Dehumidifiers?
#1
08-08-03, 09:26 PM
Dehumidifiers?
In all the years that I have given advice to people, I have never recommended a dehumidifier. I know energy conservationists, HVAC professionals, plumbers and more who on a regular basis have recommended dehumidifiers. I do not pretend to be a basement water proofing or an Indoor Air Quality expert. I base my advice and opinion on a simple fact. That is that either heat or humidity must come from somewhere in order for it to be present within a structure. This explicitly implies that a dehumidifier does not address the source of the humidity in a basement. It merely aleviates the condition and certainly does not solve it.
Improving the grade around the perimeter of the basement, providing proper drainage like extending your gutter a significant distance from the perimeter of the basement, basement wall water-proofing, insulating cold water pipes, taping the seams in your clothes dryer exhaust, covering the sump pump well and more all address the possible sources of humidity in the basement.
The concern that I have with dehumidifiers is that under certain conditions it can aggrevate the situation. Equilibrium Relative Humidity works both ways. So if the ground and basement walls are at a higher humidity level than the air in the basement as a result of the dehumidifier, the humidity inside the ground and basement walls will be attracted to the basement air in order to equalize. As the humidity in the basement air increases, the dehumidifier works harder to remove the additional humidity. It eventually gets to the point that you need a larger dehumidifier to handle the amount of humidity being introduced into the basement. As the humidity is being drawn through the basement walls and floor because of the dehumidifier, it creates paths for the humidity to flow through and in some cases be retained. This in combination with hydrostatic pressure and the fact that water expands when it freezes are one of the reasons why cracks occur in basement walls and floors.
So besides the fact that a dehumidifier does not address the source of the humidity in a basement, in most cases it is going to make things worse.
Improving the grade around the perimeter of the basement, providing proper drainage like extending your gutter a significant distance from the perimeter of the basement, basement wall water-proofing, insulating cold water pipes, taping the seams in your clothes dryer exhaust, covering the sump pump well and more all address the possible sources of humidity in the basement.
The concern that I have with dehumidifiers is that under certain conditions it can aggrevate the situation. Equilibrium Relative Humidity works both ways. So if the ground and basement walls are at a higher humidity level than the air in the basement as a result of the dehumidifier, the humidity inside the ground and basement walls will be attracted to the basement air in order to equalize. As the humidity in the basement air increases, the dehumidifier works harder to remove the additional humidity. It eventually gets to the point that you need a larger dehumidifier to handle the amount of humidity being introduced into the basement. As the humidity is being drawn through the basement walls and floor because of the dehumidifier, it creates paths for the humidity to flow through and in some cases be retained. This in combination with hydrostatic pressure and the fact that water expands when it freezes are one of the reasons why cracks occur in basement walls and floors.
So besides the fact that a dehumidifier does not address the source of the humidity in a basement, in most cases it is going to make things worse.
Last edited by resercon; 08-09-03 at 06:33 AM.
#2
08-11-03, 08:11 AM
Dehumidifers in basements
Dehumification techniques that are applicable to one area of the country don't necessarily apply to other areas of the country. For example, in Florida, dehumidification occurs nearly year round - through the operation of central air conditioning systems. In the desert southwest and the western mountains, dehumidification is almost never necessary to control humidity levels within a structure. Because I live in the northeast, I will address the dehumidification needs common to my area of the country.
resercon, the points you make in the second paragraph of your above post are right on the money. I am sure that many, many moist basements could be remediated through the proper implementation of drainage and other fixes to reduce interior moisture. However, some of what you state in the third paragraph of your post is not entirely correct.
As I see it, moisture in basements aside from moisture generated by activity within the structure (cooking, showering, clothes drying) has two major causes - moisture migrating though the foundation walls and floor due to moist soil, and moisture that comes into the house from the presence of hot, humid summertime air dominates the weather across the north and northeast in July and August.
In the summer, up in the northern / northeastern tier of the states, much, if not most, of the humidity in some basements comes from hot, humid summer-time air coming into the house through open windows and into the basement. Because basement temps in summer are relatively cooler than the air outside the house, when that outside air comes into the house and down into the basement, the cool temps of the basement result in a higher relative humidity (because cooler air can't hold as much moisture are warm air). In a house with central AC, humidity / moisture is removed by the AC system. In houses without central AC, which are very common in the northern tier of states, another mechanism is necessary to reduce moisture levels in the basement of the house - dehumidification. When the primary moisture flux is due to this mechanism, dehumidification is an excellent solution to a moist basement. For a house with central AC, a separate, stand-alone dehumidifier in the basement may not be necessary because AC is effective at dehumidification.
The other major source of elevated summertime basement moisture/humidity levels, is moisture migrating from the soil surrounding the house through the concrete and into the basement. This moisture is independent from humid summertime air. This movement of moisture happens due to moisture gradients - the laws of nature and physics dictate that moisture will move from an area of high moisture content to low moisture content. That rate of movement is determined by the relative differences in the moisture levels between the high and low moisture content areas, the permeability of the material through which the moisture will travel, and the thickness of the material. In other words, if the level of humidity outside the foundation is equal to the level of humidity inside the basement, there is no movement of moisture from outside to inside. If you reduce the amount of relative humidity in the basement such that it is less than that which is outside the basement, you will induce moisture to flow through the wall structure. But all that you need to do is to reduce humidity levels to 50%.
In the winter up in the northern tier, the air outside is cold. When that air is brought inside and heated, it's relative humidity is substantially reduced. This air is now dry enough that in most cases, dehumidification in a basement is not necessary. In those cases where the basement is so wet that humidity is a problem even in winter, the best solution is to not use the basement.
Dehumidifiers are not intended to address the source of moisture in a basement - they are a mechanical solution to controlling moisture in situations where a retrofit to the building to eliminate moisture infiltration through a concrete basement foundation wall is not practical. In modern construction, this problem is in part solved by using low permeability insulation inside the concrete wall, or alternatively, by placing an insulation layer against the exterior of the foundation wall before the wall is backfilled. In existing buildings, this is often not practical or cost effective.
resercon, you said "So besides the fact that a dehumidifier does not address the source of the humidity in a basement, in most cases it is going to make things worse." In my case, the operation of a dehumidifer in my curently unfinished basement eliminates that musty smell that comes with the humid summertime air masses we get in this part of the country this time of year.
But I just don’t understand the mechanics of how lowering the relative humidity could possibly make a basement more humid. If the relative humidity of the air in a given basement in summer is 85%, a dehumidifier should be run to reduce this level of humidity. What I don't understand is how running a dehumidifier would "make the problem worse", i.e., result in humidity levels greater 85%. It's just not logical. Also, because central AC is an effective dehumidifier, does this also mean that central AC is making things worse, too?
resercon, the points you make in the second paragraph of your above post are right on the money. I am sure that many, many moist basements could be remediated through the proper implementation of drainage and other fixes to reduce interior moisture. However, some of what you state in the third paragraph of your post is not entirely correct.
As I see it, moisture in basements aside from moisture generated by activity within the structure (cooking, showering, clothes drying) has two major causes - moisture migrating though the foundation walls and floor due to moist soil, and moisture that comes into the house from the presence of hot, humid summertime air dominates the weather across the north and northeast in July and August.
In the summer, up in the northern / northeastern tier of the states, much, if not most, of the humidity in some basements comes from hot, humid summer-time air coming into the house through open windows and into the basement. Because basement temps in summer are relatively cooler than the air outside the house, when that outside air comes into the house and down into the basement, the cool temps of the basement result in a higher relative humidity (because cooler air can't hold as much moisture are warm air). In a house with central AC, humidity / moisture is removed by the AC system. In houses without central AC, which are very common in the northern tier of states, another mechanism is necessary to reduce moisture levels in the basement of the house - dehumidification. When the primary moisture flux is due to this mechanism, dehumidification is an excellent solution to a moist basement. For a house with central AC, a separate, stand-alone dehumidifier in the basement may not be necessary because AC is effective at dehumidification.
The other major source of elevated summertime basement moisture/humidity levels, is moisture migrating from the soil surrounding the house through the concrete and into the basement. This moisture is independent from humid summertime air. This movement of moisture happens due to moisture gradients - the laws of nature and physics dictate that moisture will move from an area of high moisture content to low moisture content. That rate of movement is determined by the relative differences in the moisture levels between the high and low moisture content areas, the permeability of the material through which the moisture will travel, and the thickness of the material. In other words, if the level of humidity outside the foundation is equal to the level of humidity inside the basement, there is no movement of moisture from outside to inside. If you reduce the amount of relative humidity in the basement such that it is less than that which is outside the basement, you will induce moisture to flow through the wall structure. But all that you need to do is to reduce humidity levels to 50%.
In the winter up in the northern tier, the air outside is cold. When that air is brought inside and heated, it's relative humidity is substantially reduced. This air is now dry enough that in most cases, dehumidification in a basement is not necessary. In those cases where the basement is so wet that humidity is a problem even in winter, the best solution is to not use the basement.
Dehumidifiers are not intended to address the source of moisture in a basement - they are a mechanical solution to controlling moisture in situations where a retrofit to the building to eliminate moisture infiltration through a concrete basement foundation wall is not practical. In modern construction, this problem is in part solved by using low permeability insulation inside the concrete wall, or alternatively, by placing an insulation layer against the exterior of the foundation wall before the wall is backfilled. In existing buildings, this is often not practical or cost effective.
resercon, you said "So besides the fact that a dehumidifier does not address the source of the humidity in a basement, in most cases it is going to make things worse." In my case, the operation of a dehumidifer in my curently unfinished basement eliminates that musty smell that comes with the humid summertime air masses we get in this part of the country this time of year.
But I just don’t understand the mechanics of how lowering the relative humidity could possibly make a basement more humid. If the relative humidity of the air in a given basement in summer is 85%, a dehumidifier should be run to reduce this level of humidity. What I don't understand is how running a dehumidifier would "make the problem worse", i.e., result in humidity levels greater 85%. It's just not logical. Also, because central AC is an effective dehumidifier, does this also mean that central AC is making things worse, too?
#3
08-11-03, 10:56 AM
There are several factors that support my position, they are:
The environments we create within structures clearly defies nature and it is just a matter of time where nature will prevail.
Architects specifically design homes to offset the effects nature will have on the structure.
When choosing an application from several, ideally choose the one that has the lowest probability to create a problem.
Finished basement are never part of new construction.
The water vapor molecule is considerably smaller than the water molecule, which gives it the abilitiy to permeate all materials.
Though the rule with heat is low pressure towards a higher pressure area, it does not apply to moisture, even though there is a percentage of moisture within the heat. Equilibrium Relative Humidity (ErH%) does. This states that a mass, either a gas, liquid or solid, that possesses a higher humidity level than other masses will be attracted to the lower level humidity mass until they are equal in humidity level. The rule here is absorption. So the direction of the heat flow maybe the direct opposite of the moisture flow, depending on which mass has the lower humidity level.
Masonry rate of moisture expulsion is determined by its mass or if you prefer, permeability. Its rate determines its ability to retain a certain amount of humidity or moisture. For example a masonry wall that possesses 20% during the summer, if this amount is retained for the winter, the temperature drop of the masonry increase the percentage to 30%. At that point the wall is saturated.
When water freezes it expands about 12% in volume.
Basements are not good conditioned spaces. Musty smelling unfinished basements is a direct result of poor air exchange in the basement. This does not allow nature to take its course.
More structures have been destroyed by moisture problems than you can shake a stick at and air-conditioning tops the list for causes.
This list of the things that I base my opinion on dehumidifiers can go on and on. In fact, it is long enough for a book and it would probably come in volumes. As long as we insist on producing environments that clearly defy nature, we must look for those applications that look to offset the ravages of nature. A dehumidifier only increases the difference between the two environments and does nothing to offset the differences. Therefore the probability that problems will arise as a result of using a dehumidifier is far greater than if one was not used.
The environments we create within structures clearly defies nature and it is just a matter of time where nature will prevail.
Architects specifically design homes to offset the effects nature will have on the structure.
When choosing an application from several, ideally choose the one that has the lowest probability to create a problem.
Finished basement are never part of new construction.
The water vapor molecule is considerably smaller than the water molecule, which gives it the abilitiy to permeate all materials.
Though the rule with heat is low pressure towards a higher pressure area, it does not apply to moisture, even though there is a percentage of moisture within the heat. Equilibrium Relative Humidity (ErH%) does. This states that a mass, either a gas, liquid or solid, that possesses a higher humidity level than other masses will be attracted to the lower level humidity mass until they are equal in humidity level. The rule here is absorption. So the direction of the heat flow maybe the direct opposite of the moisture flow, depending on which mass has the lower humidity level.
Masonry rate of moisture expulsion is determined by its mass or if you prefer, permeability. Its rate determines its ability to retain a certain amount of humidity or moisture. For example a masonry wall that possesses 20% during the summer, if this amount is retained for the winter, the temperature drop of the masonry increase the percentage to 30%. At that point the wall is saturated.
When water freezes it expands about 12% in volume.
Basements are not good conditioned spaces. Musty smelling unfinished basements is a direct result of poor air exchange in the basement. This does not allow nature to take its course.
More structures have been destroyed by moisture problems than you can shake a stick at and air-conditioning tops the list for causes.
This list of the things that I base my opinion on dehumidifiers can go on and on. In fact, it is long enough for a book and it would probably come in volumes. As long as we insist on producing environments that clearly defy nature, we must look for those applications that look to offset the ravages of nature. A dehumidifier only increases the difference between the two environments and does nothing to offset the differences. Therefore the probability that problems will arise as a result of using a dehumidifier is far greater than if one was not used.
#4
08-11-03, 11:51 AM
Specifics, please
resercon, please describe how running a dehumidifier in a basement during the humidity of summer would make the basement more humid.
How does the dynamic lowering of indoor relative humidity through the operation of a dehumidifer or air conditioner result in problems for the building, and what specific problems would result from such operation?
My experience has been that operating a dehumidifier in my basement during the the summer humidity of July and August results in a basement that is more comfortable and enjoyable.
How does the dynamic lowering of indoor relative humidity through the operation of a dehumidifer or air conditioner result in problems for the building, and what specific problems would result from such operation?
My experience has been that operating a dehumidifier in my basement during the the summer humidity of July and August results in a basement that is more comfortable and enjoyable.
#5
08-11-03, 08:44 PM
Appearances can be deceiving.
Excuse me for not being more clear in my objections to the use of a dehumidifier in basements. I am not disputing that a dehumidifier will decrease humidity levels in basements. I am stating that the dehumidifier will cause the humidity level in the basement air to be lower than the humidity level in the masonry. Since nature seeks balance and the rule is absorption, the air in the basement will absorb the humidty in the masonry until they are equal. In some cases the humidity level in the masonry drops as a result. Unfortunately this is rare. What usually occurs is that now the humidity level in the masonry is lowered, it now has the ability to absorb moisture from the ground surrounding the basement. Where the amount of moisture in the ground can be described as being endless. As the dehumidifier continues to operate, the rate of moisture flow from the ground through the masonry increases. As the flow increases, so does the amount of retention of moisture. Yet the basement air is still at a low humidity level because of the dehumidifier, you just have to empty it more often. It amazes me that people do not see that the amount of water they are dumping out of their dehumidifers and not realize or even question where it is coming from. Rather they say this dehumidifier is doing one hell of job, the basement is a lot more comfortable.
I am not going to discuss the other contributing factors here such as excavated ground is far less compacted than unexcavated ground and the excavated ground can hold more than 20 times the amount of water, hydrostatic pressure and capilliary action. And if you are wondering how long will it take for excavate ground to become as compacted as unexcavated ground, more than a thousand years. There are some people who argue, never, but I am not going to be around to see who is right.
I believe in the saying, seeing is believing. Moisture meters are not that expensive today. Get one and put it against your basement wall and remember that read out. Then find a basement that does not use a dehmidifier and measure the moisture content of their wall. Theoretically the basement air with the lower humidity level should have the wall with the lower moisture content by the meter. This might not be the case because of the dehumidifier which wall might end up with the higher moisture content of the two. Then turn off your dehumidifier and open all the basement windows and run a fan for a week. Then measure the moisture content of the wall. I prefer you to not take my word on this and that you take these simple tests yourself.
One last thing I would like to add and that is, do not think that a moisture problem in a basement is confined to the basement. It will indeed affect the rest of the structure.
I am not going to discuss the other contributing factors here such as excavated ground is far less compacted than unexcavated ground and the excavated ground can hold more than 20 times the amount of water, hydrostatic pressure and capilliary action. And if you are wondering how long will it take for excavate ground to become as compacted as unexcavated ground, more than a thousand years. There are some people who argue, never, but I am not going to be around to see who is right.
I believe in the saying, seeing is believing. Moisture meters are not that expensive today. Get one and put it against your basement wall and remember that read out. Then find a basement that does not use a dehmidifier and measure the moisture content of their wall. Theoretically the basement air with the lower humidity level should have the wall with the lower moisture content by the meter. This might not be the case because of the dehumidifier which wall might end up with the higher moisture content of the two. Then turn off your dehumidifier and open all the basement windows and run a fan for a week. Then measure the moisture content of the wall. I prefer you to not take my word on this and that you take these simple tests yourself.
One last thing I would like to add and that is, do not think that a moisture problem in a basement is confined to the basement. It will indeed affect the rest of the structure.
