1 Helpful?

Will I Have Moisture Problems?

I have learned a lot from this forum and have now started to have concerns regarding my home. Our 1900 sq ft ranch style home was built in 2006 and we had little-to-no awareness of the importance of energy-efficiency and green building. We live in a moderate climate, about 35 miles south of St. Louis, MO.

Our home was built with 2x4 construction, insulated with 3.5" unfaced fiberglass insulation and a poly vapor barrier on the interior (between studs and drywall). The home was not house-wrapped, and does not have any exterior insulation. There is simple plywood sheathing on the exterior, covered by vinyl siding.

My current understanding of green building design leads me to several concerns in the home. Additionally, our air-source heat pump was oversized for air conditioning and initially lead to high RH values in the summer (~55-65%) due to short-cycling and a high fan speed. Two years ago we purchased a Santa Fe Max Dry dehumidifier (for both basement and main floor) which has worked well to maintain interior RH levels at acceptable levels (~35/~45%), depending on which of the 4 hygrometers we read. Two of the hygrometers read about 10% higher than the other two with similar temperature values when all are next to each other and allowed to reach stable temps.

My most recent concern is that I would like to add exterior foam sheathing to increase the R-values of our walls, but am hesitant due to the interior poly vapor barrier as I realize there would be no area for any moisture that may accumulate to dry. This situation is compounded by the fact that we installed a high-efficiency fireplace to heat our home and decrease heating costs. This has worked great, but has lead to higher interior temperatures and therefore, more condensation on the windows. Although the condensation is not excessive, I am somewhat concerned about moisture in the walls. I am considering utilizing the dehumidifier even in the winter to decrease interior RH levels, but 1) I am not sure how low is too low and 2) I do not know which of my hygrometer values I should believe. Currently, our interior levels without the use of the dehumidifier are ~38% or ~48%, depending on which values I believe.

I should note that our house does not have a ventilation system, but needs one based on a blower door test. We are using our fireplace in the winter, which has the ability to bring in fresh outside air (auxiliary air control) which I have done somewhat to reduce interior RH levels. Additionally, I plan to install a fresh air duct to the return side of our dehumidifier to introduce fresh air in the summer. I should also add that the construction of our home is not atypical in our area, due to the lack of energy efficiency requirements in the state of Missouri and the very low electric rates with out electric utility.

I am considering removing all sheet rock on the interior walls and then removing the poly vapor barrier, although I would like to avoid this unless it would benefit the long-term durability of the home. By doing this, it would allow me to add the house-wrap and/or ~2 inches XPS foam insulation on the exterior of the home at a later date, which would involve removing the exterior siding. However, I am not sure if this is worth the effort or expense in our climate and could use some guidance.

Is my consideration of removing the interior poly (and perhaps adding exterior foam sheathing) worth the expense in reducing the risk of condensation in winter and/or summer? Are there other issues that may arise based on the building design of the home? Are there alternate options (such as using the dehumidifier) to reduce the risk of condensation? What interior RH level should I shoot for in summer and winter, given that I have conflicting readings of my hygrometers?

Asked by Ken French
Posted Dec 1, 2012 6:10 PM ET


3 Answers

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First of all, I applaud you for focusing on these issues.

It's a shame that your 6-year-old house has such a lousy thermal envelope. It's also a shame that your walls have no WRB and that they include interior polyethylene, which is inappropriate for your climate.

To address the easiest question first: you worte, "I am considering utilizing the dehumidifier even in the winter to decrease interior RH levels." The way to lower your interior RH level in winter is by operating a ventilation system. This method is much less expensive than running a dehumidifier. If you don't have a mechanical ventilation system, you should run one or more bathroom exhaust fans until the interior RH drops.

Second, I'm not sure what type of appliance you have -- the one you describe as a "fireplace." Does it burn wood or gas? Is it vented or unvented?

I'm worried that you may be using an unvented gas space heater. If that's true, it would certainly explain the high indoor humidity. Unvented gas space heaters are dangerous (and are illegal in California).

I believe that it is possible for you to install 2 inches of exterior rigid foam on your walls without removing the polyethylene (even though the polyethylene is not ideal). For such a wall to perform well, you need to do a very good job with your flashing and the installation details of your WRB. It would also be a good idea to include a ventilated rainscreen gap between your siding and the rigid foam (unless, of course, you stick with vinyl siding, which creates its own rainscreen).

Don't do this work without installing some type of mechanical ventilation system.

Answered by Martin Holladay
Posted Dec 2, 2012 6:19 AM ET



Thank you for your response. It is good to hear you confirm that we have a "lousy" thermal envelope. It is very common in this area. I have family in the building trade (including one who recently retired as an inspector) and they all think I am crazy when I talk about how this was done incorrectly and how it should have been done.

I will definitely start using fresh air ventilation to reduce our interior humidity levels. We have had a pretty mild fall and winter so far (today had a high of 72) and we have barely touched 20 degrees as lows at night. Perhaps that is partly the reason why the fresh air ventilation has not brought it down sufficiently, along with us likely not running our fans long enough. I recently discovered, by accident, that at least one of our bathroom exhaust fans was incorrectly installed. The screw that attached the duct to the exhaust fan unit was blocking the backdraft damper from functioning, effectively blocking the outward movement of air. While I was able to fix that one, fixing the other 3 is going to be a lot of work assuming they were also installed improperly.

To clarify, our fireplace is wood burning. It is an EPA-approved Quadra-Fire 7100fp. We also have a Quadra-Fire Cumberland Gap installed in our basement, which we have not had much opportunity to use this year. Both utilize an outside air kit to provide outside air directly to the firebox for combustion, meaning that we are not pulling in outside air through cracks/windows/doors to make up for air loss through the chimney. The OAKs are probably also limiting our fresh air to effectively reduce interior humidity levels. To clarify regarding the outside air in my original post, the fireplace has a fan which makes it much more effective as a heating unit. This unit has the capability to bring in outside air through an auxiliary air duct, which is circulated around the steel firebox and blown into the house creating a slight positive pressure in the house. I have been using this with a small amount of success to reduce interior humidity levels, but again would probably be more successful if allowed to run more consistently/longer. I can certainly use this more and use the bathroom exhaust fan (the one I have repaired) to introduce heated outside air and remove interior air. Could you provide me with a formula for determining the appropriate humidity levels to reduce the risk of condensation on windows and in walls? I am aware this will be partially dependent on interior and exterior temperatures.

Finally, I am glad to hear that 2 inches of exterior rigid foam should prevent the need to remove the interior poly. Would it be more appropriate to add more than 2 inches? I have been told that there is a local source where 4" of polyiso is available at a very inexpensive price, although I have not confirmed that. Whether I use 2" XPS or 4" polyiso, I plan to put vinyl siding directly back on top of the rigid foam. Since this is vinyl siding, and you write that it creates its own rainscreen, can you point me to a reliable source that describes how to install vinyl siding directly over rigid foam? Is the rigid foam glued to the OSB, then the siding attached with 2.5"-3" galvanized nails or is there a better method? I realize the seams would need to sealed with tape/mastic. Should I add a type of housewrap or will the rigid foam be sufficient as a WRB?

Answered by Ken French
Posted Dec 2, 2012 9:02 PM ET
Edited Dec 2, 2012 9:05 PM ET.


Q. "Could you provide me with a formula for determining the appropriate humidity levels to reduce the risk of condensation on windows and in walls?"

A. Here are two links to get your started:

Are Dew-Point Calculations Really Necessary?

Rating Windows for Condensation Resistance

Q. "Would it be more appropriate to add more than 2 inches [of exterior rigid foam]?"

A. If you want to improve the thermal performance of your walls, the more foam, the better. Most of the hassle involved with this work is labor, so it seems to me that if you are going to all that trouble, you might as well install 4 inches of foam if you can afford to do so.

Answered by Martin Holladay
Posted Dec 3, 2012 8:21 AM ET

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