Replies

1. 
   4Lorn2 | Feb 10, 2004 06:57am | #1

   My understanding is that in a climate which gets cold and where a heating system is used has to have the vapor barrier between the warm moist air, inside, and the insulation, inside the wall.

   A barrier on the outside would allow the moisture to go into the insulation, likely if it is fiberglass as it is fairly permeable, where it could cross the dew point where the vapor would become liquid.

   A vapor barrier on the inside keeps the water vapor out of the insulation. Any moisture that makes it into the insulation is allowed to dissipate through the permeable house wrap.

   The foil would allow none of the moisture to dissipate to the exterior.

1. 
   AndyEngel | Feb 10, 2004 05:04pm | #2

   Which would make it an appropriate cladding for buildings in primarily cooling climates, such as Texas or Florida, where any vapor barrier should be to the outside. I think that's where the stuff is intended for, as a radiant barrier.Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.

2. 
   Mongo | Feb 10, 2004 05:26pm | #3

   Good advice so far...

   Still, your question "Would the foil be acceptable in lieu of housewrap?" begs me to ask where you live, as these two things are not neccessarily mutual. And, as the others have mentioned, your climate dictates their function and location in the structure. Also, what are you looking for? A "radiant barrier"? A "vapor barrier"? A "housewrap"?

   In general, foil is a radiant barrier that also acts as a vapor barrier due to its high perm rating. Conventional spun poly housewraps are not neccessarily vapor barriers...they allow water vapor to pass through, but not water droplets. Spun poly wraps do nothing regarding radiant. You probably know all that.

   Conclusion? If you're up north in a heating climate, foil-faced osb would not be a wise choice as an exterior wall sheathing for typical residential construction.

1. 
   Wylcoyote | Feb 10, 2004 08:06pm | #4

   Mongo, you sound like you know your housewrap physics

   Here's a question for you -

   Fiber cement siding remodel job in Sierra Nevada - snow and rain 6 months of year, 6 months of the year dry in the 70's and 80's - never air conditioned. 

   Existing t-111 over #15 Tar paper is in OK shape - planning to leave it up as sheathing underneath Fiber Cement.  here's the question:  should we put up Tyvek as vapor barrier underneath the Fiber Cement?  Worried about Tyvek in front of t-111 and Tar paper in back creating a "moisture sandwich" that will rot out T-111 if moisture comes from inside the house.

   Thoughts welcome.

   WT

1. 
   Mongo | Feb 10, 2004 09:21pm | #5

   Thanks for the compliment, but there are many, many others on this board that know a helluva lot more than me...

   First, a disclaimer...I'm in New England, so I'm not first-hand familiar with your snowy/wet, yet arid requirements...still...it does fall in lines more with a heating than a cooling climate.

   Second...remember...Tyvek is NOT a vapor barrier. The spun-poly makeup of Tyvek allows moisture VAPOR to pass through it, but the pores are small enough to not allow water droplets to pass through. It does also act as a "sort of" air infiltration barrier as well...in terms of the house being hit by winds gusts, etc, and those wind gusts translating to an over-pressure situation inside the envelope.

   Honestly, there are several angles from which you can address your situation:

   1) The T111 is an exterior product, so why protect it with an additional layer of something? Just put the fc right over it. This can work, but with wet and windy conditions being considered, trapping wind-driven liquid moisture between the fc and the T111 may not be a good idea. Read on.

   2) Same as 1), but why not create a drainage plane between the T111 and the new fc siding by adding vertical furring strips. Put the vertical furring strips over the T111 and nail the horizontaly applied fc to the furring. Any liquid moisture making it past the fc will drain down the drainage plane. Better idea...but could be slightly better.

   3) Tyvek stinks and felt is forever, so why not stick with what works and just put more tar paper over the T111, then fc over the felt. I wouldn't do this, as this could kind of maybe sort of end up being the "moisture sandwich" that you're trying to avoid.

   4) Tyvek over the T111 and fc over the Tyvek. Back to the original question, eh? Again, this can work, but with wind-driven moisture being a consideration, I'd like to revisit 2).

   My thoughts? You have six months of snowy and wet. To me, "Sierra Nevada", when I feed it through my translator, means "windy". Snowy. Wet. Windy.

   I would consider putting Tyvek directly on the T-111. Then vertical furring strips. Then the fiber-cement on the furring strips.

   For what it's worth, the existing tar paper is your vapor barrier. It may be old, it may be compromised, it may be full of nail holes, but structuraly, it still sort of falls into that designation, and you have to honor that.

   T111 is the sheathing.

   The Tyvek will then be your liquid barrier and your air infiltration barrier.

   The furring strips your rain screen.

   The fc your siding.

   When you get windy/wet/snow, some moisture is driven through the fc...through the lapped seams, the butt joints etc. This is true for most every siding...cedar claps, vinyl (especially vinyl), etc. What liquid moisture that gets behind the fc will now drop vertically in the drainage plane provided by the furring strips.

   Any vapor moisture that exits the house, through the walls, from interior to exterior, will go through the Tyvek and end up in the drainage plane. It won't be forced THROUGH the fc, but can escape via an alternative path. This will add extra longevity to whatever coating you put on the fc. Paint, solid stain, etc.

   Just my thoughts...

    

1. 
   Wylcoyote | Feb 10, 2004 09:56pm | #8

   Thanks - very good thoughts.  furring strips it will be.

   Wylie

2. 
   fortdh | Feb 10, 2004 09:25pm | #6

   Mongo, the foil faced osb I have seen has pin holes in a regilar grid fashion, and they are using a lot in Texas for radiant barrier.

   I did see one house that was having the panels removed from the roof, becaust the contractor thought the foil should face the sun to "reflect the heat back". He could not grasp that foil radiant barriers need a 3/4" air gap, and roofing or insulating against the surface eliminates their radiant barrier ability.So he pulled them off, turned the ones that survived, and brought in new for the damaged panels. Emissivity is not easily understood, so "foil side down" is the message the general contractor kept telling the sheathing sub. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home

1. 
   Mongo | Feb 10, 2004 09:53pm | #7

   Paul,

   I've never seen the foil-faced osb. Are the pinholes just preforations through the foil face? The holes don't go through the osb as well, do they?

   I think the concept that a foil face as a radiant barrier needs an air space in front of it is often lost. If it's sandwiched, the foil effectively becomes nothing more than very thin conducting layer.

   Plus, for radiant, the foil face needs to stay relatively clean. A layer of dust quickly reduces its effectiveness. 

1. 
   fortdh | Feb 10, 2004 11:16pm | #9

   Mongo, the pin holes are only in the foil, about 1/8" apart.Don't know the perm, especially since the foil is glued to the osb. Some energy wise builders are using the osb/foil on a regular basis.I heard two weeks ago, that one county in Dallas area is going to make it code for all residential roofs of the future.

   Re dust, I was told by one manufacturer that their studies showed the foil would loose is barrier effect after about 7 years in typical attic if foil is facing up. If down, and 1/2-3/4" air space or larger is maintained, the foil does not get covered with dust, and functions forever. It's good stuff, and I am impressed with the results I've had with it.

   There are a number of companies that are installing foil face up on top of attic insulation, so usefullness is limited. The foil also has pin holes for vapor transmission.

   I have found the most effective system for keeping an attic cool (to ambient ) is to staple the barrier across the roof rafters,thus trapping the hottest air between the rad barrier and roof deck. A gable fan pulling from behind the barrier and above the collar ties,will quickly exhaust the hottest of the hot.Stapled off in a long tent like fashion enables the fan pull all the way through the "tent". A continuous ridge vent works well also, if enough slope. Bottom edge placement of the barrier depends on continuous soffit or keep above insulation if not continuous soffit vent.

   In talking to a number of radiant barrier manufacturers at the Home Builders convention, sales are up and rising rapidly, as people are realizing how much attic heat they can divert, in the A/C portions of USA. Sorry for the dissertation, PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home

1. 
   Mongo | Feb 11, 2004 12:53am | #10

   Radiant gain in attics is an important topic that's not often discussed. I first figured it out in my own house.

   My house is a 2-story colonial with a full walk-up attic above. When I first built, the attic was unfinished and I simply put fg batts in the rafters. In the summer the attic temps would soar...and the 2nd floor bedrooms below would be a bear to sleep in at night. (No air conditioning, located in CT.)

   I later added AC, and in the process, because I was putting the second floor air handler behind the kneewall in the attic, I upgraded the attic as well.

   Attic with exposed framing, with 3/4" subfloor. FG batts in the rafters with air channel against sheathing for soffit-to-ridge. Attic temp was 127 degrees at the peak. Did a little detailing, but installed 2" sheets of PI. Attic temp dropped to 77 degrees. Took another night, but after all the latent heat ws lost, sleeping condtions improved markedly.

   Sold!

   Now that the attic is finished off, the summer attic daytime temp sits around 73-74 with the AC off.

   I've tinkered with the idea of sheathing a roof (exterior) with a radiant barrier facing up. Sounds like the foil-faced osb would work. Then sleepers from ridge-to-soffit on top of the radiant sheathing, with the sleepers sitting over the rafters. Then regular sheathing nailed to the sleepers, then felt and shingles. the space between the top sheathing and the radiant covered sheathing would provide the air channel. I know it's been done, I've just not done it. Not sure how dirty the foil would get being "hidden" in the roof like that...

2. 
   fortdh | Feb 11, 2004 01:57am | #11

   Mongo, What you describe is like some roofs I have seen in northern Europe and Switzerland. The double roof has about 2" in between to guarantee the top roof stays cold. No ice dams.

   However, we can reach outdoor air temp in the attic by the method I described in the earlier post for only 10 cents a sq.ft., plus $35 if a gable fan is needed. I have one air handler in the attic. Another advantage, is that as soon as the sun is off the roof, the temp drops quickly up there due to no massive cooking all day. I let the fan come on at about noon, and off at 5:30 PM.

   Do you have a pull down lid for the top of the walk up steps? Doing that takes a load off of the side walls of the two rooms that wall the stairway to the attic.

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

3. 
   Mongo | Feb 11, 2004 03:52am | #12

   My attic is finished off...part game room, part TV room.

   Before it was I did have a lid made out of...polyisocyanate!<g>

4. 
   fortdh | Feb 11, 2004 04:09am | #13

   Mongo, way to go. I think you like polyisocyanurate just a little bit too much. {GGG} You keep it behind sheet rock, I trust, in occupied territory. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home

5. 
   BillHartmann | Feb 11, 2004 05:55am | #14

   That is was someone brought up a last week or the week before that no one know what it was utill I did some searching.

   I think that it was called "curtain venting" or something like that.

   In the case of a new house they where including the outer finished walls (often brick). But it seems that it is not unknow for that part of Texas (Houston maybe) for they to do a retrofit of vented roof like you discribed.

6. 
   Mongo | Feb 11, 2004 06:21am | #15

   Bill,

   The double roofs have been discussed here for a while. I can't think of any recent threads, but we had a few several years ago. I missed the thread you mentioned. I'l try to search for it.

   I have done double roofs before, but not with the radiant barrier on top of the first run of sheathing. I've only done rafters/ply/sleepers/ply/shingles.

   Of course, all our discussions about this only bring smirks from the SIP guys.<g>