Drywall moisture problems with 2 inch Exterior Foam Board retrofit in Hot-Humid Gulf Climate
I am beginning a deep energy retrofit on a 100+ year old home in a hot-Humid climate. The home is in great structural shape with very little if any moisture problems. As typical of these homes it leaks everywhere which likely helped keep it in good shape over the years. If anything got wet, there was enough ventilation to dry quickly.
The attic and ventilated crawlspace were recently foamed and sealed and the client now would like to tackle walls.
I was planning on taking the wall to the 2X4 studs, then cover (in order) with OSB, house wrap, 2 inches of polyisocyanurate, furring strips and then siding. However I have heard from other contractors of similar projects developing moisture problems in the interior and drywall shortly after renovation.
Is this just an example of poor attention of details? Are they leaving gaps in the wall assembly letting hot-humid air in that condenses on the cooler interior walls? Are the homes not venting internal moisture adequately? Are the AC units oversized and not dehumidifying enough.
I cannot find any good long term data on this wall assembly in the hot-humid climate. On paper it seems to work but we are seeing some examples otherwise in practice.
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Your system should work, as long as:
1. You pay attention to air sealing (to minimize the chance that humid exterior air could enter your wall cavities).
2. You don't install interior polyethylene or vinyl wallpaper.
Your walls should be able to dry to the interior (which will be dryer than the exterior when your air conditioner is operating). As long as you obey the rules above, your house should perform well.
What is your city?
Can you give more details about the "failure" examples.
Terrence, exterior foam board only insulation may be a bad idea in your climate.
Instead, with the walls open you can spray foam them and caulk all the joints such as sills, headers and band joists.
Think about it. Where the foam board does leak warm moist air, it is then trapped with time to wet your inner wall. With the wall inner space foamed instead, the moisture is either in or outside the home and not in the wall.
Building Science Corporation may be your best source.
The City is New Orleans, The failures were not major problems like mold and rot. Most contractors down here learned the mold/moisture problems with the interior vinyl wallpaper and are very aware to not make the same mistake again.
The reported problems were small problems like pictures and wall attachments all falling down throughout the home as the drywall softened, as well as some buckling. Again, it hasn't been a widespread problem since very few old homes in the area have retrofitted with this type of wall assembly.
Most of the new green homes being rebuilt in the devastated neighborhoods after Katrina are using this approach. So far they are not reporting any problems. However most of them have had blower door testing and much better attention to air sealing. My suspicion is that in the older retrofits with these problems, they did not pay proper attention to air sealing the entire home or had leaks or other moisture problems.
I was just tying to fine some more long term data on the durability/stability of this system in this climate. When others are reporting these problems it makes me cautious that there might be something more insidious occurring.
Ideally we will be able to get another 100 years out of the wall when we are done and with a more efficient home to boot.
You might contact Bill Robinson, he is in New Orleans and has experience in that market and climate. He is moderator of the Exterior Detail forum over at JLC.
You would be fine with your wall assembly as long as all your water management and envelope sealing details are carried out to the T. Also, you need to have insulation, sheetrock and wall finish that dries to the inside of the wall. Above all and probably most important, you need to install an HVAC system that is designed, installed, balanced and commissioned properly.
Here is a good link from the LSU Ag Dept.: http://www.lsuagcenter.com/en/family_home/home/design_construction/Construction/Foundation+Floors+Roof+Walls/Exterior+Walls+Roof/Walls+Wall+Covering/Ideal+Wall+Assemblies+for+Hothumid+and+Mixedhumid+Climates.htm
You can also got to http://www.buildingscience.com and search information for hot/humid climate.
I've heard about the problems with glued-to-the-wall mirrors, as you point out. These mirrors are vapor impermeable, just like vinyl wallpaper, so you get mold behind them. These walls have to dry inward.
Thanks for all the advice. I will follow up with your recommendations.