Tony Billau
1/23/2012
Wrote earlier with no response. Will try again. Existing 4/12, 1900 sf. rancher in Cottonwood AZ area. Comp. roof is shot. Would like to remove HVAC units from roof, remove comp. shingles and add standing seam. Because duct work will be in attic, I was going to have open cell spray foam applied to underside of roof sheeting/rafters. Now my question. Is there anyway to tell if adding vertical then horizontal battens on top of existing sheeting vented high and low before metal would help energy efficiency enough to offset material and labor (myself) cost? If I choose this system, would you use reflective underlayment? Would reflective underlayment work without battens? Thank you for your time, Tony
Todd Miller
Classic Products, Inc.
1/23/2012
Thanks for your questions. Reflective underlayment needs to face an airspace in order to be effective. The vented airspace would be very helpful but ultimately still would not give you the R value that the spray insulation would. It is however plenty adequate for most homes.
Tony Billau
1/23/2012
Thanks, but my question was will doing the additional work above the sheeting vs put metal with no air space on sheeting ever pay off? And will the reflective underlayment do anything at all if metal is installed directly on top of underlayment? Thanks again, Tony
Todd Miller
Classic Products, Inc.
1/23/2012
I'm sorry. I misunderstood. First of all, a radiant barrier does no good unless it faces an airspace. Now, some of the radiant barriers have bubble wrap Attached to them and that helps provide some airspace. I would say that if you use the foam, there would be little reason to build up an airspace on top of that. I do not think it would be worth the extra cost.
Tony Billau
1/23/2012
If the bubble wrap is affordable to me, would you need a ridge vent?
Todd Miller
Classic Products, Inc.
1/23/2012
You only need a ridge vent if you have soffits intake vents to feed air to it.
Tony Billau
1/23/2012
Would any air really get through bubbles if I used soffit and ridge vent?
Todd Miller
Classic Products, Inc.
1/23/2012
No. For ventilation you need a minimum one half inch unobstructed gap
Info @windowsonwashington.net
An informed customer is our best customer.
1/24/2012
Tony, You will need to decide if you are going to treat the attic as conditioned and move it inside the envelope or leave it vented. That has nothing to do with using a radiant barrier or not. Radiant barriers, as Todd mentioned, need an air space to be properly installed. Using metal in lieu of asphalt shingles is going to make your attic markedly more comfortable and reduce the amount of radiant head gain from the roof and make the home much cooler. If you choose a color in the lighter range, this will be even more dramatic. Adding a system of purlins and putting a radiant barrier across the roof deck will reduce the amount of radiant heat gain that gets to the sheathing and then into the attic but its overall impact will be a direct reflection of what color spectrum room you choose. Putting the roof on purlins and allowing for some over deck venting will also have an impact in attic heat. Adding foam to the roof deck is a nice option if you want to keep that as conditioned space. If you don't need the space for anything like that, I would avoid the added expense of the foam and just use a proper metal roof. Let me know what you ultimately want to do and I will tell you the best way to go about it. Using rigid foam to the outside surface is usually a bit cheaper than going the all spray foam route as well.
Tony Billau
2/8/2012
I am going to use spray foam on the inside roof sheeting, not because I intend on using that space, but because the duct work will be in attic. I was just trying to find out if using furring strips and ventilating above sheeting would be worth the cost in efficiency. I had not thought about ridged outside. How does that work? Thanks
Info @windowsonwashington.net
An informed customer is our best customer.
2/10/2012
Tony, Turning the roof into a "hot" roof system as a result of the ductwork being in the attic might be like swatting a gnat with a sledgehammer. There are ways to properly air seal the ductwork as well as insulate them quite cost effectively. That being said, hot roofs, done properly have other inherent benefits if you are planning to take advantage of them. Feel free to contact directly and I can give you a more detailed breakdown of how to design the roof in whatever capacity you see fit. There are a bunch of ways to skin this "cat" cost effectively.

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