Help finding an article
rhl_
| Posted in General Questions on
There is an article on GBA here somewhere that shows a cross section of a wall and how temperature changes linearly with position in the wall. I’m looking for this article. I think Martin May have written it.
if anyone can provide a link to it I’d appreciate it.
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I find the search function of this web site is useless!
I go to Google enter your search phrase and search it. Then you will see the setting drop down use it to select “Advanced search” then you can limit your search to this site.
https://www.greenbuildingadvisor.com/article/vapor-barriers-are-a-good-thing-right
https://www.greenbuildingadvisor.com/article/thermal-and-moisture-control-layers
This image looks interesting but I do not find it in the article it is linked to.
Walta
I drew that! Well, I drew it and the Fine Homebuilding art department made it pretty. It's from this article: https://www.greenbuildingadvisor.com/article/why-flash-and-batt-makes-sense.
A faster way to use Google to search this site is to type "greenbuildingadvisor.com: air sealing" (or the topic of your choice) in the Google address bar. Or the search bar.
Yeah that’s the same idea. Basically temperature change is linear through a wall with slope related to R-value.
Ryan, the process is to find the total R-value of the wall, then what percentage of the total each layer of the assembly is providing. That percentage relates to the inflection points where materials change. In the image Walter shows, if the foam provides 30% of the total R-value, the temperature at the inside face of the foam will be 30% of the difference between the outside temperature and the inside temperature. Then connect the dots with straight lines.
I agree that the GBA search is useless. Should be a parametric search with lots of options - author, date range, title, etc.
Ryan,
Here is a link to an article that explains why the type of illustration you are talking about is widely misinterpreted: "Are Dew Point Calculations Really Necessary?"
Here are the relevant paragraphs:
"Building scientists sometimes talk about a wall’s “temperature profile” or “temperature gradient.” The idea is to estimate the temperature of different wall components, assuming certain indoor and outdoor conditions.
"For example, consider the wall of a house on a cold winter day. If it is 72°F indoors and 0°F outdoors, the siding temperature will be close to 0°F, while the drywall temperature will be close to 72°F. The other wall components will be at temperatures ranging between these two extremes.
" If we draw a cross-section of a wall, we can calculate the theoretical temperature of any point within the wall. However, since these temperature profiles usually fail to account for air leakage, they are usually inaccurate. Moreover, they represent a theoretical one-dimensional model; since the real world has three dimensions, this model has limited value. ...
"It’s certainly useful to know whether your sheathing will be above the dew point or below the dew point in winter. When sheathing is below the dew point, it’s likely to accumulate moisture. Warm sheathing is better than cold sheathing.
"Unfortunately, though, temperature profiles and dew-point calculations have been misunderstood and misused for years. In his excellent book, Water in Buildings, William Rose wrote, “The language ‘reaching dew point’ seems to indicate that one could plot a temperature profile through a wall, find the point where that profile intersects a horizontal line indicating indoor dew point temperature, and expect burgeoning water at that location. This impression is decidedly incorrect. If water accumulates, it does so on the surfaces of materials, not within the thickness of materials.”
"Rose goes on to explain that misunderstandings arising from dew-point calculations are caused by a failure to consider the saturation vapor pressure. I’m happy to report that, for the purposes of this discussion, understanding saturation vapor pressure is unnecessary. (For those who care, Rose explains, “A description and example of the profile method is maintained in ASHRAE Handbook — Fundamentals, Chapter 23. … If at any point the vapor pressure value exceeds the saturation vapor pressure, reset the vapor pressure at that point to the value of the saturation vapor pressure. After all, having vapor pressure exceeding saturation is quite rare.”) ...
"Anton TenWolde, a supervisory research physicist at the U.S. Forest Products Laboratory, made the same point at a workshop at a 2002 EEBA conference. TenWolde’s discussion of the issue is worth quoting at length:
" “The perceived importance of condensation has been bolstered by the wide misuse of the dew-point calculation. … Many of you are familiar with a chart like this: you project the temperature profile through the wall to calculate saturation vapor pressures. Then you calculate vapor pressures based on the permeance of the materials, and you come up with a profile like this.
" “I have seen hundreds of these profiles, and many seem to show condensation occurring in the insulation. This has encouraged a lot of research into the performance of wet insulation. But the picture is wrong, because the vapor pressure has to be below the saturation pressure. You need to make a correction, and if you do that, if you redraw it, the condensation does not occur in the insulation. We thought there would be a problem with condensation in the insulation, but all the action happens on the sheathing and the interior vapor barrier. We’ve confirmed this by opening up walls. The action is never in the insulation.
" “I have a problem with the way we perform dew-point calculations. The method cannot handle hourly calculations. It doesn’t take anything into account except vapor diffusion. It doesn’t take into account moisture storage, air movement, liquid water movement, or rain. It doesn’t take into account more than one dimension — it thinks the wall is flat. It doesn’t take into account the variability of material properties, or the effect of the sun. In other words, it doesn’t take into account the real world.
" “A moisture problem occurs when wetting exceeds drying over a long period of time. But it is important to know several things: How wet does it get? How long does it stay wet? And what is the temperature while it is wet? Because if it is cold enough the mold won’t grow well, and decay organisms won’t do well. How does this information translate into the design of a building?
" “You need to assume that the building will get wet, somehow, at some point in time. Stuff happens. So you need a moisture-tolerant design. The question is, how much water should a building component be able to handle? Which leads us to the question, what good is building science?
" “For one thing, it is not very good at predicting how wet buildings get. The dew-point calculation was an attempt at doing that. But the dew-point calculation is terrible at predicting if something gets wet, much less how wet it gets. Wetting is an unpredictable, singular event. I don’t think we should let building science anywhere near this question.”
"To elaborate on TenWolde’s point: even if a builder performs a complicated dew-point calculation to be sure that OSB sheathing doesn’t get wet due to diffusion of water vapor originating from the interior of the home, the calculations won’t prevent the OSB from getting soaked by wind-driven rain leaking through defective flashing.
"Sophisticated hygrothermal modeling programs like WUFI take into account a tremendous number of variables, including the orientation of the wall, the width and height of the roof overhang, the amount of rain striking the wall, the amount of sun hitting the wall, the amount of air leakage through the wall, and differing indoor conditions. Compared to a WUFI simulation, a simplified one-dimensional model based on a temperature profile through a wall and a dew-point calculation is of limited value.
"It’s nevertheless worth performing the calculation, because we really don’t want our sheathing to be cold enough to accumulate moisture. Field studies have shown that Lstiburek’s simplified dew-point calculation method is adequate to avoid diffusion-related moisture accumulation in OSB or plywood sheathing. That said, we shouldn’t pretend that this calculation can predict the actual moisture content of the sheathing; at best, we can say that the method works well enough to avoid problems from moisture originating from the interior of the house."