Metal roof // unvented // cathedral // exterior 1 1/2" rigid insulation

Where Roofs Meet Walls is a Critical Connection

Corners and connections are where insulation and air barriers can have trouble. Compressed or insufficient insulation can cause cold spots, which lead to condensation, mold, and rot. Air leaks at this connection can cut the effectiveness of the insulation substantially. In cold climates, this is where ice dams begin.

To keep the air barrier continuous, span the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. over the framing connection and use adhesive or sealants at framing connections as shown.

Roofs and walls need to dry
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Moisture in roof and wall assemblies is inevitable, so it's a good idea to design them so that they can dry. Roofs and walls that can dry to either the outside or inside are good, but those that can dry both directions are even better.

  1. Designing to dry out means doing two things well:
  2. 1. Choosing materials carefully—each layer affects the vapor profileA vapor profile is an assessment of the relative vapor permeabilities of each individual component in a building assembly and a determination of the assembly's overall drying potential and drying direction based on vapor permeabilities of all of the components. The vapor profile addresses not only how the building's enclosure assembly protects itself from getting wet, but also how it dries when it gets wet. For a detailed treatment of this subject, see Building Science Corporation's article Understanding Vapor Barriers. of the assembly.
  3. 2. Planning the construction to be forgiving—flashing keeps water out, and ventilation removes water vapor.

Unvented roofs can perform well as long as they are properly detailed to limit moisture transfer from the interior. Construction details vary depending on climate, but closed-cell spray polyurethane foam (specifically allowed by Section R806.4 of the International Residential Code) can be used anywhere.

Exterior insulation keeps the framing warm and dry
By moving the insulation outside the framing, the chances of condensation are almost eliminated. Another benefit is that you can get a superinsulated roof without increasing the size of the rafters, or furring the framing down and encroaching on the living space.

For detailed information on this topic, read Unvented Roof Systems at BuildingScience.com

Venting above the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. allows drying
Conditioned attics with unvented roofs can still provide a drying path through ventilation. You can ventilate the roof claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. above the roof sheathing. You can do this the same way you ventilate wall claddings either with a drainage mat or with furring strips positioned above each rafter. A second layer of roof sheathing installed above the furring provides backing the for shingles or shakes.

Learn more in the Green Building Encyclopedia

Enclosure overview
Exterior walls
Roofs: Attics, Structure, Claddings

DRAWING DETAIL

Download: PDF | DWG

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5.
Aug 6, 2017 8:53 AM ET

Is the self-adhered ice and water membrane needed?
by Andy Alden

I realize its standard practice to install a self-adhered ice and water membrane at the eves to handle ice dams. That said, I can't see why one is needed with this type of roof design. Its hard to imagine any scenario where ice damming would occur.


4.
Nov 22, 2016 7:53 AM ET

Response to Kris Burkett
by Martin Holladay

Kris,
Welcome to GBA. Happy to help.


3.
Nov 22, 2016 7:36 AM ET

Thanks Martin
by Kris Burkett

Martin, thanks for clarifying my project. I'm in zone 5a and plan to option 2 of your article How to Install Rigid Foam On Top of Roof Sheathing. 4" xps rigid with R-29 unfaced batt insulation. Construction will start next year and I will have more questions but joining GBA has been nothing short of outstanding. A good friend who has been in the trade for 40 years (I'm a part timer) will be helping but he is still stuck on old techniques. Your help with GBA will be greatly appreciated.


2.
Nov 21, 2016 12:33 PM ET

Response to Kris Burkett
by Martin Holladay

Kris,
If you haven't read it yet, I urge you to read this article: How to Install Rigid Foam On Top of Roof Sheathing.

Your plan will work in Climate Zones 6 or warmer, depending on the R-value of the rigid foam you specify. But you would need more than 4 inches of rigid foam for Climate Zones 7 or 8, as explained in that article.

Where the self-adhered membrane belongs depends on the purpose of the membrane. If you are installing it as an air barrier (to limit air leaks through the roof sheathing), then it belongs directly above the roof sheathing. (If you tape the roof sheathing seams with a high-quality tape like Zip System tape, you won't need an additional air barrier in this location.) If the self-adhered membrane is being installed to fulfill code requirements for protection from ice dams, it belongs above the uppermost layer of rigid foam. (Usually, code requirements for ice dam protection can be satisfied with a 3-foot-wide or 6-foot-wide band of membrane at the eaves and valleys.)

Elsewhere on the roof, where ice dam protection is not required by code, you'll need roofing underlayment. The roofing underlayment needs to be installed between the uppermost layer of rigid foam and the furring strips.


1.
Nov 21, 2016 10:44 AM ET

Roof Detail
by Kris Burkett

I'm planning on building a roof similar to this detail. The differences being: 4" rigid insulation instead of 1.5" and 1x4 furring strips, to secure the metal, instead of the 1/2" sheathing. Where would you put the self adhered membrane and the roof underlayment?