Temperature change through an exterior wall
Reading the “Hybrid insul….” and other threads here about potential condensation on the exterior sheathing of an exterior wall have caused me to wonder about a couple of issues. Question #1, regarding the dew point in a thick wall: Does the temperature through an insulated wall change linearly? In other words, if it is 70 degrees inside and -10 outside, there is an 80 degree delta T. Is half way through the wall going to be 30 degrees? I’m trying to get a handle on where my “dew point” is going to occur, and see if I need 2″ of XPS outside the 1/2″ plywood exterior sheathing to prevent condensation. A linear temperature change does not seem likely, but that is only a “gut”. I realize that the dew point will change depending on ambient air temperature, but let’s look at “Temp Average”. I will likely have to have the exterior 1/2″ CDX for seismic reasons. House is in Fairbanks, AK, w/ 14K HDD. Wall insulation will likely be some kind of BATTS (why? that’s for another thread) about 16″ thick, and I will have a Tenoarm vapor barrier under the sheet rock. I’m trying to keep the exterior part of the wall vapor permeable, so I really don’t want the XPS out there, unless needed. Too, vinyl siding. Question #2: I’m still confused on whether or not to have the Tyvek under or over the XPS if I need the XPS. I believe I was told earlier that either way would work, and it seems to go on the outside up here, typically, but whether or not that is right or tradition I don’t know. Question #3: 2″ R-Tech (which I think is EPS) is typically used here instead of XPS on the outside of walls. Tradition? Cheaper? As good? Thanks again; one more whack on the head and I’ll get this straight. john
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JK,
Temperature gradient is linear through a homogeneous insulation layer. The gradient slope tilts as it passes through different materials, like light refracting through a dense medium. You can calculate the temperature at any point in the wall by the cumulative R-value to that point (inside to out) as a percentage of total R-value. That percentage will be the percent of the total delta-T to that point.
But that's all irrelevant. If you install a sealed air/vapor barrier (remember that air sealing is always more important than vapor diffusion sealing), then there will be so little moisture getting into the wall from the inside that no point in the envelope will experience condensation.
With a wall that thick (and don't use fiberglass batts - they're the worst insulation on the market - if you must use batts then use bluejean cotton batts or wool batts or hemp batts), and an interior vapor barrier, you have to keep the outer skin vapor permeable. EPS, such as R-Tech, is a lot more permeable than XPS, so if you feel you need more insulation on top of a 16" double wall, then go for it. But each additional layer will reduce the overall permeance. Vinyl lap siding is highly permeable even though the vinyl itself is not.
The best place for a weather barrier is always toward the weather, since it's the secondary drainage plane after the siding.
Robert: Thanks for the info on the wall temperature gradient and the location of the Tyvek; peace of mind. As for the insulation type,the cotton has looked wonderful since I first saw it, but then I read about it "absorbing water". Perhaps that is when it is not properly protected from vapor in the first place. The mineral wool (Roxul) actually sounds like the best alternative to 'glass, but I don't see it discussed much anywhere I have searched. What is your (or anyone else's) take on it? It seems good for the environment, is allegedly hydrophobic, has good R value, is easy on the body during installation, and is good at sound deadening. Anyone's experience w/ rock/mineral wool would be appreciated. Thanks. john
I am also interested in Roxul as an alternative to fiberglass, has a lower embodied energy and a bit higher? R-value. I think something like 90% of all new homes in the US use fiberglass batts in the walls, it has served me well over the years. Keep the stud cavity airtight and the insulation type becomes less important.
Yes, Roxul seems superior to fiberglass in every way I can think of but it's a bit more expensive too. I plan to use it in my own building project. Roxul drainboard is another very interesting product for insulating and water-proofing exterior basement walls. Roxul products are easy to come by here in Ontario but I've heard that it isn't available everywhere in the states.
As far as I'm aware, neither Roxul, wool batts, nor hemp batts are available in the States. Pink Panther rules! Sprayed or rigid petrochemicals are the hottest "green" item.
Canadians have always been more sensible than us 'Mericans.
To our friends to the East: Send info on where to get Roxul, as far west as possible, pls. I'm in Alaska and can take a trailer to pick up a load, and fish! BTW: Good luck in the hockey game today, but I hope you lose, naturally. john email to [email protected]
Doug: "...(fiberglass) has served me well over the years. Keep the stud cavity airtight and the insulation type becomes less important." Those are my sentiments, too. I am certain that the cellulose advised by several people here is one of the best, if not the best, way to go, but I can't find anyone who pumps it into walls; they pump fiberglass, which is surely better than batts, too. However, at $1.70/sf pumped in vs $0.50 for batts, I can put in an extra layer of batts, staggering joints on all layers, to really minimize "crack losses", have a higher over-all R and be cheaper.... minus my labor. My 3 layers, installed in '80, are doing real well and I did not do nearly as good a job as I will next time. HERE IS A COMMENT that may generate some chatter: "It seems that the unavoidable gaps in installing batt insulation has no real effect on heat loss." I am looking for that study, but it was a reputable outfit. If I can find it, I'll post it for discussion. I feel (gut) that if the air is not whistling through, a small gap is still dead air, which has SOME R value, and if gaps are a small per cent of the wall, then the differences start to become academic. The issue of water vapor transport, however, may still be a factor worthy of consideration, and 'glass is apparently not the best at that. In the absence of cellulose, I guess I'll have to pick my poison, but Roxul seems to be the best alternative. Comments appreciated, as always. j
JK,
It's true that if you air seal the envelope well, then the liabilities of fiberglass become less important. But that "comment" about unavoidable gaps having no real effect on heat loss sounds like Pink Panther Propaganda.
In the conventional wall cavity, the typical 5% voids and compression of kraft-faced fiberglass batts can reduce rated R-value by as much as 30%. In your double wall envelope with a tight air/vapor barrier, minor voids will have far less of an effect as you suggest. But you will certainly not get more R-value with batts than with blown-in fiberglass, particularly if you use one of the new products like Johns Manville Spider, which is formaldehyde-free, has a mold inhibitor, and reduces air-flow.
Conventional fiberglass batts have a density of about 0.6-0.8 pcf, while BIBS (blown in batts) are installed at up to 2.2 pcf for an R of 4.1-4.2/inch, compared to batts at R-3.15 to R-3.7/inch.
Can I assume ICF is safe to moisture related issue? what about concrete wall with exterior EPS board + stucco(or brick veneer)? Thank you for any help.
Robert: "But that "comment" about unavoidable gaps having no real effect on heat loss sounds like Pink Panther Propaganda." It was a surprise to me, and don't quote me on it, but that was the essence of the statement. I was some place like a DOE study, but I can't find it again. I will check w/ the guys here that do BIBs and see if they use Spider. Roxul claims that their batts have R's from 3.8 to 4.1/inch, depending on thickness, which competes w/ blown-in glass, sans "mistakes" w/ batts. Maybe by the time I build someone will be blowing cellulose into walls here. That would be my preferred insulation.
Harry: I am not sure what you question is, but the ARXX ICF supplier here mandates applying bituthane (spelling? they call it "bitchathane" because it is so sticky) on the outside of the ICFs, at least those under ground. I think most ICF's are EXP, which is not water proof at all. Ask your supplier, for sure.
oops... make that "EPS", not "EXP". I was talking and typing....
Mineral Wool is readily available in the US through commercial construction supply houses (for shaft walls and other fire rated enclosures / party walls). If you can't find Roxul, Thermafiber is another main brand: http://www.thermafiber.com/
Our local lumberyard had flyers all over their sales desk last week pushing Thermafiber, it's getting a lot easier to source here in Maine. We're starting to use it instead of XPS on exterior of foundations.
Jesse: Thanks. I found Thermafiber and have emailed them for a supplier. Glad to hear it is becoming more popular; it looks like real good stuff and may even wonder NW some day. It will be interesting to see how the drain board (I assume that is what you use on exter foundations; a rigid board stuff?) compares to XPS in tests and real life.
JK - shop around some more - we get blown in fiberglass and blown in cellulose for about 20 cents more a square foot than fiberglass batts - not that much of an upgrade butwell worth every penny.
As for the gaps not mattering - I doubt it was a DOE study - they are upgrading their insulation requirements for 2011 Energy Star which I do not think they would do if they believed the gaps did not matter. Also - no studt needed - borrow a thermal camera from a buddy and check our a batt wall on a cold day - that will convice you.
Danny: Thanks for the reply. I've exhausted the installer list (2); I have not called Anchorage yet, but since they are 350 miles away, I doubt that they'd be any cheaper. If I could find blown-in glass for even 50 cents/sf more than batts, I'd go for it. As for the gaposis, an Owens Corning rep sent me an internal study which shows the gap losses as pretty tiny, (3% or so?) FWIW. Getting info from the lion's den can be good or bad; dunno in this case. I THINK that the gap-effect will be somewhat subdued because of the batt-to-batt layer inside my studded wall. Too, since I WILL do a better job w/ my vapor barrier and I WILL have tight Tyvek on the outside, I am HOPING that the gap effect will, in fact, be minimal. What I DO know is that we are currently losing 9 btu/hr/sf w/ my present house (includes heating water), and that is not too bad for here. $4K extra for blown-in will be tough to recoup. Maybe I need to start a company..... That thermal camera is a great idea; I'd love to check my present walls. j
$500 extra for blown-in would be tough to recoup, carefully placed fiberglass batts perform very well, completely fill the space.. Sloppy workmanship does not perform from an energy efficiency standpoint or any other for that matter.
All evidence is to the contrary. Perfectly-installed batts test at lower R-values than their ratings. And typically-installed batts have at least 5% voids and significant areas of compression with gaps between insulation and drywall for convective loops.
Fiberglass batts that are not surrounded on six sides by an air barrier are highly vulnerable to convection degradation. Kraft-faced fiberglass actually reduces a walls fire resistance. Insects and rodents love to tunnel and nest in fiberglass. And they have too little density to be effective sound barriers.
Blown fiberglass can offer a significant improvement in air tightness and thoroughness of installation, but still offers none of the fire, vermin, fungus and moisture protection of dense-pack cellulose.
If the difference in installed cost is only $500, you couldn't make a better investment in the thermal efficiency, livability and durability of your home.
See https://www.greenbuildingadvisor.com/book/export/html/14020 for more.
Here is some interesting "data" on the subject of batts. This is from NAIMA, who are 'glass/rock wool people by definition. (See their Insulation Facts #14.) "Cellulose promoters claim that installations using batt insulations have voids which cause reduced thermal performance. They cite the ASHRAE Handbook of Fundamentals which references a test showing that a 4% void area in wall insulation
increases heat loss by 15%. However, the 4% void test is not representative of typical installation, or even very sloppy workmanship. In sidewalls, a 4% void is equivalent to approximately a 4 inch space the entire width of every stud cavity." My questions here would be: (1) Did they cite the ASHRAE info accurately? (2) What is the real accumulated gaposis in a wall that is done w./ extreme care? 1%? 2%? I wish I could find that out. I'd still like to see unbiased testing of a real-world batt wall vs blown in 'glass (since cellulose is not available to me, I don't care about it right now.) I am going to ask Owens Corning and Johns Manville that very question. If anyone has any study links, please lay 'em on my. Surely SOMEBODY has some thermal images of walls insulated each way, but who? Thanks. john
Robert: I was off-line for a while and posted before I refreshed and saw your answer. I read the report by Martin that you cited, and it frankly did not do much to discourage the use of fiberglass batts. The tone was gnerally "if done poorly, as most contractors do, it won't work as well as planned". OK, agreed; poor workmanship in anything has less than desirable results, and it is apparent that that holds exponentially true for batt installation. I'd still like some hard numbers comparing "The absolute best we could do" batt and blown-in walls. I suspect that is too much to ask, as it is hard to define and replicate "the best we could do". Maybe the guys at NAIMA have that answer. ????
JKLINGEL,
Did you follow Martin's links to http://www.bestofbuildingscience.com/pdf/Insulation%20inspections%20for%20home%20energy%20ratings%20HEM_22-1_p20-23.pdf ?
And check out http://bpi.org/documents/Yellow_Sheet.pdf
Don't accept anything that comes from the fiberglass industry - they have a long history of misleading propaganda, both in support of their nearly worthless products and against their competitors.
Robert: No. I did not get as far as Martin's links yet; wilco. Yes, reading on the NAIMA web site, it is clear that they are not cellulose fans. BTW: I am working tonight on incorporating a tweaked Larsen truss into my house. At first I thought it would not work, given how the concrete slab is, but I am tinkering w/ making a go of it.
Some of the netherworld claims about cellulose insulation approach those of Icynene in terms of performance gains over fiberglass insulation. All I can say is I have had very good efficiency and durability results using fiberglass insulation, the extra cost of blown cellulose attic insulation in the Twin Cities at least does not begin to pencil out. Weight is also a factor when superinsulation and PH standards are desired.
I've seen no "claims" made about cellulose that haven't proven true in the field and in laboratory tests. The FTC rule about R-value truth-in-advertising was created to rein in the fiberglass industry, just as the code prescription for vapor barriers was inserted as a necessary response to the poor performance of fiberglass in buildings.
And weight of cellulose is not an issue. I use 18"-20" of cellulose in attics over ½" drywall on 24" oc framing with no sagging.
The other benefits of cellulose over fiberglass are also well established: fire-resistance (even certified as a fire stop), acoustic resistance, fungus resistance, insect proof, vermin resistance, air resistance, and hygroscopic advantage.
There is only one reason to even consider using fiberglass - it's cheap. But you get what you pay for. The only reason that it's become the industry leader is the Pink Panther and the millions of dollars of advertising behind him.
I should add that fiberglass is a known carcinogen, that most of what's on the market contains formaldehyde which is the primary chemical sensitizer that can initiate multiple chemical sensitivity, and that kraft-faced fiberglass - like foam - must be covered by a fire barrier.
I don't have the breadth of knowledge and/or experience to argue about 'glass vs cellulose, but I tend to think we are getting into the Ford-Chevy debate, to a degree. Fiberglass has served me real well, despite my inexperience/ignorance in 1980. That said, we have a pretty dry climate up here. It is hard to imagine that FB is rubbish, as millions of homes have relied on it for decades, and are apparently doing OK. That is not to say that cellulose is not a better product; I have the feeling that C is better. The question is, does it always "pencil out", esp when you can't even get it. Oh, sigh.... I wonder why the guys here won't install it in a wall. Maybe they just don't want to develop the fine-tuned technique that Robert has. Robert: Why don't you come up here and train these guys, and I'll take you out halibut fishing?
More like a Porche-Pinto debate.
In addition to all the rest, fiberglass has 17 times the embodied energy per pound and 5 times the EE per cubic foot installed as cellulose, which is almost 100% recycled material and diverts all that newsprint away from the landfill.
I've never opened a fiberglass-filled wall more than a couple years old that wasn't riddled with mouse tunnels, full of mouse middens, stinking like mouse urine and droppings, and not uncommonly finding dead mice entombed in their favorite indoor habitat. Installing fiberglass in a house is like putting out a sign saying "MICE WELCOME".
Cellulose with boric acid is the only commonly-used insulation that not only won't attract mice and won't grow mold but will actually kill all common household insects, including termites, carpenter ants, cockroaches, and earwigs.
Anyone who would argue about an additional $500 (which is about a quarter of a percent of the cost of a typical new home) for that kind of value is penny wise and pound foolish (except, of course, those who live so remotely that there are no local cellulose installers).
Disclaimer: I don't now, nor ever have, worked for or been paid by anyone in the cellulose industry. For the first part of my building career, I used only fiberglass. For the last 20 years, I've used cellulose almost exclusively. And in my classes, I teach the advantages and disadvantages of all insulation materials and systems.
For "green" building, there shouldn't even be a debate.
Robert: I read those links. Very good. If one is using batts, you better be top-notch at it. The one comment "In general, no exterior sheathing should be visible through gaps in the material" was a bit startling. Duh! I guess that is when you get out some spray foam and fill 'er in. Your comments about animals is 'glass is right on; I've taken bags of dried mushrooms out of my garage ceiling, and see little burrows in there. I have no idea how many voles are in my walls; ignorance is bliss. I'm going to ask a new question, as this topic is drifting.... but with great information!