Replies

1. 
   BossHog | Jun 28, 2014 06:18pm | #1

   Insulation is what should keep your house cool.  Not ventilation.

2. 
   deadnuts | Jun 28, 2014 07:23pm | #2

   Depends...

   You're talking about the most important part of your homes thermal envelope--the roof. Just like your head, things can and should be done to control what happens to it; particularly when it really hot or cold outside.  I think the most important thing to remember is that you can only control the transfer of thermal energy through conduction, convection, and radiation means.

   A vented air gap, in and of itself, cannot control the transfer of thermal energy through your building envelope by means of conduction (changing the R-value), but it can help with convection by bringing in cool air at the soffet and venting hot air (in proposed sheathing gap) at the roof's peak. Venting is very modest form of  thermal energy control and is, quite frankly, a more effective moisture control mechanism. The thermal control aspect of venting your particular project assumes that you thouroughly air seal your existing 1x12 sheathing plane to reduce convected heat gain (or loss) by means of wind wash via soffet to ridge venting.

   The only way I see you more effectively controling heat gain (without increasing your roof's thermal R-Value) is by  incorporating a reflective thermal barrier into your new construction. This would allow you some ability to block some of the suns radiated energy. The most widely accepted method of successfully doing this is to install a highly reflective surface downward toward your building envelope (so dust doesn't settle on it and block its effectiveness) and make sure it is in minimal direct contact with another building material (which you would be doing with the channel). This is to prevent short circuiting of the sun's reflected heat by means of conduciton. Huber industries, the makers of "zip panel" now makes an OSB sheathing panel with a foil face bonded to the backside just for radiation barrier purposes. I would recommoned using this product to get an easy, effective, and integral radiate barrier in place with your new construction.

   Keep in mind that this is not as effective as increasing the R-value of your homes thermal envelope (conduction control). I believe the biggest "bang for your buck" comes from keeping thermally conditioned air inside your building envelope--not keeping thermal energy out. However I also believe radiant barriers are effective and worthy--especially now that we can easily incorporate them into our projects.

   SIDE NOTE: If you do choose to investigate increasing your homes R-value (conductive control of energy transfer), then I would caution to do it in such a way as to not develope a dew point ( where moisture vapor turns to liquid water) within your thermal enevelope during extreme temperature swings inside and outside of your home --especially when working with materials that "care" about moisture (i.e. wood, drywall, moisture sensitive metal fastners, etc.). That's a whole 'nother story.

3. 
   DanH | Jun 28, 2014 10:45pm | #3

   It would probably keep it a little cooler, but how much is very hard to predict.

4. 
   Mike_Mahan | Jun 29, 2014 10:29am | #4

   Fire codes.

   Be aware that some locations in Calif. have some very restrictive fire codes that govern eave vents and soffit construction.

1. 
   deadnuts | Jun 29, 2014 11:05am | #5

   Can you tell us what some of those restrictions are Mike?

1. 
   Mike_Mahan | Jun 29, 2014 12:13pm | #6

   In San Diego County

   

   Vents

   : Attic vents shall not be installed in eaves, eave overhangs, soffits, cornices or between

   rafters at eaves. Vents are permitted in the eave assembly only under the following conditions:

   o

   Eave or cornice vents are constructed to resist the intrusion of flames and burning embers

   into the attic area of the structure; or,

   o

   When allowed by the building official and the Fire Authority Having Jurisdiction, enclosed

   eaves may have strip vents on the underside of

   the eave closest to the fascia provided the

   closest edge of the vent opening is at least 12 inches from the exterior wall and the building

   is protected by a fuel modification zone at least 100 feet wide

   All vents (roof, foundation, combustion-air, etc.) sha

   ll resist the intrusion of flames and embers or

   shall be protected by louvers and 1/8” non-combusti

   ble, corrosion-resistant mesh. Turbine attic

   vents shall be equipped to allow rotation in only one direction.

1. 
   deadnuts | Jun 29, 2014 02:38pm | #7

   very interesting...but

   That's all very interesting. I can definitily see where a municipality in a dry climate such as San Diego would be particularly concerned about the spread of fire and inmplement these sort of building code measures. However, isn't San Diego in the very southern part of California rather than central (where the OP is located)?.

1. 
   Mike_Mahan | Jun 29, 2014 02:58pm | #8

   The whole state is in extreme drought.

   As you can see from the maps in the links below even the central and northern parts of the state are extremely vulnerable. In terms of drought the central part is especially bad.

   http://droughtmonitor.unl.edu/Home/StateDroughtMonitor.aspx?CA

   http://www.fire.ca.gov/fire_prevention/fire_prevention_wildland_zones_maps.php

2. 
   deadnuts | Jun 29, 2014 07:47pm | #9

   So is this not only a San Diego County code as you quoted from, but also a statewide adopted measure?

3. 
   Mike_Mahan | Jun 30, 2014 08:52am | #10

   Yes

   There is a state wide code for wildfire exposure. Local jurisdictions may write their own codes using the state regulations as a minimum, define areas at risk, and are charged with enforcement.

   
4. 
   deadnuts | Jun 30, 2014 07:43pm | #11

   Thanks for the additional info Mike.