On a remodeling project I am faced with the problem of increasing the R-value of a north facing wall (that never gets any sun) without creating a condensation problem. The wall is std. 2×4 construction in the Mediterranean climate of the West Coast (2×4 studs, fiberglass R13 between studs + 3/8 sheathing then siding. I fear that if I add rigid insulation to the outside of the studs the air gap between the rigid insulaton and the fiberglas facing will be perfect for winter condensation to develop. Has anyone pulled this off ??
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The very act of adding insulation decreases the odds of having a condensation problem, but what you describe is the chance of trapping moisture.
In a cold heating climate, the moisture would be moved from heated siode of wall towards the outer, and the exterior foam would be wrong because the problem you anticipate would be assured.
But in your moderate zone, Foam panel insulation on the e3xterior is fine. The only problem is that it is wise not to have a double VB that can trap moisture in between. This all depends on the kind and the integrity of the existing VB. If it is just the kraft face on batt insulation, I wouldn't worry too much.
You could get other opinions though, and the building science website might give you further evidence or information
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Thanks. The Kraft vapor barrier is 20 years old and expected to be very leaky from the standpoint of being a continuous vapor barrier. If I understand your point, this application would have the rigid EPS barrier act as the REAL vapor barrier and the leaky Kraft fiberglas would only add R value with little effect on actually trapping moisture.
Winter vapor drive is from inside to outside. The first condensing surface is the inside of the exsterior sheathing. From the standpoint of condensation inside the wall, the act of adding foam insulation to the exterior side of the assembly will raise the temperature of the first condensing surface and lower the risk of condensation.
In the summer, the Kraft paper will allow some drying to the interior due to it's elevated perm rating in a high RH environment.
The question then becomes an issue of rates. At what rate is moisture introduced from the outside. What is the material to the exterior of the plywood sheathing? Can you count on it or the foam to act as an effective drainage plane? If not then you may end up with bulk moisture trapped within the wall.
A good system is to add a rubberized membrane to the exterior of plywood and use the foam on the exterior in an amount sufficient to entirely insulate the building with no insulation in the stud cavity. The first condensing surface then is held to room temperature and no VB of any kind is used on the interior. All drying is to the interior. This is a very durable and forgiving system as well as one that exhibits superior air tightness.
It is obvious from the detailed reply you understand my concern. To respond to your question - the exterior surface is redwood horizontal ship-lap siding and is being replaced by Clear-Lam an engineered ship-lap plywood. Right now the exterior wall (on the outside surface) is wet all winter and is obviously a condensing surface. What I am trying to avoid is moving that surface to the interior of the sheathing. I have no problem with the VB on the foam being the first condensing surface and based on the current leakiness of the fibreglas (which I surmise is why the exterior siding is currently the first condensing surface) it would be the interior side of the foam - which means I will need to protect the sheathing with a membrane.
I am a bit concerned if I remove the insulation in the wall cavity that I would need 2-21/2 inches of Foamboard which is a bit much for a 2x4 wall.
I think that if you are worried about the facing on the batts acting as a second vapour barrier you could easily take care of that by poking a whole bunch of little holes in the facing. A small skewer ought to do the job.You could also put down your foam board and after removing the batts blow the space with cellulose which is cheap and will stop air movement if you use it at a high density. For that matter you could apply it as a wet spray if the wall is completely open. Let it get as thick as is convenient.Cellulose is also capable of adsorbing water vapor and releasing it later without the stuff ever reaching a liquid phase.If my house wasn't a 100 years old... there would go a really fine hobby.
If you insulate the interior and the exterior, you will need to be able to calculate the temperature of the first condensing surface in January. The more insulation you put inside of this surface,(the sheathing in your case) the colder this surface will be in winter. The delta T through the wall will need to be calculated. It is safer to put all the insulation outboard of a superior drainage plane. 2 inches of polyiso is sufficient for Canadian winters with this method. Remember that the effective R-value will be substantially greater due to the reduction in thermal bridging, the lack of air leakage and the dead air space in the wall cavity, in that order.
Sounds like you really know your stuff, what is it you do? I'm not familiar with the formulas you use, not that I'm disputing it just never seen it put quite that way. I used to live in Detroit MI. about 400 miles south, when I moved here I brought all my building methods with me not realizing a slight difference in climate can produce drastic results. Consider me a graduate of the school of hard knocks, I know what works in my neck of the woods. Another thing to consider is workmanship. A crappy install will yield poor results even using the best of materials. Now I use wet spray cellulose and avoid the condensation and air infiltration headache altogether.
I think that I'll start a thread this weekend addressing the exterior air barrier approach to wall construction. To avoid cross posting the information here, I'll just refer to it after I've had time to compose the initial post. It will be a little lengthy. Thank you for your patience.
Thanks, I'll be looking forward to hearing what you have to say, always good to see an old problem through a diferent set of eyes.
Looking forward to it. Great. I was just looking ofr some ideas and I've got plenty.
I see no problems associated with sheathing, membrane, 2" of foam and 1.5" of furring strips and siding outboard of a 2x4 stud. Is this your concern?
Edited 5/24/2005 1:03 pm ET by RayMoore
Yes, that was my concern. I was trying to avoid re-calcing the jam extensions for the windows + polyiso foam and furring. Just means I will have to re-do some moldings on the east and west sides of the house. Nothing serious.
put the ridgid insulation on the inside. Here in northern Michigan condensation is a huge problem. Every remodel I have ever tore into with styro on the outside was bad news. Thus when I built my own place I put the styro on the inside under the drywall. Since then I have added on to the house several times, no mold, no problems.
Can you explain this a little better? Are you referring to interior condensation or exterior. Interior condensation will not form due to the warm interior surface temperature of the sheathing. Exterior condensation is not an issue due to the rubberized membrane. If the styrofoam is applied over felt paper then the sheathing will rot from absorption of liquid water that gets behind the foam but this is not condensation and will not occur with a rubberized membrane. Also styrofoam is a poor choice for this application due to the thickness required. Rigid foam(polyisocyanurate) has a much higher R-value per inch.
I'm not totally clear on your question so I'll answer as best I can. Several projects I opened up had one inch styro applied on the exterior of the studs, wall cavity 6 inch unfaced fiberglass with 3 mil. poly vapor barrier under drywall. Mold formed in the corners of the stud styro connection every place there was the slightest amount of air leakage. Mold covered a great percentage of the exterior sheating (inside face). My method of choice when forced to use fiberglass bats. OSB exterior sheating, 2x6 studs, 1/2 styro on room side, tape seams then drywall. It's been 25 years and have not seen any problems.
i have done a fair bit of remodeling in out mediterranean northwest climate. bear in mind that most of the heat loss is through the lid. walls aren't usually responsible for heat loss. windows are. if opening the north wall from the interior and adding rigid insulation is not an option, i am not sure i'd worry too much about heat loss through the wall so long as the r-13 batts are in good condition and the bldg paper is doing its job. in our climate though, i never use a poly vapor barrier at the interior.
Thanks. Nothing will be added to the interior since the project is adding double-glazed, high performance windows and new siding. This all started when I had the crafty idea that while I had the north wall open I might as well add some insulation since the north side never gets any sun - I wanted to minimize heat loss. As it stands right now, I either re-calc materials et. al. for adding polyiso foam to the north + furring strips or make sure the R13 batts are in perfect shape when I close the wall. The easiest might be to rip out the R13 and add unfaced insulation there. but I am still thinking about this option. I just know that all winter this wall will be wet and any moisture that gets trapped inside of the sheathing will rot the wall.