Replies

1. 
   gdavis62 | Jan 21, 2004 05:06am | #1

   In large parts of the midwest, the insulation contractors are large operations, sometimes with one location running 25 to 35 trucks.  Here is what they do there pretty much as standard practice.

   Seal all your windows and doors with special low-pressure urethane foam.  Use the same foam to seal all wall penetrations that can be seen and accessed from the inside.  Spray all the stud bays with a light coating of urethane foam, which rises to a thickness of maybe 3/8".  Pack out the rims with about 3" of the same foam, making certain to come down over the mudsills onto the foundation to seal that crack.  It's a real big cause of leakage.  I know, you used that sill seal stuff, but you need more than that to keep out the breezes.  Then, in the studbays, fiberglass batting is done to your thickness spec.

   Sealing things up tight in walls and paying attention to details at openings, is of primary importance, and the foam products take care of that pretty well, when done by trained mechanics.

   "We don't need no steenking housewrap!"  Sealing up with a skim of urethane from the inside, then using urethane caulk at any suspicious gaps, and going over all openings with a watchful eye, will do you just fine.  Flash your windows and doors per good practice, and shinge-lap your 15# felt all over, under a good siding job, and you will be toasty in the winter, and dry in the rainstorms.

2. 
   Dinosaur | Jan 21, 2004 06:24am | #2

   If the foam guy thought your arrangement would move the dew point inside the Fibreglas, I assume you were planning to lay in the foam-board as the inner element in the sandwich. Why not do the reverse? Put the extruded polystyrene just inside the plywood sheathing. Maybe glue it in with urethane foam caulk to seal each stud bay?

   With the Fibreglas batts as your inner layer, you'll want a vapour barrier; there's a reflective mylar bubble-wrap called Reflectix that's supposed to add about R-10 in 3/8". It makes an excellent insulating vapour barrier over batts. Tape the joints with metal duct tape and it'll be so tight you'll need an air-exchanger....

   BTW, nobody builds exterior walls in 2x4 up here anymore. Maybe you're in a more clement climate, though, LOL!

   Dinosaur

   'Y-a-tu de la justice dans ce maudit monde?

1. 
   xman | Jan 21, 2004 11:21pm | #3

   The foam panels were to go outside the sheathing according to the FHB article.

   The more I think about it, the more I question the "dew point in the fiberglass" comment. IF that is the case, then without fiberglass, condensation would still occur within the wall but against the sprayed in foam. That is the first condensing area. I also think that stemming air flow could make this a moot point. It seems that bulk air flow is the greatest source of moisture, while vapor diffusion is tiny. Sealing it all up with spray foam takes care of bulk air. SO your moisture source is internal. Does painted drywall and kraft-faced batts take care of the moisture flow from inside the room into the wall cavity?

   Still so many unknowns!

1. 
   Dinosaur | Jan 22, 2004 05:24am | #4

   If this is drawn from an article in the current issue, that explains why I haven't seen it. I just got the previous issue about a week ago.

   I can't see why the dew point would move 'inside the Fibreglas'. Extruded polystyrene is one of the highest R-value elements you can put into a thermal envelope, per inch of thickness. So, since I haven't seen the article, can you give me a brief rundown on the reasons stated for the mixed-bag of insulators in this wall?

   Dinosaur

   'Y-a-tu de la justice dans ce maudit monde?

1. 
   skids | Jan 22, 2004 08:01am | #5

   i have to say that a 2x6 wall is not "the perfect wall" unless the issue of thermal transmission is addressed. i would think the "mooney wall" would do a better job of this. another alternative would be to stagger the studs so that none of them go all the way from interior to exterior. 

   i also think that a SIP is hard to beat for insulation, with no thermal breaks it just has to be properly sealed. i know i was advocating light gauge steel studs before, but have changed my position largely because of the dewpoint issues, and the fact that the dew might condense on the metal. i do think that they have their place as interior partitions etc. with spray foam insulation. without the spray i wouldn't use the metal.

     

2. 
   xman | Jan 22, 2004 08:59am | #6

   Dino:

   The spray foam/fiberglass combo was something Paul Hayden suggested--sounded like a good idea to me. The FHB article had cross sections of two homes, one using a "Good System" , the other a "Better System". The better version went like this from outside to inside--Back primed wood siding, 1x3 furring strips, housewrap, 2 layers of 1 inch XPS foam with staggered seams, plywood sheathing, 2x6 at 24 o.c., filled with damp spray cellulose, non-paper faced gypsum wallboard, plaster skim coat, latex paint. Spray foam was used on sill and wall where finished in basements. I am not a builder, so I have to ask about windows in this FHB example. Where are they nailed? Right through the foam? YOu then flash OVER the foam?HOw about brick ties(if wall was used with brick)--they just puncture the foam as needed?

   Edited 1/22/2004 1:06:09 AM ET by Xman

1. 
   Dinosaur | Jan 26, 2004 06:07am | #14

   Anytime you start laying 'soft' goods on the outside of the sheathing, you have to get creative when installing windows and doors. That wall looks to be close to 8" thick, so you're going to need to order the windows with that in mind, too.

   As for the flashing, just remember to flash the same way you would lay roof shingles: you want to shed the water from inner-upper to outer-lower....

   For the brick ties, yes, they would have to be attached to solid, behind the foam board, so they would pierce the envelope. You could spot foam those penetrations with canned foam.Dinosaur

   'Y-a-tu de la justice dans ce maudit monde?

2. 
   csnow | Jan 30, 2004 01:00am | #15

   I'm not seeing a lot of advantages with the 'foam on the outside' approach.  Seems like a lot of hassles with windows, doors, siding, and trim.  If you put the sheathing outside the foam, you lose your shear wall bracing.

   I designed a home in New England with sheets of 2 inch foam on the inside under the drywall.  Due to the extra thickness, extra work was required trimming out the windows and doors on the interior, but the carpenters commented that they didn't think it was so tough.  The deep windowsills actually look very nice.

   Anyways, it worked out well.  Just seems easier than the outside solution, and there is no doubt which side the VB is on here.

3. 
   mmoogie | Jan 30, 2004 02:48am | #16

   Hi all,

   If your first condensing surface (the inside (ie:warm) side of the sprayed foam is below the dewpoint you can get condensation on it. That can happen if there is too much additional permeable insulation (fiberglass) on the warm side of the foam and/or if the foam is not sprayed thick enough. It all depends on temperature differentials and relative humidity.

   This is a link to a paper by Joe Lstiburek on this very topic. The dewpoint can indeed wind up on the backside (warm side) of the foam. The article has graphs to show at what temperatures and under what condition this can occur. It's fairly dense reading. It's also available as a PDF download.

   Steve

   http://216.239.41.104/custom?q=cache:eBnv6b2xCLgJ:www.buildingscience.com/resources/walls/insulation_sheathings.pdf+insulation_sheathings.pdf&hl=en&ie=UTF-8

   Edit: I just looked through the html version and on my browser anyway, the graphs and illustrations don't show up right. I suggest the PDF download. You don't get to the good stuff till page 7 or so...

   Edited 1/29/2004 6:53:08 PM ET by STEVENZERBY

4. 
   xman | Jan 30, 2004 06:59am | #17

   Steven:

   Thanks for bringing that article to my attention again--I had read it months ago, but it makes a lot more sense now. What it tells me is that you have to lay on the foam thick, or else you will get condensation. There essentially has to be a lot of R value outside of the point of first condensation, or condensation will occur at cold temps. The dew point in my area, which was the one used in the examples, would be about 40 with an inside temp of 70, and a 35% relative humidity. The temp at the point of first condensation can be calculated using his formula on page 5. When I do that, using 2 inches of R-7 foam + an R-15 batt,  condensation can occur within my wall on the foam surface anytime the temps drop below 10:

   Temp at interface=15/30(70-10) + 10=40

   Drop the temp to 0, and the same formula gets you 35 as the temp inside. The rigid foam externally helps a ton in bumping up the interface temp. Let's add R-10 outside my plywood sheathing and take the outside temp to 0:

   Temp at interface=25/40(70-0) + 0=43.75=no condensation

   So if I desire to use a spray closed cell foam/BIBS combo, it would have to be approximately 2.5" of foam(R-17.5), and 3" of BIBS(R-12). I add another R-1.5 for plywood, brick, etc. beyond the foam. So, here's the computation at 0:

   Temp at interface=19/31(70-0) + 0=42.9=no condensation

   Food for further thought.

5. 
   Dinosaur | Jan 30, 2004 07:09am | #18

   I agree in general that insulation outside is a PITA. I wouldn't do it for new construction; only for retro-fit or remedial situations where you don't want to destroy the existing interior finish.

   Dinosaur

   'Y-a-tu de la justice dans ce maudit monde?

3. 
   pinemarten1 | Jan 23, 2004 01:18am | #7

   To All:

   What about this option.  The Mooney Wall concept - 2x4 construction, strapped with 2x2s horizontal, and then open cell spray foam all the way (r-3.7/inch) = R18.5 or close.  Strong wall (studs vs "glue" wall SIPs), minimal thermal breaks (only where the 2x4s & 2x2s cross), ultimate air sealing (no in wall convection), yet open cell foam will allow structure to "dry" if it gets wet, all insulation completed inside one day for an average size house by one sub - minimizing schedule conflicts.

   Add 6 mil plastic vapor barrier on interior for cold climate?  Or "go naked"?

   Thoughts?

   Pinemarten

1. 
   xMikeSmith | Jan 23, 2004 01:42am | #8

   if you're using foam or cells, you don't need the vapor barrierMike Smith   Rhode Island : Design / Build / Repair / Restore

1. 
   pinemarten1 | Jan 23, 2004 12:46pm | #10

   Mike:

   No vapor barrier - even with open cell foam? - perm rates of over 10 for 5 inches?

   I think you are right though - the good news is it could then "dry" to both sides if it just happened to get wet.  But there should be no significant moisture drive with the air seal.

   Now if we could change the MN Energy Code......

   Pinemarten

1. 
   xMikeSmith | Jan 23, 2004 02:56pm | #11

   pine... gene leger used to post here a lot... he wrote a nice book on construction.. his technique  ( and others ) is called "air diffusion" or dry-wall air barrier..

    his drywall gypsum is continuous on the outside envelope before any of the partitions go up.. the drywall forms an air seal everywhere including at the base.. with special attention to electrical and other penetrations... basically .. no gross air movement.. no condensatin within the walls..

    we try to acheive the same thing with poly vapor barriers... but that is almost impossible... densepak cells or continuous foam will acheive the air diffusion desiredMike Smith   Rhode Island : Design / Build / Repair / Restore

2. 
   xman | Jan 23, 2004 02:59am | #9

   That's a good idea as well.

   I finally did find an insulating company that has been using a combination for 10+ years. They spray just a 1/2" of closed cell foam to seal things up, then add the R-19 batt of fiberglass. They have always used a visqueen barrier inside as well(although he admitted that doing tight glued drywall is probably enough).  He gave a rough price of $1/sq. ft for that combo(R-22 in the wall) vs. $1.75-$2 for my proposed combo(R-27). Perhaps a good compromise?

3. 
   aimless | Jan 23, 2004 07:40pm | #12

   I've been reading for quite some time about the Mooney wall, wondering when Tim is going to publish in FHB? But I'm a little curious as to how this could be adapted for my local conditions. Around here, a 2x6 wall is a basic requirement for building a second story. But to do that, I'd lose the savings benefit of using 2x4, and I'd now have to do non-standard jamb-extensions. Or am I just looking at this wrong because I'm a dumb homeowner?

4. 
   skids | Jan 25, 2004 07:54am | #13

   Paul Hayden had some remarks to me when I asked this question, "what about just filling it up all the way with spray foam" and he said that it may be a problem pulling away from the wood as it cures when you spray it that thick. I dont know if this is correct but I think Paul knows what he is talking about. it may be solved by spraying foam in 2" lifts, letting it cure before applying 2nd and 3rd applications but now you are adding 3 times the setup costs to already expensive foam application.

   i like what mike said about a previous poster, gene i think, did by putting all the drywall up before building interior walls thereby eliminating leaking through the walls for a better sealed envelope. that technique combined with mooney walls and blown in cell sounds like the ticket to me, just hope you dont put holes in it or get it wet. drywalling ahead of rough electrical inspection is out of sequence too, there must have been a few logistical problems.

4. 
   fortdh | Jan 30, 2004 08:30am | #19

   To Xman and the rest of you.

   Some great posts on the subject.

   I returned last night from Dallas, after spending a few days at the Las Vegas Home Builders show, so only now catching up. If you want to see a bunch of insulation manufacturers scatter, just ask them where to put the vapor barrier in a mixed climate. Bottom line, it is a tough question to answer, and from a perfect placement, we would have to rotate our walls with the thermostat.

   They do agree, that the saving grace is to keep the moisture out of the wall, if possible, and at least minimize air flow that helps carry moisture. Next in line for protection, is to cross the dew point within the foam, and that will require specific specs for the climate, understanding that that is a compromise for some days of the weather year.

   Sips do not seem to have a condensation problem within the foam portion, but seam fit can, if not sealed well, as many of you have noted.

   Exterior foam board certainly assists in moving the dew point outward, and it also breaks the thermal bridge that the studs create.

   One could ask, "where is the dew point within a 2x6 stud that is only r-6 ?" We have all seen dew lines on the outside of walls that indicate the exact location of studs, or bathroom walls that form condensation on the stud placement, before the rest of the sheet rock sweats.At some point in a mixed climate, the dew point must be inside the stud itself...

   I have been invited to visit some of the foam makers, and listen to their latest in wall design.They have done a lot of research, and are learning new approaches. It was obvious that some want maximum sales, while others were interested in specing a good package for the customer, cost/performance effective.

   There still seems to be some discussion about foam application, thickness, pulling out from studs etc. I asked about this again, and got mixed answers. Some agree that too thick, too fast can be a problem, but going around a room twice is not a big problem, and it takes about the same time, ie., its time per inch either way.Others just say technique and care by the applicator is key to a good job.Re: combo, many insulators have a variety of products, foam, fg batts, cellulose etc. Some recommended 2x4 walls sprayed full, (more foam sold, and trim work needed) vs 2x6 walls with a combo of foam/fg batt. Others said to put 3" in a 2x4 wall and that would be enough at r-21 for closed cell poly.

   Some like 2x6 walls for strength, others for insulation thickness. I like it for both reasons.

   I will update all, after some factory visits this spring.

   I have got to tell about a guy in Dallas. A/C bills have been terrible every summer, so he finally took a look in his attic. He had counted 6 roof "turtles" from outside, could only find 3 from inside the attic. The roofers had just nailed them on the roof, never cut them through!

   Now he is having powered roof fans installed. Some day I may be able to convince him to put in a radiant barrier system and knock 40 deg out of his attic, where all of his duct work for three systems lives.

   Paul

   Energy Consultant and author of Practical Energy Cost Reduction for the Home