Asphalt Shingle Roof and Attic Insulation
SergioD
| Posted in General Questions on
Currently I have an asphalt shingle roof, which has reached its end of life. At the same time, I need to redo the insulation in my attic because it is very inadequate. So I am looking for advice on an effective design for the roof and attic insulation that will mainly keep the temperatures in my attic at a reasonable level. Note that my air ducts are in the attic, but the air handler is located in my conditioned space. And I am located in Huntington Beach, CA (US Climate Zone 3; CA Climate Zone 6). For most of the year I experience a mild climate.
I’ve had a few roofing contractors come by and give me quotes on a new roof, and most of them push for asphalt shingles. I am guessing it’s because it takes less skilled labor, and less time and material. So in the interest of keeping things simple and cost effective, if I do go with asphalt shingles, is the following design acceptable:
1) 7/16″ plywood sheathing (this is existing)
2) tape seams with Zip flashing tape or similar (is this necessary?)
3) close off gable, eave, and dormer vents (to convert to attic to conditioned space)
3) 2 layers of #30 felt paper
4) “Cool” roof asphalt shingle
5) BIB system or fiberglass batts on the underside of the roof deck to R-38 or better
With this design will I experience any condensation issues? I have attached a picture of the underside of my existing roof deck. The plywood sheathing is currently installed on top of 1×6 battens, which were probably for the original wood shake shingles when the house was built in 1972. So there will be an air gap between the areas that have no battens and the insulation that will be installed. Will thermal bridging be a problem?
In case I do decide to increase my budget and go with a more expensive roof material, is a lightweight concrete tile ($$) better or a metal shingle ($$$)? And would I benefit from installing them on battens? I have to go with a lightweight roofing material because I have a 2×4 truss roof which would mean that it would have to be reinforced for a heavier material like normal concrete tile.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
There is no reason to move to an unvented roof in your climate. People have unvented attics for various reasons, none of which apply to you in your climate. Usually it is because they live in more extreme climates than yours where it is imperative to keep the ducts in conditioned space for the HVAC to work efficiently. It is very expensive to go to an unvented conditioned attic and you are lucky enough to live in a moderate marine climate where it isn't necessary.
There are three things you can do:
1. Staple a radiant barrier to the underside of the rafters and/or get low emissivity shingles that do not absorb very much heat. This will cut down heat in the attic considerably.
2. Blow enough cellulose into the attic that the ducts are completely buried and secured so they won't "float" to the top over time. Ever heard of old tires floating up to the top after burying them deep in garbage dumps?
3. If you currently have good roof venting then you can get a whole house fan and that will evacuate any hot air that accumulates in the attic in the hot months. They work in your climate.
I would always do item 2 and do at least one of the other two items. If you do all three items you shouldn't have any problems. If you did all three you'd be really good and could cut your air conditioning bill also. Very few climates in the United States will let you get away with this alternative to conditioning the attic when there is HVAC stuff up there.. You are very fortunate to be in one of those few areas. Take advantage of it.
Hi, What Eric said plus
You need to air seal the attic floor / ceiling prior to insulation installation.
Air sealing at the ceiling will be difficult in my situation since I have a lot of recessed lights. In fact I just changed them out to the low profile lights in the following link. So I don't want to do blown-in insulation at the ceiling level either because whenever I'll have to replace one of these lights the insulation will rain down on me. I also have some blown-in cellulose insulation right now and I hate the dust that it produces.
https://www.homedepot.com/p/Commercial-Electric-Slim-Baffle-6-in-Color-Selectable-New-Construction-and-Remodel-Canless-Recessed-Integrated-LED-Kit-91479/310114087
Tim,
We've been outvoted.
I'm beginning to realize that these new surface mount LEDs sold by Home Depot and others are not what they're cracked up to be. Surface mount LEDs that are mounted to standard electric boxes can be air sealed by sealing the electrical box to the ceiling. The sealing mechanism is separate from the LED. You can do a blower door test with those electrical boxes and be confident the ceiling will stay at the same level of air tightness no matter what changes.
When you install these new LEDs you get a simpler installation but give up that air sealing independence when replacing the LEDs. Plus you get insulation falling all over you when you take them out. The bottom line is that a very good remedy for insulating one's house is drastically compromised. It ends up being a very bad set of compromises on the house as a whole.
I wish I did not have so many recessed lights in my house, more specifically the 6" diameter ones. If the recessed lights were the 4" types then I would have replaced them with 4" junction boxes and installed "surface mount" low profile LED's.
I actually asked here if these lights can be air sealed and some people responded that they passed blower door tests with no air sealing around these types of lights. I still plan on applying some rope caulk or foam tape around the lights so that the gaps between the lights and the drywall are filled in as much as possible. https://www.greenbuildingadvisor.com/question/air-sealing-low-profile-led-lights
But getting back on topic. If I leave my attic vented, how can I bring my air ducts into the thermal envelope? I was thinking that I install a BIB system or fiberglass batts on the underside of my roof deck, but will I experience condensation issues since the asphalt shingles are vapor impermeable? Actually from what I've read, leaving the attic vented may be required to prevent condensation issues.
Sergio, I'm glad you are open to venting the roof. It really is a much simpler way to manage the elements in the location you're at. You don't have to worry about condensation issues where you are located. It can be confusing when one is new to green building because there is all kinds of seemingly conflicting advice that is geared to the kind of construction for individual climates. Many people have to worry about condensation in their climates but you do not, especially when the roof is well vented.
You can make your already purchased LEDs work. It really isn't a problem to make it airtight using the previous advice you have gotten. You have to work carefully and just be precise. If you had to replace the LEDs as often as one replaced incandescents then you'd definitely have a problem. But you don't have to. Most of them will probably last 15 years or more. I'm guessing here, but not a problem.
The advice you got from Tim and me really is good advice for your location. Cellulose blown on top of the attic floor is definitely the way to go. It will stop migration of heat much better than fiberglass, BIB or batts. Make sure all penetrations at the ceiling, except for the LEDs, are sealed with caulk or canned foam before you blow the cellulose in. If you really want to avoid being around the cellulose just pay someone to blow it in. It's not a hard job and there will be many people with the skills to do it.
You house will be very well insulated with around 14" of unsettled cellulose on the ceiling. If you decide to install either a Whole House Fan or a radiant barrier on the rafters, both of which are good ideas, make sure you do it before you have the cellulose blown into the attic.
Very important: If you elect to install a radiant barrier on the rafters make sure it is PERFORATED..