Replies

1. 
   DickRussell | Sep 12, 2008 07:13pm | #1

   I followed the formulas through, and of course they are fine. Comments:

   Basis for last column says it is for fixed 50% RH inside humidity, but the formula is based on a 45 F dew point, presumably calculated for a certain air temperature at a given RH. If the indoor air temperature can be dropped into the spreadsheet as a variable, then the dew point will change.

   I have seen remarks about a particular wall construction being fine with respect to moisture as long as inside humidity is below a certain level. The 50% might be high for the middle of winter up here in NE, although it would give a conservative result. Advice is to keep the humidity down to 40% max for health reasons and to avoid wall problems. Doing a lot of cooking, with water vapor generation, could drive it higher, of course.

   The YES/NO decision is based solely on the temperature at the outside of a wall layer being below or above 45 F, the presumed dew point of the inside air. This presumes that the air barrier at the inside is faulty, so that the air within the cavity is open to the humidity of the inside air.

   With a well-sealed inner wall, preventing convective flow into the cavity from inside, the only means of moisture transport is by diffusion, a relatively slow process. There will be a moisture concentration gradient across the wall, just as there is a temperature gradient. The drop in moisture content from inside to outside (dry winter air outside presumed) goes according to the perm values of the various layers in the wall. A FG batt will have essentially no resistance to moisture diffusion, painted drywall and foam fairly high.

   As long as there is no very low perm layer on the outside, the actual dew point of the air within the different layers of the wall structure will drop also. A NO result will of course be accurate, but a YES result is really a MAYBE, depending on how well sealed the inner layer is to leaks.

   I guess the use of the spreadsheet would be to evaluate wall assemblies for condensation potential if the inner layer isn't well sealed. But shouldn't new construction be based on making it tight against convective movement to/from the cavity?

1. 
   michael_maines | Sep 12, 2008 11:54pm | #2

   Good points Dick.  I will make the RH a variable.  IIRC Riversong had suggested using a high value to cover high-moisture days.

   If I was installing instead of drawing I could make sure the interior vapor retarder was done properly.  Instead I have to make sure the system will work well even if not done perfectly.  We've been doing a lot of 2" foam with the balance of the cavity filled with fiberglass, and I wanted a way to make sure the system will not cause moisture problems. 

   The way I was thinking of the YES/NO answer was that by playing around with the delta T you could guess how often the dewpoint would move to a "bad place."  There aren't a whole lot of 0° days so if the dewpoint moves to the fiberglass on those days, good air sealing means that it shouldn't be a problem.  

   Could you expand on As long as there is no very low perm layer on the outside, the actual dew point of the air within the different layers of the wall structure will drop also.?  We do have a very low perm layer on the outside, and I am trying to fully understand the dynamics of this system.

   One problem I found with my spreadsheet is that with dense-pack, the dewpoint always occurs within that material.  When is that a problem?

   What other tools are out there to determine what a good assembly is?

    

    

1. 
   DickRussell | Sep 13, 2008 07:31pm | #7

   Mike, to calculate a dew point for variable temperature and relative humidity, you need a curve for vapor pressure of water. Use this to give vapor pressure in psia for temperature in degrees F:VPW=EXP(14.4055-6936.5635/(A2+379.591))where "A2" is whatever cell holds T; for your current spreadsheet, the T in degF is in B3. This equation is good to about 1-2% accuracy. A five-constant equation gives a better fit, but can't be solved easily for getting T from P (will need to do this below for dew point).Anyway, that gives you the partial pressure of water in psia for saturated air at the temperature. I at first thought that multiplying by RH gives partial pressure at the given RH. Finding dew point would then be a matter of finding the temperature that gives saturated partial pressure equal to the actual partial pressure at room temperature. Inverting the above equation:TDP = -6936.5635/(LN(VPW*RH) -14.4055) -379.591However, I'm not sure of the definition of "relative humidity." It is supposed to be how much water is in the air relative to what the air could hold at that temperature. I've got to look up how humidity is calculated. What I have above doesn't take atmospheric pressure into account, and that has to matter. I'll edit in a fix when I've found it, unless someone beats me to it.Edit later today:
   OK, I Googled up a couple of relative humidity definitions, and the following URL gives some formulas for intercalculating between actual temperature, dewpoint temperature, and relative humidity:http://www.gorhamschaffler.com/humidity_formulas.htmIt turns out what I posted earlier was right, and relative humidity doesn't depend on altitude (or atmospheric pressure). The equations given in that URL use different Antoine equation coefficients and work with temperatures in degrees C and pressure in millibars. But that's OK. The end result is about the same.Using my own equations or the ones in that URL, I don't get the 45 F dew point your spreadsheet uses for 68 F and 50% RH. I get a dew point just under 49 F. Big deal. The purpose of your spreadsheet is to get a feel for the possibility of a condensation problem within a proposed wall assembly. It doesn't matter if you pick a particular temperature and RH. They just have to be feasible numbers. It's just as good to just pick a dew point temperature to represent the humidity of the air inside the house. At 68 F, 43% RH gives about a 45 F dew point.
   ----- end of info added by edit ------Then I guess I still owe you a response to your asking about the dew point of the air within the layers of the wall dropping. As your spreadsheet shows, there is a temperature profile across the wall (big drops across good insulating layers, small drops across good conductors).Similarly, there will be a moisture concentration profile across the wall, assuming typical winter conditions with higher humidity inside, dry (low dew point) air outside. In the absence of convective air flow, with only diffusion of water molecules through the wall layers, you'll see big concentration drops across layers with high vapor retardency, small drops across very permeable layers. A FG batt is so porous, with no real resistance to the diffusion of water vapor, that the moisture concentration within a FG batt will be almost uniform.Cellulose also is quite porous, although probably dense-packed it may have some very slight resistance to diffusion compared to FG.Just as we use R = 1/U to relate "R" value of insulation to the "U" (heat transfer coefficient) seen on the stickers of new windows, we probably could invent something like "M" to represent the resistance of a material to moisture diffusion, the inverse of "perm." This is reasonable, since the permeability of a material is cut in half if the thickness is doubled.We probably could estimate the water concentration profile across a wall where movement of water vapor is only by diffusion just as we do for heat through insulating layers. Add up the total M values of the layers, and the fraction concentration drop across a layer would be M for the layer divided by total M for the wall assembly.Geez, I hope Martin Holladay chimes in on this to give his opinions.2nd Edit later today:
   Ok, I reread your questions, particularly about dew point occurring within densepack cellulose. Since both FG and cellulose are very porous with respect to moisture diffusion, there will be a fairly flat moisture concentration profile across a layer of FG or cellulose. That really explains why there shouldn't be condensation within a FG batt or cellulose, if there is no convective air flow from inside the house to the cavity. As more moisture diffuses into that layer from inside the house, the concentration of water vapor within the layer will rise mostly uniformly until the dewpoint rises to the coldest temperature in that layer. That of course will be at the outside boundary of the layer.Usually that boundary would be a surface of markedly lower perm value. That would be the inside surface of the sheathing or of a foam layer. As moisture diffusion into the cellulose layer increases, the concentration can't rise above what the outer boundary temperature will support. The added moisture either is absorbed by the boundary material (plywood or OSB) or forms liquid water that runs down the surface.As mentioned in a prior post, calculations by John Straube have shown that the wood materials can safely absorb the relatively small amounts of diffused moisture coming across the cavity, even over a month or more of very cold outside temperature. If there is convective air flow from inside the house to the cavity, all the calcs are voided.Moral to the story, as noted by others: make an effort to slow down the movement of moisture into the cavity, but don't put up such a barrier that when moisture does get in under other circumstances it can't dry out again.Edited 9/13/2008 7:16 pm ET by DickRussellEdited 9/13/2008 7:19 pm ET by DickRussell

   Edited 9/13/2008 7:40 pm ET by DickRussell

1. 
   BillHartmann | Sep 14, 2008 03:49am | #8

   I thought that there was a University sponered (or maybe DOE) that handled all of those calcualtion.But I am not sure, but I think that I hve heard something about it..
   .
   A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

1. 
   DickRussell | Sep 14, 2008 05:31pm | #11

   There is program called WUFI that is used to model moisture movement through walls over time. I haven't used it, though. I imagine it would have to calculate temperature profile across the wall as part of the effort.

2. 
   BillHartmann | Sep 14, 2008 05:55pm | #12

   That is the one.http://web.ornl.gov/sci/btc/apps/moisture/ibpe_sof1.htmlWUFI-ORNL/IBP - What's that?HygrothermicsHygrothermics Besides the thermal properties of a building component and their impact on heating losses, its hygric behaviour has to be considered, too. Permanently increased moisture content in the component may result in moisture damages. Elevated surface moisture levels in living rooms can lead to hygienic problems and health risks due to mould growth.
   In addition, thermal and hygric behaviour of a building component are closely interrelated: an increased moisture content favours heat losses; the thermal situation affects moisture transport. Therefore, both have to be investigated together in their mutual interdependence; the research field of hygrothermics is dealing with these problems.
   Out of Date: Dew-Point(Glaser)Glaser The Dew-Point method as detailed in ASHRAE has been a common method to assess the moisture balance of a building component by considering vapour diffusion transport in its interior. However, this method does not allow for the capillary moisture transport in the component, nor for its sorption capacity, both of which reduce the risk of damage in case of condensation. Furthermore, since the method only considers steady-state transport under heavily simplified boundary conditions, it cannot reproduce individual short-term events or allow for rain and solar radiation. It is meant to provide a general assessment of the hygrothermal suitability of a component, not to produce a simulation of realistic heat and moisture conditions in a component exposed to the weather prevailing at its individual location.
   Up to Date: WUFI-ORNL/IBPWUFI-ORNL/IBP The menu-driven PC program WUFI-ORNL/IBP developed by IBP and ORNL and validated using data derived from outdoor and laboratory tests, allows realistic calculation of the transient hygrothermal behaviour of multi-layer building components exposed to natural climate conditions.WUFI-ORNL/IBP is based on the newest findings regarding vapour diffusion and liquid transport in building materials.WUFI-ORNL/IBP only requires standard material properties and easy-to-determine moisture storage and liquid transport functions.WUFI-ORNL/IBP can use measured weather data - including driving rain and solar radiation - as boundary conditions, thus allowing realistic investigations on the behaviour of the component under exposure to natural weather. .
   .
   A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

2. 
   cargin | Sep 13, 2008 06:43am | #3

   Mike

   I don't understand everything in your worksheet.

   I don't have to. Simple mind.

   But my comments would be cosmetic.

   I redid your worksheet and change the color scheme.

   I like to use yellow for fill, or requires user input.

   Gray I reserve for cells that have formulas in them. Don't touch.

   I like to see clearly where I enter data, and clearly where the result is. That's why I use color.

   Otherwise nice worksheet. I like seeing this kind of stuff on the board.

   When I copied and pasted on to a blank worksheet the formulas did not follow. Usually they do. So what I am posting is not a sheet that works.

   Rich

1. 
   pete123 | Sep 13, 2008 07:47am | #4

   I think you may need to do a "Paste Special" and "Choose formulas and number formats". Then it should work...

1. 
   cargin | Sep 13, 2008 02:14pm | #5

   pete

   I didn't know what you were talking about.

   So I did cut and paste again. I found paste special and uthen I clicked on xml worksheet.

   Now it works. Thanks for the lesson.

   Rich

2. 
   michael_maines | Sep 13, 2008 06:23pm | #6

   Thanks Cargin, that's much clearer.  I can't take credit for coming up with the format, I stole it blatently from Riversong, but had to add my own formulas.

   I don't know if our interior decorator would approve of your color scheme but I like the concept--really brings it to life!

3. 
   Clewless1 | Sep 14, 2008 04:27am | #9

   Are you saying you assume the RH inside the house is 50% at 68 degF? Just making sure I understand what you are doing. And you determined the dewpoint will be 45 degF using this moisture content (not sure I said that psychrometrically correct).

1. 
   michael_maines | Sep 14, 2008 05:17pm | #10

   I would assume the interior relative humidity is usually 30 or 40%, but 50% is a worst-case scenario. 

   I'm not sure about the 45° dewpoint.  Those are numbers I stole from Riversong, and one of the things I would like advice on.

1. 
   Clewless1 | Sep 19, 2008 05:35pm | #13

   There's probably an on line psych chart somewhere ... or use a paper one and you can look up anything you need on that.