*
I’ve been told that doubling the vapor barrier can cause condensate problems between the barriers. Is this true? It doesn’t make sense to me that plastic on Kraft faced insulation would condensate anymore than drywall on Kraft faced insulation. Anyway, when I started out I thought that I was supposed to so I’m 1/2 covered. Should I ripp off the plastic or keep going? Dilema, please help, wife wants this project done asap.
Thanks,
Dan
Replies
*
You only need one or the other...It should take just a few minutes to change I would think...
The wife knows her stuff huh!]
near the stream,
aj
*I'm told it's advisable to cut slashes in the kraft paper before covering it with the plastic.
*if you believe in vapor barriers on the ceiling.... or if you just believe in air retarders.. it comes down to the same thing...the kraft-faced sucks... er..rather.. leaks..there is a continuous joint every 16 or 24 inches... how is that going to seal ?if the kraft faced were continuous.. then the poly would be redundant... but it ain't... as a matter of faact , it has so many joints and penetrations ,, it's like it ain't there at all..put up the poly.. and let it lap the walls .. watch out around all penetrations,.. and think about a second layer of insulation on top of your first layer.. at right angles..or , better yet.. blow a cap of cellulose on top of the fiberglass...if you insulate.. go up in the attic witht he lights out .. before they put the board up on theceiling.. look at all of the light leaks.. and just imagine them as air leaks...you'll convince yourself to do 2 things... put up the poly vapor barrier / air barrier.. and blow a cap over the glass....b but hey, whadda i no ?
*http://www.certainteed.com/pro/insulation/index.htmlCertainteed at their site says to pull off all the paper instead of slashing....and Mike they say the kraft works and that it's OK and tested that it be stapled to joist sides...I'm not big on batts but I am firmly not for doubling barriers...especially if one of the barriers is plastic.near the stream almost agreeing with ya Mike,aj
*aj.. not trying to put words in fredl's mouth...but stapling kraft to the side of studs .. or rafters... or joists... goes against his theories of sealing the ceiling....wether you like F'glas or not... batts just suck...high-density unfaced is what we always try to use...if we're using f'glass..the slashing (recommended by mfr.) comes from the tendency to use batt on batt...you go into an attic.. and there is an old 3 inch batt up there...so the homeowner overlays it with a 6 inch kraft faced... what he /she has done is create a condensing surface in the zone where it might actually be below the dew point ..so.. their problem is not with a double vapor barrier on the back of the ceiling surface.. it's with a vapor barrier buried in the dew point zone.. they recognize that this MAY trap a lot of warm moist air right where it can't be handled by anything..in the average non-fredl. home.. the ceiling leaks air like a seive (or sieve) .. so the warm moist air is flowing into the attic, hitting the kraft layer and getting trapped.. and condensing...ugly..eh ?if the poly is on the back of the sheetrock surface and it has sufficient insulation above it.. it cannot become a condensing surface..so what is the slashing gonna do for anyone.?..or to look at it another way.. if the mfr. wants a vapor barrier ( the kraft-face),.... why would the mfr. care if the vapor barrier became MORE perfect ?kinda flys in the face of reality , eh ?anyhooo.. we ain't got that problem.. i can count the number of ceilings that had kraft facing stapled to the face on one hand.. (that weren't furred also).....b so typical for us:.. in the old days .. furring under 6mil poly,... under 6" unfaced batts.. under 6 inch unfaced batts laid in the other direction.. nowadays.. furring under 6 mil poly, under 18 inch to 24 inch of cellulosethe furring is just too cool, the electricians love it.. they can go right over the top of the furring and under the joiststhe insulators love it... they can blow the attic before the blueboard goes up if they want.and the board hangers love it.. they get a smoother ceiling and a bigger target for their joints and screws...take the money you spend on stapling batts to the side of the joists and put it in furring...the only walls you erect before furring are the bearing walls.. the partitions go to the bottom of the furring...why would anyone willingly build without a vapor barrier and furring..?some of our early solar homes had 20,000 gallon tanks of water built into their foundations.. and were earth sheltered too.. no moisture problems even 25 years later....control the air.. do it the genel. / fredl. way or try the old fashioned gold standard.. the 6 mill poly vapor barrier....[which is conicidentaly.... an air barrier too] ... then get rid of what you don't want with a bath fan.. or a kitchen range hood....b i love it when a plan comes together
*Seems that stapling the Kraft-faced to the sides of the studs almost guarantees a series of floor-to-ceiling chimneys, encouraging air circulation within the wall.If the kraft-faced fills the bay, you wouldn't think putting poly right up against it would be a problem. If the poly works as it should, moist winter-time area can't get through it anyway. And if the kraft works as it should, moist summer air can't get to the poly. Right?If Mike's right (and he probably is) the problem would be in the summer, with moisture finding it's way through the kraft and condensing on the cool poly. Cutting slits will just encourage this. So I'd have to say, after you've stapled the insulation up, strip the kraft off (center) if you're going to use poly.Mike, what do you use for furring? What is your spacing (with 24" truss spacing)? Appears to me this would solve a problem I have with some floor truss sags and waviness. Do you shim the furring to ensure a flat ceiling? Do your furr completely around the perimeter of the room?
*Mike, the furring is no better than the side stapling.Both create a misalignment of the air barrier and insulationthis allows for free convectionfree convection hnegates the insualtionI realize this is horizontal, but that's the way it is.If you want to sheet the ceiling with foam and tape joints then fur down, you have an air barrier to prevent convection between and thru the batt.-Rob
*Rob...I agree with you, and even a least expensive thickness would probably do more than seal the lid by adding a radiant surface to the inside....But Mike's idea I would think is better than stapling to the inside...near the stream,aj
*rob.. are we talking about furring the ceiling ?..((Both create a misalignment of the air barrier and insulation ))whoa!you sound like a chiropractor...'splain yourself, lucyyou're spending money you don't need .. the only time it's cost effective to add foam is when the space available is limited..like in a 2x4 wall..or in a cathedral ceiling..and if we do add insulation board to the heated side (which we do if we're trying to overcome those defiencies) then we furr that too..crusty... furring is common practise here... always has been, at least since the lath and plaster days..we furr 16 inch OC, perpendicular to the joists..1x3 furring..no furr blocking at the end walls.. but the furring starts and ends at the walls.if you coordinate with your elec. you can get a lot fewer penetrations of the air barrier / vapor barrier..another benefit of furring is the devices in the ceiling..got track lights ?run a furring right where the track is going to go..drill a hole at the exact location of the device box, and pull the wire thru...the rockers will thank you, the elec. and the homeowner..got a ceiling fan... put a piece of 3/4 ply in the fan location.. mount your fan outlet...if you need additional support anyplace.. you can substitute 3/4 board or ply for the furring.. there are a lot of things you can do in that 3/4 inch space..b but hey, whadda i no ?
*Let me see if I have this right. I put the poly up against the ceiling joists. Then I put up the furring, and then the drywall. So I will have a 3/4" space between the poly and the drywall. So Rob, you say this 3/4" space with drywall on one side and poly on the other, and 18-24" of cellulose on top of the poly, is going to allow convection?What happens to the poly once I start pouring in the cellulose? Right, it sags down right against the drywall, except right next to the furring where I will now have a 3/4 x 1/2" air space. Not much convection gonna happen here I don't think.
*Crusty...I think Rob is saying the bats are the problem with fit no matter with or without kraft and or strapping...He is in favor of cellulose...near the complicated thread restudying the stream of yak,aj
*
I've been told that doubling the vapor barrier can cause condensate problems between the barriers. Is this true? It doesn't make sense to me that plastic on Kraft faced insulation would condensate anymore than drywall on Kraft faced insulation. Anyway, when I started out I thought that I was supposed to so I'm 1/2 covered. Should I ripp off the plastic or keep going? Dilema, please help, wife wants this project done asap.
Thanks,
Dan