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Community and Q&A

How to insulate my roof… no attic

Bryan Wilber | Posted in Energy Efficiency and Durability on

Hi everyone,

I live in a house built in the 60s. It is a California Modern design (or so I’m told). It has very low roof pitch with 2 simple gable end roof sections. Inside it has exposed beam / vaulted ceilings. When you look up inside the house, you see 4 X 10 beams every 6 feet running as rafters would and then you see tongue and groove 2 X 6 roof decking running between the beams.

Bottom line is: there is no attic, there is no insulation whatsoever. There is only rolled roofing, felt, and one layer of 2 X 6 between the sun/rain etc… and the living room. It’s in Atlanta and when it’s over 85 degrees, the AC runs continuously.

The interior is beautiful… the beams and 2 X 6s are stained and polyeurethaned. I can’t insulate from the inside. It would ruin the look and the ceilings are already low.

I need to build up the roof from the outside. I’m very open to all suggestions…

Here’s one suggestion from a friend.

Remove existing rolled roofing. This will expose 2 X 6 tongue and groove. Glue and screw down 1/2 inch OSB. Lay 2 X 4 parallel to ridge sleepers every 2 feet with 1 1/2 inch foam board between them. Lay another layer of 1/2 inch OSB over the 2X4s and foam board and then either put roofing down… or add another set of 2 X 4 sleepers perpendicular to ridge on 16 inch centers with no foam board to create a ventilation space from soffit to ridge.

Comments / Suggestions are appreciated.

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  1. Riversong | | #1

    You're in climate zone 3. The current Energy Code standard for your zone for both new construction and renovation is R-30 ceiling insulation. This would require 5" of polyisocyanurate foam board or 6" of XPS (styrofoam) board.

    I would suggest considering 6" SIPs (OSB/foam sandwich panels). The materials are expensive but the labor is less than installing multiple layers.

  2. GBA Editor
    Martin Holladay | | #2

    If you decide to install multiple layers of foam, it isn't necessary to install OSB between each layer of foam. The first layer of foam can be installed directly against the existing 2x6 sheathing (or on top of #30 asphalt felt). Only one layer of OSB or plywood sheathing is required -- on top of your final layer of foam.

    If you have more than one layer of foam, stagger your foam seams in both directions. Taping the foam seams improves the air tightness of your roof assembly.

    Finally, if you decide to go the SIP route, you don't really need a conventional SIP (which usually includes OSB on both sides of the foam). The product you need is called "nailbase." It's a commercial roofing product consisting of rigid foam glued to a single layer of OSB. The OSB goes up; the foam goes down over the existing sheathing.

  3. Expert Member
    Michael Maines | | #3

    Nailbase is the simple solution, but it's hard to properly seal between panels. You can either take extra care to seal those joints, or do the two-layers-overlapped-and-taped which to me is actually easier to do properly. My experience is limited to doing each of these methods only once, so there may be other techniques or products I'm not aware of, but I have seen several examples of SIPS that exhibited failure at the joints.

    In your hot, humid climate, vapor drive will be primarily to the inside, and any mistakes in sealing your insulation could result in condensation on that nice ceiling. Venting the roof would marginally increase the lifespan of your roofing but it causes as many problems as it cures so it's not really necessary.

  4. guy | | #4

    you got the right idea, just to many steps. You do not need so many layers.
    Strip off the roof. Run a 2x4 (flat) around the parmiter of the roof. Fill the area with 1.5 inches thick Polyiso. Chauk line to idenify where your rafters are. Running with the slope, lay 1x4 lumber flat onto the insulation 2ft O.C. starting at one edge accross the roof to the other edge. Use construction adhesive between 1x4 and insulation board. Use an appropreate screw length not to penitrate thru the roof sheathing. In this case a 3" screw would be fine. The 1x4's that fall on the chaulk line can have a random 4" screw placed which will enter into the rafter. Leave the space at the bottom open to allow venting to the ridge.

  5. guy | | #5

    I decided not to finish and tried to cancel out but it posted. Any way you can use the plastic sign board, cut into 3'strips and clue together 3 or 4 layer to make a 3/4" thick vent screen. You can run these accoss the bottom edge between the 1x4s to make a vent screen. Run your new facia baord around the parimiter to hide the insulation staying below the 1x4s. Install your plywood over the 1x4's flush with the outside edges. At the low edge install your drip edge flashing to hide this open area, but space it out from the facia 3/4". You can use a 1x4 laying flush on the facia to keep your metal straight. Be sure to remove it after the metal edge is installed. to allow venting to you ridge vent you install with you new shingle roof. The 1x4's need not be a soild run to the ridge a little space here and there on the vertical allows air to flow freely.
    You can increase the thickness of this desing to meet any r vaule. Just increase you boarder frame to match. Polyiso and i.5 thick is about R-9. Ventilation is a must. Do not let anyone tell you diffrent.
    It allow for the insulation to release gathered heat in the cooler evening. Allows moisture to escape and creates a buffer from heat penitration by allowing a air to flow through freely.

  6. guy | | #6

    buy the way the 3/4 inch space at the bottom to allow air in gives you 9 inches of net free air to flow throught to the ridge vent.

  7. rustyjames | | #7

    You could also go right over the deck with the required amount of foam board and install a standing seam metal roof system, with concealed clips/fasteners on bearing plates. For this, a J-channel (18 ga. galvanized) would be required on the eves and rakes as a pocket to hold the insulation, and screw the panel clips and trim to.

  8. Aj Builder, Upstate NY Zone 6a | | #8

    Green Building Advisor?

    Foam is a green product?

    At least in my area one can get ahold of recycled flat roof foam. Recycled foam is as close to a green product as one can get. New foam is at the opposite end of the spectrum... and IMO should not be advocated on a site with green in it's moniker.

  9. rustyjames | | #9


    You're right, foam isn't green, neither is the metal roof. What's your suggestion to the OP's question?

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Hi Rusty... Recycled foam.... as I stated. Can't get any greeener than reuse. But foam definitely creates moisture issues, so go with venting the roof over the foam... Not that easy to build green... I like cellulose... if I take the whole list of items built into a home... green is difficult. I agree.

  11. rustyjames | | #11


    I'm with you on reusables but the only problem with the foam boad is the edges tend to get beat up during removal, transport and storage. That makes it difficult for tight joints. Of course, you could trim it, but now it becomes a labor cost issue. As for the metal panels over foam, there's systems available where the clips suspend the panel, (typically about 3/8"). Not much of an air space but it's better than nothing; It seems to work well on commercial applications.

  12. Todd | | #12

    I have the same home and roof-ceiling. I love the look of the interior of our home but in Florida in the summer my AC runs constantly and its still hot. Down here I havent come across an easy solution. I like some of the ideas posted but I would love to hear what you decided. I have to find a soulution..

  13. Bryan Wilber | | #13

    Hi, I'm the original poster.

    Haven't implemented a solution yet. It's 93 degrees in Atlanta today.... AC running continuously... I go out front and squirt the hose on the roof occasionally and watch the steam come off... :)

    I just haven't had time or money to implement a solution, but here's a link that may be interesting to you...

    My house is an "EIchler" house... named for the architect who designed this style. Thousands were built in California.

  14. Riversong | | #14

    The "solution" at the web link you shared is a reasonable one, but keep in mind that the article was written by the owner of a foam room insulation company, hardly an unbiased source, and it includes some incorrect statements about the R-value of various foam options.

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