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Ventilation of a flat roof

R

Rio

Silver Member
Joined
Mar 31, 2010
Messages
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We are doing a 2nd story addition consisting of a deck over an existing 1st story with a 2nd story work studio behind the deck over the rest of the existing 1st story. The existing structure has a flat roof, that is balloon framed with about 18" from the ceiling joists to the roof joists; there is no ventilation on this part. The new 2nd story will have a flat roof (1/4:12 pitch) behind a parapet, with the ceiling applied directly to the roof framing members.

I checked in the 2007 CBC (California's version of the IBC), article 1203.2, and it appears that ventilation will be required. We have done these previously without any ventilation and have them inspected with no problems coming up so I checked in the 1997 UBC and in 1505.3 it says words to the effect that the building official may require ventilation on flat roofs if he/she thinks the environmental conditions warrant it. I have looked in the 2007 CBC and don't see this exception.

I asked the contractor who is building the addition his opinion as he's had a lot of experience and he said he always thought that ventilation is only required if there is an attic access.

As it stands now it appears to me that attic ventilation is going to be required for both parts of this addition. Any insight on this would be appreciated.
 
2009 IBC (looks like a new section): Enclosed attics and enclosed rafter spaced formed whee ceilings are applied directly the othe underside of roof framing members shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain and snow..
 
Thanks for the feedback Peach. From what I've read it looks like we're going to have to provide the ventilation. The contractor is meeting with one of the field inspectors in the next few days so we'll see what he says. If I get a different answer I'll post it.
 
Quote from: IRC FAQ: Conditioned AtticsBy Building Science Corporation "Unvented attic assemblyAlternately, an unvented attic assembly can be implemented, based on the requirements of Section R806.4 Unvented attic assemblies. This section was originally appended in the 2007 Supplement to the International Residential Code (IRC).The underlying goal behind this section is to raise the temperature of the cold-side inner surface (i.e., underside of the roof deck, or inner foam surface) sufficiently that condensation will not occur, if interior air comes in contact with that surface. This is done by using what is referred to as “air impermeable insulation,” such as rigid foam board or spray foam.Two acceptable assemblies are shown below: one with air impermeable expanding spray foam insulation installed at the underside of the roof deck (as shown in Figure 2), and the other with rigid board foam plastic insulation installed above the roof deck (as shown in Figure 3).In both assemblies, air permeable insulation (such as batt or loose fill) is used to increase overall insulation value. This is by no means a requirement; however, it is typically the most economical way to achieve target (or code minimum) R-values. Alternately, “air impermeable insulation” alone could be used for the entire insulation thickness, assuming that all requirements below are met."

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Thanks JM................. I know we never had to ventilate when doing flat roofs previously down here in this neck of southern California but I need to see where it says it in the 2007 CBC or IBC as stupid California hasn't adopted the IRC yet.
 
Rio:We drill lots of holes, and then we install baffles to keep the insulation clear of the air flow. California has adopted the stupid IRC, it will take effect on January 1st, it is for sale now on the ICC website, you may have to be a member to follow that last link, I don't know since it works for me.BTW, J. Morrison's link to the Building Science Corporation is considered valid building science, but Joe Lstiburek can be a real nut at times, he (in pursuit of energy efficiency) at times ignores areas like toxic substances, we in California do have Prop 65 which makes many products like spray foams illegal, he will maybe wake up when H.R. 5820 becomes law (or what's left after Congress gets done with it) and the EPA starts enforcing such prohibitions nationwide.If you seal a roof structure with foam and get a roof leak (ever see that happen?) you are in real trouble, maybe even a tear-down because of mold, form a recent case I just had, mold seems to grow like crazy in styrofoam.

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I used to have alot of respect for Joe until he apparently jumped on dupont's bandwagon
 
\ said:
I used to have alot of respect for Joe until he apparently jumped on dupont's bandwagon
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Me too, he testified in court against Tyvek and won many times, I know an expert in North Carolina that drove to Massachusetts to hear how he did it and he was great. All of a sudden DuPont became a "sponsor" of Building Science Corp and he started recommending it. At this point Building Science Corp has removed their link to their list of sponsors. Note through that when he does use Tyvek it's always in a rain screen application, so water never touches it.
 
still.. well, frankly R703.2 calls for one layer of No. 15 asphalt felt, free from holes and breaks (or other weather resistive barrier - approved is a building official call, of course) shall be applied over studs or sheathing of all exterior walls......

better than any of the other "manufactured" products... and what we should start requiring... easy enough.. don't "approve" any other WRB.. make them rip 'em off.

Approved (Chapter 2) is acceptable to the building official.. period..

felt is the best product out there.. (to heck with new technology). Yup, it's heavy (which is why you see it in 3' rolls, not 9' rolls).. it's a little fragile, so to be free of holes and breaks.. I guess the siding contractor needs to use some CARE when installing it.

I feel better now.. even though I strayed dangerously off topic.
 
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Thanks for the feedback and the tips about using foam. If I could ask, what don't you like about the IRC, Conarb?
 
Rio:

  1. Part of it is that I grew up with the UBC and learned and followed it from the 50s when it was about a ¾" thick book.
  2. It is admittedly an oversimplification to gain acceptance in areas that didn't have codes, the idea being that areas would amend it as they progressed, to date only Elmhurst Illinois has done a good job of doing that. BTW, I know two structural engineers who worked on the writing of it. The code as written is a huge step backward for California, it will be interesting to see how they have amended it to create the 2010 CBC and retain old UBC provisions for health and safety.
  3. I don't like the whole ICC concept of "performance based codes", this is industry promulgated crap to gain acceptance of new products, I've read that historically 9 out of 10 new building products fail and are withdrawn from the market within the first 10 years, many of these failing products are showing up in buildings now. At times it's not any one product that fails, but products in combination with other products, many of them new as well. As we see here, it sounds great to seal a building with foam for insulation purposes, but what if there is a leak? In the old days a leak was a problem to fix, now if there is mold you might just as well tear the building down, insurance only covers sudden leaks, if you don't know there is a roof leak and it gradually saturates the foam causing mold growth, you have no insurance coverage and are in for hundreds of thousands of dollars in remediation and rebuilding.
 
As both a contractor and insurance adjuster, I'll jump in here.

First there is no blanket statement that the insurance only covers sudden leaks. There are many policy variances and whether or not an insured could have had knowledge and corrected the problem is a main factor on many policies.

I agree that the flat roof spaces must now be ventilated and this presented a problem on a new design I had come up with. My final design solution was this: I am using 2x8 joists 24"oc with 3/4" T&G decking with the sheathing joints sealed with glue. The insulation consist of tapered foam on top of the deck. Below, we are painting the underside of the sheathing white (before installing it to save labor) and then staining the 2x8s and leaving them exposed. We solved the code issue by NOT having a sealed unventilated space; saved the cost of having any ceiling surface finish material and have exposed rafters as a design/architectural feature.
 
That's a good idea to take care of the unventilated space, eliminate it. I might be able to do that for the new roof but for the existing one it isn't an option.

Any other ideas on how to deal with an unventilated space or any ideas on getting an exemption?
 
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