| Author |
Message |
Martin Hartmann (Unregistered Guest)
Unregistered guest
| | Posted on Tuesday, June 30, 2009 - 07:37 pm: |    |
The architectural firm I am working with is designing a winery production facility in a hot, arid part of Southern California. Although the building is mostly based on function, the design is very high-end, modern and the owner is looking for a clean look inside and out.
The roof framing is flat with a tapered insulation system on top of a wood deck with a modified bitumen membrane. The tapered insulation is for drainage purposes only. Fiberglass batt insulation is being called out between the wood roof framing members for thermal protection. Because the owner wants a clean look, we are proposing that gypsum board be installed directly to the roof framing as a ceiling finish. This essentially creates a sandwich of gyp bd, insulation, and roof sheathing.
IBC Sect 1203.2 calls for ventilation at enclosed rafter spaces, but the interpretative manual only mention how this applies to cathedral ceilings; not flat. Our deputy building official stated that the ventilation is not required at flat roofs. The construction manager is inclined to eliminate any proposed ventilation since the building department will not require it. I feel pretty strongly that the ventilation should be there or else we will have some enclosed space that is prone to all kinds of moisture, thermal, and pressure-related problems.
Does anyone have any experience with ventilating flat roofs? I know one of the latest Construction Specifies issues had an article, but it seemed to deal mostly with ventilation the space between the membrane and the deck.
Thank you in advance!! |
Nathan Woods, CCCA, LEED AP
Senior Member
Username: nwoods
Post Number: 296
Registered: 08-2005
| | Posted on Tuesday, June 30, 2009 - 08:19 pm: | |
If you fill the cavity between the wood framing members with full-depth insulation, there is no air space to ventilate. That is a very common implementation in Southern California. |
Steven Bruneel, AIA, CSI-CDT, LEED-AP
Senior Member
Username: redseca2
Post Number: 185
Registered: 12-2006
| | Posted on Tuesday, June 30, 2009 - 08:53 pm: | |
If this project is in California I assume you also need to conform to the CBC requirement for a white reflective, "cool" roof. That will mitigate some of your concerns. |
Wayne Yancey
Senior Member
Username: wayne_yancey
Post Number: 218
Registered: 01-2008
| | Posted on Wednesday, July 01, 2009 - 10:22 am: | |
Martin.
Delete the batt insulation and use closed-cell spray polyurethane foam to create an unvented roof assembly that works in all climate zones.
Closed-cell medium-density spray foam (2 ound per cubic foot) has a higher thermal resistance and low vapor permeance and also acts as an air barrier (at 1 inch minimum thcikness) and therefore does not need an additional vapor retarder in cold climates.
Go to http://www.buildingscience.com/doctypes/digest and look for digest BSD-149
You could also consider SIPs and less roof framing. |
Mark Gilligan SE, CSI
Senior Member
Username: mark_gilligan
Post Number: 173
Registered: 10-2007
| | Posted on Wednesday, July 01, 2009 - 12:37 pm: | |
There are concerns about the diaphragm strength of SIPs and for the time being their use should be limited to single family house construction. I would not use SIPs in high seismic regions untill there is a concenssus regards design and detailing requirements in earthquakes. |
Phil Kabza
Senior Member
Username: phil_kabza
Post Number: 392
Registered: 12-2002
| | Posted on Wednesday, July 01, 2009 - 04:29 pm: | |
Wayne mentions the concern for vapor permeance above; vapor performance of this system should be modeled. Winery production and storage will entail fairly high levels of indoor humidity that may require some control for proper barrel aging.
Why not enhance the roof board insulation on the exterior where it would naturally go - the dewpoint stays outside, a vapor retarder can be placed on the exterior side of the wood deck, the interior is free of insulation materials and finishes for the clean look you are seeking. Several mod bit systems are available that will provide a CRRC cool roof rating. |
Ron Beard CCS
Senior Member
Username: rm_beard_ccs
Post Number: 306
Registered: 10-2002
| | Posted on Wednesday, July 01, 2009 - 05:00 pm: | |
I have a couple comments on the previous comments:
When using foams, they cannot be left exposed but rather have some protection such as plywood or GWB which has some minimum fire-rating per the applicable code. I love the CCSP foams but they don't always work on all applications.
I also like SIP panels and have seen and used them in several non-residential applications [Microtel Motels for example]. Based on my non-structural engineer readings of many SIP tach data they are heavily tested under seismic conditions and do well. Mark can you elaborate a little more as to your concerns with SIP's?
Wayne's comment about vapor drive raises another point: when reviewing the ceilings/roofs in a possible hostile space environment do not forget the walls in your analysis.
Finnaly, correct me if I am wrong, but the definition of a rafter is defined as a sloped member. |
Mark Gilligan SE, CSI
Senior Member
Username: mark_gilligan
Post Number: 176
Registered: 10-2007
| | Posted on Wednesday, July 01, 2009 - 07:08 pm: | |
The primary concerns regarding SIPs are:
The panel connectionss are not as strong or as ductile as corresponding connection that nail the plywood off to sawn lumber.
SIPs consist of independent panels that must be tied together much like precast concrete elements. This is harder to do than with stick build construction, so untill we figure out how to do this there is a concern that they will suffer more damage in earthquakes and high winds.
SIPs are relatively new technology and while they have found their way into the IRC they are not yet in the IBC in part for the reasons I noted. Until they mature I recommend caution in how they are used |
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