Comment: from a reader
I had hardy board siding installed on my house when we had it built 6 years ago and we have had a problem with moisture along the walls on the interior floors. the builder installed the hardy siding against the house wrap and the back side of the siding was not sealed or primed. do you think this might be our problem? The problem is only on the exterior walls inside the house.
The house is six years old, located in Mississippi Our location has had two rain falls in the past two months. The relative humidity for this time of year is the highest @ 70% t0 80% on the outside of the house. I am not sure what the humidity would be on the inside. We leave the heat and cool pump set on 75% when we leave and set it at 73% when we are home. The temp outside 80-90% during the day and 60-70% during the night. Paper back roll insulation in the walls and blown in insulation in the ceiling. We
noticed the problem when the house was a year old, we have re-caulked everything on the exterior, we have painted the exterior again, we have sealed the brick on the bottom portion of the house, I have set the fan motor on the heat and air to low, so it would run longer.
I’m assuming that the house has a go air conditioner with the power to drop the house temp and to maintain it at 72°F when the outside air is 110°F . This problem occurs mostly when the inside temp is at 72° to 95°, if so than what is happening is to much humidity from the outside is coming into the house though Fresh air intakes(makeup air) for the appliances and or exhaust fans as well as an air exchanger. What is happening with the Air conditioner it sounds like it is to large of a unit. When the air conditioner runs constantly(45 minutes per hour) it has the ability to remove the water from the inside air. But if the AC unit only runs 15-25 minutes per hour and make up air coming into the house, this excess hot air coming in to the house brings with it a high concentration of water in a vapor form.
Now the hot air carrying humidity is in the house it rushes to the ceiling , while the AC unit is running dumping cold air into the house. ( A quick lesson in physics; Hot air is light and rises and cold air is heavy and drops to the floor) As the hot air looses its temperature it must loss volume of water its holding to do this it has to condensate on something cold. In a house it is the A- coil inside the furnace. but if the temp inside the house is already cool and the AC unit is not running the A-coil in the furnace is warm now the air has to find a cooler surface to condensate on. In this house it is the floor next to the exterior walls because gravity is holding the coldest air at the floor line. The exterior wall location is because the heating and cooling supply ducts have already dispensed the cold air there now making condensation.
1. Run 1-3 dehumidifiers I now they are ugly and noisy
2. Down size your AC unit and install a 2nd AC unit in the Attic for 90° + days this will allow you to run your primary AC unit in your furnace longer and remove the excess humidity from the house.
3. If you have a air exchanger run the fresh air through a dehumidifier before bringing the air into the house. You can do this with any fresh air intake flex tubes.
4. Keep the house doors shut as much as possible A max. of 1 door opening per hour, 10 second open.
The siding is not the problem. the house wrap is not helping. In hot climates you must protect the house (wood structure) from excess humidity. That means you must seal wall sheathing surface with something that will not allow moisture to pass through even if the siding is nailed on. that means all nails and nail holes must be sealed. Than a layer of moisture control fabric must be installed that removes the excess humidity. tarpaper works best. The insulation with the paper on it is not good because it has the ability to hold the humidity in the stud cavity. In hot climate locations fiberglass, or mineral wool insulation is great because air can pass through the insulation allowing the stud cavity to dry out. Do not use poly on the inside walls in a hot climate because you will trap humidity in the stud cavities and will rot out your structural walls.