pvc roof: vapor barrier or not

brandonm78

I am getting bids to get my roof replaced. I have had 5 or 6 bids and narrowed it down to 2 companies. All but one has included a vapor barrier, but one of the two finalists says its not needed.

Its a mostly flat roof, in the Seattle area (marine environment). The house has no duct work (we open the windows to get fresh air). Radiant heated, and has a real wood burning fireplace. Tongue and Grove ceiling.

I am wanting a PVC roof since the home is on the ocean and gets heavy wind and is in a salt water environment.

One bid is for (from the bottom on up).

vapor barrier.
6" polyiso insulation (2 x 3" stagerred)
1/4" denz deck
fully adhered pvc membrane

The other bid is for (from the bottom on up) *prefferred installer based on price.

AF 50 fire sheet.
6" polyiso (2 x 3" staggered)
1/2" recovery board
1/2" CDX plywood
fully adhered pvc membrane

The question I have. Should I have the bottom bid guy, use a vapor barrier like all the others, or trust him and not have one installed? If so, should he use the AF 50 sheet as well (for fire protection), and where in the assembly.

Any other concerns on either of their setups?

Brandon

OldNBroken

To start with: On your second bid you have 1/2" cdx being added directly under the membrane??? HUGE NO NO!!! Either there is a miscommunication here or someone doesn't know what they are bidding. The membrane will be adhered to the recovery board. Other than that it's a clean spec.

From the look of both bids you are requiring a class A fire rating. The densdek is not really required and is more expensive than the alternative fire sheet and recovery board. With the fire sheet base generally a vapor barrier is not required.

Does look like you are having a very good full system installed though, kudos. What mfr? IB or Sarnafil?

brandonm78

Forgot to mention. On bid # 2 the PVC is a fleece back. Not sure if that makes a difference on the recovery board / cdx plywood installation. They definitely quoted the recovery board beneath the cdx.

The two PVC's.

First bid: sarnafil 60 mil or 72 mil option.
second bid: fibertite xt 50.

Preferences? The sarnafil bid is a good chunk of change more.

Windows on Wash

No more of a vapor barrier required that what is provided by the membrane in this case.

+1 to old and broken's advice.

__________________

OldNBroken

Doesn't matter the product. I can't see any reason for that extra layer in any scenario for a singleply. Maybe one of the other guys knows something I don't. Not sure what effects it has on your attempt at a class A either. I guess you could ask the contractor his reasoning for it.

On a side-note. Heading to Fort Columbia to start a job on Monday...can y'all keep the moisture away for a few weeks for me? For once

brandonm78

I think he was trying to get it so he could use 8" screws. So he did recovery board then cdx to add 1" of thickness. Is recovery board + cdx more expensive then 1/4" denz deck. Looking online I see denz deck as being about $16 a sheet. 1/2" CDX looks like its more than denz deck alone. I may be looking at outdated prices though.

Just wondering if I should tell him I want to use vapor barrier / denz deck as well. Like the first bid, and then proceed.

OldNBroken

Honestly I would go the opposite direction as you. Most of us don't much like messing with densdek if we don't have to. Firestop, 3 layers of 2" polyiso staggered (best configuration for insulation), recoveryboard and membrane. IMO probably the better system and slightly less costly but not by much. Either way will work, you just don't need the cdx and it bothers me that the contractor spec'd it.

Still waiting for anyone to chime in as to a legitimate reason why one would do that on this system. Like I said, they may know something I don't.

brandonm78

I contacted Fibertite, the manufacture, and without saying much they questioned the need for CDX as well. Saying they saw no reason to add the extra weight and add another layer of combustion to the roof.

So I emailed the roofing company and said I had concerns about adding a layer for those two reasons. They said they could easily change things up and offered:

"To take care of the fire rating either Densdeck or Securerock can be used in 1/4" thickness to achieve a class B rating. The assembly below that would be, 2 layers of 3" Poly Iso over an underlayment of 1/2" Fiberboard.

Regarding screws, the approved type only come in 1" increments so we have to design the assembly depth for the 8" screw"

What do you think about this new setup?

Thank You for your help.

OldNBroken

Well, no need for the wood fiberboard if you are using densdek. How long has this company been in commercial roofing?

You want one of two things:

1.
firestop or other fire-rated base sheet
polyiso (minimum 2 layers)
1/2" wood fiberboard
fully adhered membrane

2.
polyiso (minimum 2 layers)
1/2" densdek
fully adhered membrane

And who says screws only come in 1" increments? Even if they do, an extra 1/2" of penetration isn't going to go through the ceiling or anything. I'd plan on 8" screws for that anyways.

Anyways. Always good to do some homework before decisions on capital improvements like this.

brandonm78

Well, I finally got a good answer on the CDX.. it was "The reason I suggested the use of 3/4" CDX was threefold, first to bring the roof deck height up to a point where the 8" screws would work without penetrating the car decking below, second to avoid the use of insulation over the unheated spaces at the roof perimeter and thirdly to provide a solid base for anchoring the edge metals to the roof."

He said he can do it without the use of CDX, and using 3 layers of insulation (6.5 inches) instead of 6", but the roof perimeter framing would have to change to: "a simple pony wall would satisfy the anchorage requirements for the sheet-metal edge trim.".

The other thing I asked him about was taping the insulation continuously to provide more of an air barrier / water proof barrier. He said that was not needed and that staggering pretty much does as good of a job as taping the joints.

I am asking so many questions because the last roof I had installed (in March of last year) is leaking and the contractor went belly up. So I have to do it over again.

One good thing, is that this roofer offered that I can buy a manufacture labor warranty for 20 years, as they are an approved installer of fibertite. It requires an inspection after it goes on. I will probably take him up on the offer.

brandonm78

I'm leaning toward option # 2 you gave me.

6.5" of polyiso (3 layers staggered).
denz deck prime.
membrane

I have questions now still about the vapor barrier, if I should use duraguard or prime? and if insulation should be tapped / denz deck taped, or even the car decking joints tapped before they put it all back together. I want this thing as efficient as possible.

Thank you for your help
B