Moisture problem
I built a fully insulated 8’x12’ shed last 2 summers using 2x4walls with R13 FG insulation + 1.5”EPS on the walls then 1/2”OSB sheathing. I put a small heater inside during the winter just to observe its performance during the cold months. For the past winter, there was no problem, so I built another extension in the house last summer using 2×3 double stud walls with R19 FG, gypsum bd on the inside. I installed 1/2”OSB sheathing then 1” EPS on the outside the T-111 sidings. I used house wrap both on the inside and outside walls. The inside walls are properly sealed but no final painting. This winter I put a small heater to observe also and to my surprise is there was a lot of moisture coming on the walls as evidenced by water at the bottom of the walls. I am a little puzzled what did I do wrong? Any suggestions is appreciated before I proceed with the final finishing work. Thank you.
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Big difference between a shed and a house is there are no moisture sources in a shed (people, cooking, showers). Usually, any insulation or stackup works in a shed, not all work in the house.
Depending on the climate zone you are in, the 1" of EPS outside your wall is not enough for condensation control with plain drywall. For example, around me Zone 5, an R19 wall needs R7.5 of exterior rigid. If it is less rigid, than you need a warm side vapor retarder.
If you are in similar cold climate, it could be as simple as air sealing the drywall and painting it with vapor barrier paint.
Thank you for your reply. Actually I have a similar extension in the house using only 2x4 and R13 FG + Gypsum + housewrap and T-111 sidings but have no problem with moisture as compared to the othe extension with double stud walls. Actually I heated up the room to about 72degF and had my dehumidifier running just to see the results. Though I was able to reduce the moisture on the walls, the problem is stll there. As far as air sealing of the room, I would say the sealing is quite good as evidenced by the suction/possitive pressure the moment i open the door. Hopefully as summer is approaching, I will still monitor the condition before I do my final finishing work.
The rigid insulation on the outside acts as a vapor barrier, without sufficient amounts of it to warm up the temperature of the sheathing, you'll get condensation. The rigid won't allow this moisture to pass to the exterior to dry thus the issue you are seeing.
The reason your other wall is fine is that there is no rigid on the outside.
Your options are either to add more rigid insulation, you can see the ratio needed based on climate here:
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
Or add a smart vapor barrier behind the drywall to control moisture movement and air seal the drywall.
If you go the vapor barrier approach, make sure to run a dehumidifier set on the lowest setting for a couple of weeks to dry up the existing walls, you don't want any moisture in the walls before the vapor barrier goes on.
Thank you for your reply. I think adding rigid foam is not an option anymore unless I have to remove my sidings and add more foam insulation. Actually the R19 insulation has a kraftpaper faced then I add Tyvek over it before I installed the gypsum bd. Perhaps I can use a vapor sealing paint before I do the final painting with the dehumidifier running continuously. I live in Juneau Alaska. I have a lot of similar construction except the double stud walls and this is the results.
That explains it. In Zone 7, even with a vapor barrier, you need much more rigid than what you installed.
Take a look at table 2 here:
https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf
Your choice is now either strip off the siding and add rigid insulation or add enough spray foam to the cavity to bring the overall R value up to the guidelines. You don't need to fill the cavity fully with SPF, just enough to get the overall R value of the exterior rigid plus SPF to the ratio shown in the table. The rest of the cavity can be filled with fluffy insulation.
Unfortunately, in very cold climate, there is no easy way out of the moisture issue you are seeing.
More foam or less. The wall can also be made safer by removing the exterior foam and adding a rain-screen gap behind the siding, which will allow the moisture to dry to the outside. It still needs a good interior vapour-retarder and air-sealing either way.
Thank you both of you. I will take a look on the links you sent me. The T-111 sidings are not direct contact with the rigid foam. I put 1/2” thk furring strips 16”o.c(stud
location) where I also nailed the sidings. All joints where sealed properly. I will see what I can do as soon as the weather improves here. I have some thoughts of ripping the interior gypsum wall, remove the FG insulation and replace it with sprayed foam. It is easier for me to remove the gypsum wall than the sidings. zi will appreciate very your comments/suggestions. Thank you again.
Exterior low perm foam vs vapor retarder class needed is here:
https://www.continuousinsulation.org/content/2021-ibc-irc-adopt-improved-vapor-retarder-requirements
Looks like you need a Class I on the interior.
Verify air sealing with a blower door.
Sounds to me like the new addition to your home, was not heated above the dew point of the air in your home so lots of water was removed from your air.
Walta
I tried to monitor this problem by putting a heater, dehumidifier and a digital thermostat with RH function. I don’t have a hygrograpgh so i wasn’t able so see the actual dewpoint temp in the room.Thank you