A Few Follow-Ups to an Older Thread?


Old 03-27-03, 10:29 PM
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A Few Follow-Ups to an Old Thread?

This is in follow-up to a threaded discussion that took place in January of this year; A complete copy of this thread is available via the following url; http://forum.doityourself.com/showth...hreadid=119150

But, because the thread is already quite long and several parts of the discussion are irrelevant to the topic at hand, though this is a query in follow-up to the above thread; I elected to link the discusssions but not resurrect the archived thread for what should be a few, fairly simple questions.

In a nutshell; After consulting this and the other relevant forums to interlock the various pieces of the puzzle; I am planning to install 1x3 furring strips on the interior of my perimeter walls which are currently constructed of plaster-covered, cement block. On top of this "frame", I will attach 3/4" of rigid foam insulation and 1/2" sheetrock. As was determined in this forum, I will gain insulative value against diffusion by leaving the 3/4" void between the existing wall and the foam/drywall.

Basically, I have three follow-up questions; Though the answer to the first may be better determmined after examining the information that follows this short list;

1) As far as can be determined, there is no code requiring a vapor barrier between the block wall and the furring in my local jurisdiction. It sort of makes sense that one probably wouldn't hurt, but because I am in a desert environment would there be any gain or loss from installing the barrier, considering the information posted, below?

2) Is there any reason to allow for movement between the sheetrock and the foamboard? I am strongly inclined to use a single screw to fasten both to the "studs" and I am considering building my own version of panel construction by gluing the foam to the wallboard, especially in the areas above the doors and windows. Is there any reason why this would not be advisable?

3) Because I'm primarily battling diffusion; Would it be advisable to even consider pumping the void full of cellulose after all is complete and after the installation has passed inspection**?

Pertinent Info;

The installation will be approximately 15" above grade and above a crawlspace approximately 3', deep.

The mean relative humidity based on a 10-year timeframe for my area breaks down as follows;

January: 49.2
February: 40.1
March: 34.1
April: 27
May: 25.3
June: 22.5
July: 40.1
August: 48
September: 42.8
October: 36.9
November: 42.4
December: 52.8

And the mean percipitation based on the same timeframe and from the same research site, breaks down as;

January: 0.23"
February: 0.30"
March: 0.61"
April: 0.35"
May: 0.50"
June: 0.41"
July: 1.90"
August: 1.82"
September: 1.51"
October: 0.73"
November: 0.36"
December: 0.61"

And the Climate Center records the following HDD info using a base of 65 (Avg 1914-2002);

January: 865
February: 625
March: 485
April: 231
May: 62
June: 2
July: 0
August: 0
September: 22
October: 220
November: 594
December: 855
Total: 3961

Thanks in Advance;

**Note: The UBC is the relevant national code and there are no local codes which require an insulation inspection. The inspection to which I am referring are the framing, sheetrock and electrical.

For the information of others considering this same feat; The insulation discussion took place via the url cited in the first paragraph; The electrical discussion is archived at http://forum.doityourself.com/showth...hreadid=125531 and the discussion of installation methods and the relevant codes can be found at http://forum.doityourself.com/showth...hreadid=126271. Also, I have posted a reply to the numerous suggestions that I contact the AHJ at http://forum.doityourself.com/showth...hreadid=126606

Last edited by magister; 03-28-03 at 02:55 PM.
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Old 03-28-03, 07:28 AM
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Equilibrium Relative Humidity (ErH%)

Is the process where air and/or an object that possesses less humidity will absorb humidity from other air and/or objects that have more humidity until the humidity levels in both the air and objects are equal. Furthermore the percentage of moisture in air is relative to the temperature of the air. For example, air at 32 degrees F. with a Relaive Humidity (RH%) of 60% will drop to 10%RH when the temperature is risen to 65 degrees F. This explains why the inside of homes are so dry during the winter. And why most people add humidifiers inside their homes. But the reverse is also true with RH%. So if you increase the RH% inside your home to improve the comfort from 10%RH to 25%RH at 65 degrees F. When this heat transfers through your wall that now possesses a 25%RH, the RH% increases with the drop in temperature. At 32 degrees F. the RH% would probably be 100%.

At first glance one would think that providing a vapor barrier would prohibit the formation of condensation but it depends on the RH% of the air and the materials it comes in contact with (ErH%). So if the masonry wall possesses 65%RH at 32 degrees F. and the heat transfer possesses 100%RH at the same temperature, the masonry wall will absorb the moisture from the heat and will prohibit condensation from occuring.

Nature always seeks balance and when it comes to materials either too much humidty or not enough humidity damages the material. Including the fact that your area has a low RH% throughout the year, I would not recommend the use of a vapor barrier under these circumstances.

The aforementioned explicitly implies that moisture can be transported by heat and/or air. Compound this fact with the air tightness of the wall and that whenever you heat air it expands, the probability that heated air will by-pass the insulation to the masonry wall is extremely high. It is literally impossible for you to seal every nook and cranny in this wall, especially around the edges and penetrations like with electrical outlet/switches. This also explains why many in this industry contribute moisture related problems in homes today more so to air transported moisture than heat transfer.

If we look at the gap between the insulation and masonry wall as either being filled or left void as it applies to the aforementioned, if the gap is filled, the heated expanded air with moisture will have no place to go. Its likely path is through the masonry wall. Yes, air can permeate a masonry wall. Even with the low % of humidity in the masonry wall and outside, the probability of condensation occuring increases dramatically.

On the other hand, if the gap is left void, this will reduce the amount of air that will permeate the masonry wall and this air in the gap will be capable to absorb the moisture that is being transported through the expansion (heating of the air inside the home). Though the probability of condensation does increase, it is no-where near the probability if the gap was filled. I would not recommend filling the gap between the insulation and masonry wall. Furthermore a dead air space means little or no air movement. Air under these circumstances have very good thermal values. In other words you will not gain much R-value by filling the gap verses leaving it void.

Live and dead loads induce movement of the structure. Hydrostatic pressure, wind, rain, soil, expansion and contraction of materials and a whole lot more cause the structure to be in constant movement. Failure to properly secure the wall to the masony will only result in a lot more work for yourself by spackling and painting that wall constantly.
Old 03-28-03, 11:54 AM
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Thanks Resercon; I was hoping you would come through...

I came to basically the same conclusion as you, of course with only a limited understanding of the subject, when it came to the vapor barrier; But, it seems to be local code in some places that get much more rain and whose humidity levels and HDD are multiples of my own. Plus, you add this to it's requirement as part of the IRC (which doesn't apply to me) and I wasn't sure if I was missing something.

Of course my situation was even more compounded because of what I'll call a discrepancy between the HDD value for my area. In my first post of this thread, I passed the official NOAA HDD value which is an average of the years 1914-2002 (3961). But, if you just looked at the year 2002, you'd find that using the same base 65, I had a HDD value of 2981. The DOE info on vapor barriers uses the HDD value as part of the determining factor when it comes to whether or not they recommend an outer barrier and they do speak of "border" values, but they only give an example of bordering downward and though they refer to the importance of relative humidity, I could not find any guidelines. Compound this with the fact that 2981 would qualify as a "border" value, but possibly not 3961 and the info about the IRC; I was at a loss of what to do.

When thinking of it logically; The fact that my walls are currently constructed of 50+ year old, hollow-core cement block with an internal frame width of 1.5 inches; They are covered on the inside with multiple layers of plaster and on the outside with multiple layers of stucco that even though I am going to eventually take-off some of the outer layers, I find it very doubtful that I'll get it down to bare block before I recoat the outside. My walls are solid as a rock, so not only could I see no real benefit from the presence of a barrier, but I had to wonder if it would be detrimental because as you more knowledgably state; The little bit of moisture has to have a place to go...

So, because I know that moisture content is obviously a field of study for you; I certainly wanted to seek your input; Thanks.

As for the "movement" question, perhaps I didn't make myself clear and maybe I have reached my own conclusion; I am planning to secure the furring, most likely with tapcons (screws) to the block wall. And on top of this furring, I am going to screw a layer of foamboard and a layer of sheetrock. My inquiry was whether or not I could use a single screw to pass through both the foam and the wallboard, or whether or not I needed to fasten the two. separately. I've seen quite a bit of literature on panel construction and my understanding is that they simply glue the foam to the rock and then, screw the entire unit into place.

Any way you look at it, everything will be secured with screws; It would just be easier to fasten the two layers to the furring with a single screw, as opposed to screwing them both, separately. So I guess my real question was whether or not there is a significant enough variation, between the expansion and contraction rate of the two materials to justify going to the extra effort. But upon further reflection, I see no reason why I couldn't "tack" up the insulation with fewer screws and the sheetrock screws will still puncture the foam, marrying the two; So, I guess the real answer to this question may be; Whatever works...

(Though, if there might be some insulative value gained by gluing the foam to the back of the board before screwing the unit in place...)

Thanks again. As usual, you've been a lot of help and I welcome any additional thoughts or comments from you, or from anyone else who may be reading this thread.

Peace, Love & Happiness;

Last edited by magister; 03-28-03 at 08:39 PM.
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