Polyisocyanurate Rigid Foam Insulation Boards Over Plywood Sheathing & Vapor Barriers

cloves

Hi all, I am currently in the mist of getting new vinyl siding for my house (I am located in North NJ Zone 5 btw). I was thinking of adding 1" Polyisocyanurate Rigid Foam Insulation Boards over the plywood (Dow Tough-Stuff R). On the inside of the stud walls, I am going to be using fiberglass batts and it comes with a paper vapor barrier (which will face the inside of the house, warm side).

I had 2 questions:
1. Is the poly-iso and fiberglass a good combination?
2. Does anyone see issue with moisture when using the poly-iso and fiberglass with vapor barrier inside? I still am not understanding if the poly-iso also has a vapor barrier or not.

porky150

the poly outside will only form a vapor barrier if it is either tounge and groove or you seal it manually,(i.e. dust tape the all the seams), remember the house has to breath some. i mean you can make it too tight, but if you choose to seal the out side as well you can create weeper holes in the bottom of the starter strip behind the foam. adding an inch of poly behind your siding is always a good idea, but i think that if your plywood or osb was installed tight and you over lap the seams between the ply and the poly that should create the desired atmosphere inside the home withoput worry about moister build up. as far as the weeper holes you would space them 4-6' apart and about 1/4" dia. good luck and i hope this helps p.s. if you used any kind of house wrap, you need do nothing further than install the foam and siding, that would seal everything and create a moisture barrier, it just depends on how much you want to spend. remember if its worth doing, its worth doing right

jklingel

I do not think you want any rigid foam w/ foil on it, as it will be a vapor barrier, either a second one or one in the wrong place. Other than that, foam is OK out there. Greenbuildingadvisor.com recommends R5 over a 2x4 wall, R7.5 over a 2x6. NOTHING is a good combination with fiberglass insulation; it is the worst insulation on the planet (I assume you are talking about batts). Try to locate cotton, mineral wool, or cellulose batts instead. You should have an air barrier on the inside of the wall (in house), like the airtight drywall approach, and you probably do not want an interior vapor barrier. VBs are usually advised only in very cold (zone 7 and 8) places, and some very hot, humid places (where it goes on the outside). Your walls need to breathe as much as they can.

cloves

Thanks for the reply fella's.

Porky, yes I planned on taping all the seams with tyvek tape or something equivalent. I want to make a super tight building envelope. If I can save money and not use roofing tar paper over the plywood and or house wrap that would be great. Just to clarify you are saying if I add house wrap on top of the plywood and then my polyiso foam sheathing and then the siding I should be ok? But if I don't add housewrap I will not be ok.

I am trying to find some imagery of the weeping holes you mentioned. Not finding anything at the buildingscience site

Jklingel, wish I had another option for the inside but I purchased the fiberglass batts already. I am in zone 5 and I just looked up the code and vapor retarder is required on the inside warm side. From my reading so far, the batt insulation (Johns Manville) that I got has a vapor retarder. Which isn't a vapor barrier like plastic. The vapor retarder allows moisture to pass thru it from what I am digesting.

"402.5 Moisture control. (Mandatory). The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation."

creamaster

http://www.buildingscience.com/docum..._download/file

check this out has nice info for what you are thinking of doing.

Gary in WA

This has the foil faced foam in your area. I may have missed it in the B.A. one.... Page 8, and figure #10: http://www.buildingscience.com/docum...researchreport

Gary

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Tom Struble

what i like to do is tape the top of the foam to the wrb,then install the next insulation board over top of the tape,[i don't tape the horizontal seams in the foam closed] so that any water that gets in can drain out

i use the same technique under all windows and other penetrations

i add a wrap ''skirt'' under the penetration and weave it over the foam

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cloves

creamaster and GBR thanks for the pdf links! I have read and searched that site alot and never came across that particular set of pdfs. Version 1 is very similar to what I wanted to do.

tomstruble, thanks for the images. It took me a bit to understand exactly what you meant. But after soaking it in, basically starting on the first bottom row of very thin foam you used, you taped the top of the sheets to the WRB with typar tape. Then I am assuming that the next sheet you put above you didn't add tape to the bottom. This way if moisture did get inbetween the WRB and the foam sheets, it could weep out the bottom inbetween the front of the foam and behind the siding. Great pictures. I wonder if the fact that the foam insulation I am looking at is 2inches thick if this method could still work?

Tom Struble

yes this foam was thin,i was hired to replace the existing materials
the thin foam while not very high in r value makes a nice base for vinyl siding

you can use the same method with thicker foam using a strip of thin poly and taping that to the wrap and letting the bottom drape over the top of the foam

cloves

hi all, sorry with the cold I have been out of commission on the renovation.

tom thanks for the update! I was reading up on the other issues when adding 2 inches of foam and then siding. The most painful one of course being that I am going to have to somehow extend the window frames and doors. Then I started to think about the window trims. Tom do you by any chance have any photos that somewhat show what 2 inch foam and windows/doors looks like?

Tom Struble

no sorry but asuuming you pull the old siding it really wouldn't look much different than many of the houses that are probably in your nieghborhood where the homeowner went over the exising siding