Greetings-
I am getting a proposal together for an exterior remodel.
house has kraft faced fiberglass insulation in the stud bays.
client wants to try and and more R on the outside.
siding will be fibercement.
I am trying to figure out the best approach to avoid big moisture problems.
help, and thanks in advance.
aaron
Replies
What moisture problems are you talking about???
not sure, but I have a vague concept that creating a double vapor barrier, will pose a problem, and my wall will end up eating itself from the inside out. I have looked through the archives, but have been unable to find a thread that details a system that the Breaktime Heavy Hitters Crew seems to think works best......thanks for responding.aaron
As far as water vapor condensing in stud bays, are you in a northern climate where you mostly concerned with heating, or a southern where air conditioning is the main concern? Maybe oversimplifying, but, in general, if in north, keep VB towards heated space, in south, towards outside (humid outside air in summer with air conditioning, condensation is a problem). If there is already a VB on the inside face of pre-existing insulation, you can use unfaced or slash the kraft paper on the insulation you install.
what a fool I am not to say that I am in upstate ny.sheetrock is already up. I took over this project from another man.thanks againaaron
Hey, no problem, can't remember everything. Anyway, I'd think then that you would not want another vapor barrier on the outside.
Edit: In re-reading your first post, it dawned on me that the easiest way to insulate would be to use Styrofoam or the like, hence your concern about double vapor barrier. If you use just about any other insulation, you will have to monkey around with building a place for it--furring or strapping, etc.
I don't know what to tell you except that people seem to do it all the time (put Styrofoam on under the siding). It is easy and has a fairly good R value, but it could present problems with not allowing any moisture that gets into the wall cavities to get back out. One thing I thought of was to fur out with strips about a half inch or so thicker than the foam insulation and allow a vented air space behind the foam so condensation can get out. Sounds like a lot of horsing around to me though. Maybe others who know more will reply. (I tend to come up with overly elaborate fixes.)
Edited 11/12/2005 7:54 pm ET by Danno
I have been doing research to do this to my own home - or at least see if it is a good idea. I live in Portland, Oregon, where moisture need to be dealt with. The best information I have found so far in in the PDF I have attached.
Canada Mortgage and Housing Corporation (one of our 2 national housing/building research agencies) has been recommending the rainscreen for my area (the Atlantic Provinces of Canada) since 1985 and it's been mandatory in our code in Nova Scotia for 4-5 years. (things do move slow in local gov't!!)
In then document you refer people to, figure 9 has an ommission:
If the clapboard is wood and not something like "Hardiplank", the back of the siding should be primed so that it will not absorb any water that may get behind the sidng by wind driven rain or poor execution of flashings/caulking.
You don't need an air space under the insulation. This will set up a convective and air movement cavity for infiltration/exfiltration which will lessen the effective R value of the foam you install.
People are worried about the "theoretical" vapour diffusion that is moving from interior from fall to spring. This is very small (about 1-2%) compared to what can be moved by exiting air leakage. (98-99%) But air leakage is not continually one direction in walls- one day it may be inward (infiltration) and the next it may be outward (exfiltration)due to wind pressures......so what may get in the wall one day may be dried the next.
I think we tend to get this "wall picture" from looking at water/mould staining on roof sheathing. With attics, moist, warm air movement from the house is happening 99%+ of the time due to stack or chimney effect (warm air rises; in a 1000 sq ft bungalow in a very cold still night, the total pressure on your ceiling from the warm bouyant air will be about 600 lbs or about 0.004 psi) or by suction created by winds. Thus if you have a wet/damp house and a loose ceiling, you get a lot of moisture on your sheathing all winter leading to staining or in the worst case, severe surface mould leading to longterm sheathing rot.
To reduce the chances of moisture in the walls/attic, control house air moisture levels by dealing with damp basements/crawl spaces and basement leaks, vent produced moisture the the exterior (dryers, baths and kitchen) and airseal the walls/ceiling/attic. this is also a big energy saver.
In our area (Atlantic Canada) with 7300-10,500 degree days, extruded polystyrene and other foams have been added under retrofit vinyl siding for years with few problems except in wet houses and for poor installation leading to leaks into the walls. My own house has a double foil (the perfect vapour barrier) faced polyurethane foam as sheathing directly under the siding. By theory, my 17 year old house should be a pile of rotted studs by now!!
You are right on. Putting a vented space between the added insullation and the existing would pretty well nullify the benffits of the added insullation. Imagine a layer of cold outdoor air flowing by convection between the new insullation and the heated space. In rainscreen walls the venting is outside the insullation.
Also correct that air leakage, not diffusion of vapor through the wall is the big moisture problem.
There is a temperature gradient in the wall, about 70 Farenheit onthe inside and outdoor ambient temp on the outside. As air flows through the insullation it cools off and reaches its dew point, where moisture condenses out or forms frost.
One rule we use here (Canada) is to have 2/3 of R-value outside the air/vapor barrier (usually polyethylene sheeting). If exixting is 2x4wall with fiberglass batts, that would be R-12 or so. You could add about 2 1/2 inches of styrofoam (R-20 per inch), to meet this requirement.
Also if the styro is will sealed, it will reduce the flow of air through the wall, and reduce the potential for moisture problems.
If you are taking the wall apart from the outside, you will often see dirty areas in the fiberglass, where fine dust is filtered out of the air as it leaks out. You might be able to seal these leaks with caulk, urethane foam, tape, etc. Also have heard of spraying a layer of isocyanurate foam on back of drywall, around electrical penetrations, etc. just to seal these leaks.
Another option might be Larsen trusses (see FHB index). This is an economical way to create space for more insullation. Also a layer of poly can be added between the Larsen trusses and the old wall to seal air leaks.
What do you all think of tesse iadeas?
You say >> client wants to try and and more R on the outside. << To me that means rigid foam insulation. Here is something to think about... Different types/brands of foam have different perm ratings. Perm rating in this case means a materials ability to pass vaporized moisture. Also if you use rigid foam, look into getting some that is treated against insects - borates or similar...
Is this really a bad thing? Here in MN, I see this all the time...anytime someone 'upgrades' their old house to vinyl siding (blech) they get a layer of the pink stuff first.My parent just had that done to their house, in fact...by a crew that refused to use Hardi due to moisture problems. Of course, none of that is saying it's a proper thing to do...just that I see it done a lot.
No - I don't think it is a bad thing... All I was saying is that all rigid foam is not created equal.
Edited 11/13/2005 11:42 am ET by Matt
Check out http://www.buildingscience.com/ for a wealth of information related to insulation etc. for different zones. A warning, though: there is so much interesting and useful information on the site, that it may take you some time to find exactly what you are looking for-- but it is well worth the effort. Also, check out these two articles from FHB archives: "Does Fiberglass Insulation Still Make Sense", and "Built Wrong From the Start."
Good luck with your project, and keep us posted.