. Understand why the local practice works, and detail your work accordingly.
I should also add that, in site evaluating buildings prior to developing renovation drawings, I have come across existing assemblies that were rotting because they had insulation but no vapour retarder, or that were rotting because somebody had carved a big hole in an otherwise tight and well insulated wall. Take a crisp and cold winter night, turn up the heat inside your house, have a hot shower and put a couple of pots of water on the stove, and the difference in relative humidities and the capacity of air to retain moisture between the inside and outside of your house can be quite remarkable.
I have also conducted site examinations of older brick and stone buildings which have no vapour retarder, and no insulation. Moisture in the air condenses in such assemblies at points where it dissipates quickly, and can't cause rot. As a result, these walls work adequately, often having stood for a hundred or more years. Comfortable interior humidity levels in summer and winter are difficult to maintain, however.
Hope this helps.
I appreciated your post. Since you have some experience with stone I was hoping you could give some advice on the following:
I live in a climate equal to a northern coastal climate--lots of percipitation (about 75 inches per year) and lots of freeze-thaw cycles during the winter.
The main building materials here are concrete block, concrete, stone, and autoclaved aereated concrete (light weight, r 1.2 per inch, permeable).
I am going to build a house and wanted to add a lot of insulation for a total of r30+ in the walls. I am thinkng of a cavity wall with the insulation in the middle. The walls will be quite thick--about 20 inches (not uncommon here). People here stay away from vapor barriers as the philosophy is to let the walls breath so they can dry out better. I will have a wrap around porch to keep most rain of the walls
For the insulation in the middle I am primarily considering rock wool or perlite, I have considered rigid foams but am conserned about insect, vermin problems--and the possible negative effect regarding breathability/drying
What are your thoughts of a vopor barrier or use of foam in this situation?
Stan
Stan,
Just a quick question. You mention a local reliance on masonry and concrete, which, I'm guessing, is due to termite or hurricane concerns, or a combination of both. Does this mean you intend on finishing the cavity with a masonry veneer of stone or brick?
Sincerely,
Richard
Richard,
Hurricanes are not a problem. I live in Montenegro, Yugoslavia and massonary and stone are what is typical--NO ONE here does stick frame building, though you do see some partial timber frame work occasionally. Labor is inexpensive as are basic local building materials (imported and unusal materials are expensive).
I will finish with a stucco type finnish or stone (either veneer or an actual second stone wall.
Thanks,
Stan
Stan,
It appears that there are significant differences between our local building practices and our construction economies.
That said, basic principles will still apply- if one introduces a significant temperature drop across a building envelope and (through maintaining confortable levels of humidity within the envelope) introduces a significant drop in the capacity of air to carry moisture across the envelope, moisture vapour migration and condensation should be accounted for. Masonry construction may not rot, but moisture accumulation may lead to failure of your insulation (which produces its own problems), efflorescence, and possibly spalling.
We use the following detailing regularly- perhaps it is similar enough to what you are considering doing to be of some use. This is low rise masonry veneer exterior wall detailing only, moving from inside to out, metric dimensions.
- load bearing masonry, 190 concrete block. Need a clean, even and dry surface, complete with ties to support the masonry veneer. We typically call for veneer ties every 400 vertically, and every 600 horizontally, and for horizontal reinforcing at every course in the concrete block wall
- air/vapour barrier, in one of several forms. Usually in form of an elastomeric bitumen, a liquid membrane that is applied by trowel or by spraying to the concrete block surface. It dries to form a single tough rubber-like skin that adheres the block well and seals around projections like brick ties and reinforcing. This stuff is usually used in conjunction with a sheet material, some sort of rubberized asphalt bonded to a film of high density polyethylene or glass scrim. This stuff comes in 450 wide rolls, about 1 thick. It is usually adhered to the wall with the liquid membrane itself. We typically require such sheet materials be applied to the entire wall surface, laid horizontally, starting at the bottom and working up, with upper sheets lapping 50 over lower sheets. At a bare minimum, these sheets should be applied over trouble areas- known cracks, openings, door frames etc.
In Canada there are several tested-and-true manufacturers of such vapour/air retarders- Monsey Bakor, WR Meadows, Advanced Coatings to name a few. Though it is not my market, I would be surprised if similar European manufacturers do not exist. Special details are required at construction and expansion joints, lintels, etc., but manufacturers will supply appropriate detailing for these conditions.
Cavity insulation- polyisocyanurate foam boards, 400x1220 sheets. This material is a rigid closed cell board that comes in a number of forms- faced with fibreglass scrim, foil, kraft. We usually leave the facing to the contractor's preference. We prefer polyiso because of its durability, its dimensional stability, and its low flame spread rating. The insulation is usually glued in place with an adhesive recommended by the vapour barrier manufacturer, or can sometimes be adhered to the wall with hot-mopped asphalt. Can be applied in sheets of a thickness to suit your insulating needs up to 4". 1" often provides an R7.5 insulation value (manufacturer dependent).
25 airspace for ventilation and drainage
Face veneer of brick or stone, standard unit sizes, tied by brick ties to the back-up wall for lateral support.
Make sure you flash from behind the insulation across the footing and under the veneer brick. Provide weep holes (clear spaces) between bricks of the lowest course every couple of feet.
The operating principal of this assembly is the cavity. Many think that its only purpose is to allow the brick to dry from both sides. It also serves a second, and equally important purpose. It allows wind to enter the cavity and thus equalize any pressure difference that may otherwise occur across the brick veneer. With no pressure difference, little water is forced toward the interior of the veneer course. Only surface wetting and wicking occur, apart from the penetration forced by the momentum of the droplets themselves. In windy and wet locations, one should always endeavour to employ this principal of cavity construction.
The backup wall is protected from water penetration by the veneer wythe of brick, the layer(s) of insulation and the membrane, in that order. Any water that enters past the veneer wythe drains down the insulation and out the weep holes, and wetted interior surfaces dry by evaporation from both sides. Combined with your wrap around porch, you should have few problems, if any.
This is a strong, durable and well performing assembly if constructed and detailed properly.
I suppose the remaining question is its relevance to your situation. Are you able to obtain such vapour retarding and insulating materials?
Sincerely,
Richard.
Thanks for your detailed reply.
I have had a catastrophic computer failure (I spilled a glass of wine into my new Mac laptop) and am slow catching up to emails (I'm using another computer until I get mine fixed).
What is your opinion of massonary-vapor barrier-insulation-stucco vs. the massonary-vapor barrier-insulation-air-masonary you detailed?
We don't have polyisocyanurate boards but we do have polystyrene, polyurethane, and mineral wool. I have been nervous about insect and vermin problems with foams--it sounds as if you have had no problems.
We do have various bituman products as well as the torch on membranes.
Thanks.
Stan S
Stan,
Sorry to hear about your computer.
Regarding the question of stucco on masonry, I'm afraid context makes it difficult for me to relate any useful information.
We often do use stucco on insulation over concrete block back-up walls.
The performance of this finish, however, relies a great deal not only on proper detailing, but on the quality and suitability of the products used, and the training of the installers.
There are a handful of stucco finishing systems that we usually specify (again, we are in southern Ontario) including Sto, Dryvit and Durock.
These companies have programs for training installers, tried and true detailing, meshes, base coats, finish coats, etc. that have been independently evaluated, and product warranties with infrastructures behind them that are unlikely to collapse. To translate our details and assemblies over to your situation directly is probably not appropriate.
That said, our assembly is usually concrete block back-up wall, vapour retarder, rigid insulation (to thickness required for insulating value), sheathing (which typically is dependent on the stucco mfr's spec's, often, in our case, asking for Dens-Glas Gold) air/moisture barrier (again, usually proprietary to the finish mfr) then the final layer of insulation and the stucco finish itself which involves a number of different coats and meshes depending upon the application.
Again, most of this information if mfr dependent, and the detailing/ location of reinforcing meshes/ etc is stock from mfr's literature. As a measure of caution, we send our wall sections for review to the mfr's technical department, to address any oversights, practical concerns that don't show up in drawings, etc, before the job begins.
You may or may not be aware of the (still ongoing) fiasco some years ago in British Columbia, Canada, where a huge number of condo's were built with stucco on insulation (we generically call it eifs, exterior insulating finishing system), almost all of which were improperly detailed by architects who did not understand the basic principles of assemblies that employed the product, a lot of which were improperly constructed by workmen who were uninformed and left unsupervised, all of which lead to a disasterous failure of housing stock that totals somewhere in the billions of dollars. Rot leading to structural failure, mold, leaks, often times more expensive to repair than tear down and replace.
Again, it is perhaps most useful for you to understand the reasons for your local practices, and adopt the best aspects of it into a suitable solution. I had a Scottish materials prof who was fond of yelling at us weekly that the building profession was so lucky because it was always surrounded by its own successes and failures, free for the examining. I think there is a lot of wisdom his words.
But, I suppose, in answer to your original question, our stucco detailing does employ a vapour retarder, on the warm side of the insulation.
Not sure this will help, but there you are.
Best of luck with your computer.
Richard
I am working on a remodel of my living room. We installed fiberglass batts with kraft paper facing in the exterior walls, and now wonder if it would be worthwhile to add 6 mil poly for additional insulation? We could only fit R-15 batts due to the older 2X4 construction, and are looking for any way to add insulation possible. We were able to put R-38 batts in the ceiling, so that should help. It is a cold desert climate, hot in the summer but can get quite cold in the winter, and little precipitation (10" a year). The room is an addition to the house, on a concrete slab, with wood siding. It had no sheetrock, just paneling.
Thanks for any help anyone can give me on this.
the 6 mil poly would not add any insulation value per se but will act as a vapor barrier, so you do not end up with condensation in the wall cavities on cold days) and cut down on drafts into the house.
Would you put the 6 mil poly right over faced batts, or do I need to do something to the batts? Seems I saw somewhere that condensation might get trapped between the paper facing and the poly?
Kraft facing is a vapor retarder. You don't need to add another vapor barrier to the wall. You might have a tendency to trap moister in the middle.You get out of life what you put into it......minus taxes.
Marv
Since you are already down to the studs, you might consider adding rigid foal to the inside of the walls. Extruded foam panels would add R-5/inch, foil faced polyiso will add ~R6.5/inch. You can drywall or panel right over the foam.
I built my home with 'Blue Maxx' (now AAB) forms. The nature of this type of construction is that a vapour barrier is not required due to the temperature profile of the wall (I think the dew point is going to be somewhere in the concrete inner layer), but I read about it a while ago. I think AAB has a paper on the subject on their website. Besides, condensation in styrofoam or concrete is not a problem.
Obviously, this would not asuage you concern about termites, although I think there are some concrete building systems where the foam is bug proof by virtue of formulation. Furthermore, when I took the course, the trainers showed how you could make the structure termite resistant by establishing a break in the outer insulation layer, although I don't know whether that actually works. I believe these systems are used a lot in Florida, so they should have a solution.
By the way, in my analysis of permiability of plastic with screw holes, I should have pointed out that most of the hole is filled with screw and therefore the damage to the protection is even less. And the house 'breathes' by the outside, not the inside, in such a case.