Book Storage: Dewpoint or Humidity 2

[gatt]

All;
We have recently turned a general storage area into a book storage area. There are currently two fan coil units and no reheat. I am trying to keep it dry in there by cooling the space to 60-65 degrees. The relative humidity increases, of course, and that is the parameter that everyone wants to hear about. Am I wrong in trying to assert that the dewpoint is the important number? I have 64% R.H. at 62 degrees - dewpoint about 50 dgrees.

 

[DRWeig]

I'd suggest asking a museum director or an engineer experienced in library design.

My gut feeling is that humidity control is the issue, you should not be approaching dewpoint conditions under any circumstances... You don't want to excessively moisturize the paper (high humidity) or the books will swell and distort, but low humidity may dry them out too much and make the pages brittle.

Let's see if any book-preservation experience is out there on the forum, though --

Good on ya,

Goober Dave

 

[AbbyNormal]

you can try the ASHRAE guide on Humidity control there is a chapter on museums, libraries and archives.

Books can be like sponges the absorb mositure but do not like releasing it until you get the RH down below 35%.

Sometimes rare books you have to worry about the bindings more than the paper.

I would say in most cases if you stored the books at room temperature and kept the humidity more or less constant below 60% RH you would be fine.

I would suggest you use a dehumidifier to give you independent temperature and humidity control.





The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[walz]

When the issue is to care about keeping things dry .... like your book in the library ..... yes its RH that matters

When issue is to care about condensation that occurs on the cold surfaces, like the outside surface of cold water glass or pepsi cane. it has to be dewpoint.

No way in your case, it should be dew point. Dew point has its own significane but where you need things to be kept dry, you have to care about RH.
I will add something which i feel is the confusion that brought you to write ask this question is that these two things are related .... when RH increases Dew point also increases. So may be you are thinking that as humidity levels increase in the book store, and so the Dew Point also. What does a Dew Point temperature means? It means that condensation is more likely to happen at even higher temperatures.

For this to happen, you have to have a surface/body at or colder than dew point temp you mentioned. If this cannot happen condensation will also not happen. But RH is what that will keep the quality of paper.

 

[marcoh]

Design conditions for museums are typically 21oC +- 0.5oC and 55%RH +- 5%RH. My understanding is that constant RH is the most important criteria and minimising the rate of RH change.

 

[gatt]

Thanks everyone.

My confusion comes from thinking about the moisture content in the air. At the conditions I state above, I can reheat the air and lower the R.H. but still have the same moisture content(?) in the air.

Is the R.H the "driving force" for the books to absorb moisture or the actual moisture in the air?

I have found that a recommended Temp/R.H. for an unoccupied book (braille in this case) storage room is 60F / 50%.

I will install 2 portable dehumidifiers that we already have available to lower the R.H.

 

[DRWeig]

Sounds like you have a good plan...

You're right that you can heat the air and lower the RH, but still have the same amount of moisture in that volume of air --

But, the rate of moisture exchange with an absorbant material (paper in this case) depends on the partial pressure of water vapor in the air versus the equilibrium partial pressure for that material. In other words, if everything is at equilibrium and you lower the RH of the air by heating it, moisture will evaporate from the material until a new equilibrium is reached. This is the principle behind your clothes dryer...

On the other hand, raising the RH will create a new equilibrium point in which the material contains more moisture. When we measure moisture content of hygroscopic materials, we look at water activity of the surrounding air for one method. The goal is to find the equilibrium point.

I hope I haven't confused -- it's a little hard to visualize. My comapny deals in humidity sensors for the building automation market, and I know how this stuff works but I'm not so good at putting it into words.

For some real deep humidity technical support, go to

http://www.vaisala.com

(not my company, Vaisala is more into the instrument / industrial / weather grade of moisture measurement).

Good on ya,

Goober Dave

 

[toughguy1]

I just wanted to offer my 2 cents. First, when you reheat, you force the air conditioner to run longer and condense more water out of the air while NOT over cooling. The result is dryer air that is at a more comfortable temperature. Also, reheat allows you to control humidity within a defined range. Cooling the air down to 60 degrees will condense water out of the air, but raise the RH. This is because colder air has less ability to hold moisture. I would recommend that you install reheat controlled by a humidistat and hold to 50%+/- 5%.

 

[Compositepro]

Relative humidity is a measure of the moisture level in air relative to air that is fully saturated with water at a given temperature. At 100% water will not evaporate and vapor will have a strong tendency to condense. This measurement is what is important when analyzing any situation where there is any equilibrium, like the moisture content of air versus the moisture content of paper. If the reaction is irreversible, like between water and portland cement, it is the absolute humidity that is important.

 

[SAK9]

Compositepro has put it clearly.What is important is the RH because absolute humidity can not be measured. .Air at 64% has a greater chance of condensation than air at 50%.I had in the past the opportunity of investigating mould growth in an archives room that had RH of around 65%.We brought it down to 55% to stop the growth.

Dewpoint control can be used where RH control is required ie measure the RH and vary the dewpoint to maintain required RH .Using heaters is also an option.Is any fresh air being supplied to the space?