Cedar shakes, unvented attic, SPF – underlayment alternatives
I am building a new home in the Northeast on Cape Cod (Climate Zone #5) and my research for the proper roof assembly has caused me to seek assistance from this group. I have listed my building specifications below and they are followed by three questions. Thank you for any assistance you can provide.
Building Specs
Roof:
· 5/8” CDX Plywood
· Plywood seams taped with Zip tape or equivalent
· ASTM D 226 #30 (Type 2) Asphalt Felt Paper
o 50% overlap
o Apply using plastic round cap roofing nails
· Ice & Water Shield (I&W) for eaves and valleys. For eaves, two courses of 36” wide I&W with minimum 3.5” side laps and 6 inch end laps.
· Benjamin Obdyke Cedar Breather
· Certi-Sawn Western Red Cedar Tapersawn Shakes – 7/8” butt x 18”
o 5-1/2” exposure
o Install using stainless steel type 316 nails
· Valleys to be closed and woven with concealed red copper flashing
· All exposed flashing to be 16 oz. lead coated copper
Roof Envelope Insulation:
High-density closed-cell SPF to R-49 applied to underside of roof sheathing resulting in an “unvented” attic.
Side Walls – Exterior:
· 1/2” CDX Plywood
· ASTM D 226 #30 (Type 2) Asphalt Felt Paper
o 2” horizontal overlap
o Apply using plastic round cap roofing nails or “Stinger” plastic cap staples
· Benjamin Obdyke Slicker Classic Rainscreen (6mm)
o Install using stainless steel staples
· Certigrade Alaskan Yellow Cedar Shingles, Number 1 Grade – 18” R&R Kiln-dried
o 5-1/2” exposure
o Install using stainless steel type 316 nails
Side Wall Insulation:
High-density closed-cell SPF to R-21
Side Walls – Interior:
½” Blueboard with veneer plaster, 2 coat system
Other Items Under Consideration:
– AeroBarrier installation at the finish frame / pre-insulation stage by New England Air Barrier LLC
The specifications above are consistent with those recommended by the Cedar Shake & Shingle Bureau (CSSB), however there are concerns with this approach from a practical perspective given the increased use of unvented attics, closed-cell SPF insulation and the length of time required to apply the shingles. The length of time to install the shingles results in the #30 Felt being UV exposed beyond the manufacturer’s recommended time. Additionally, in high wind areas, such as the Cape, keeping the #30 Felt intact is challenging. Based on observation, it appears many high-end custom home builders have abandoned the use of #30 Felt and have turned to newer, more advanced products.
Questions:
1. Cost aside, is the use of a self-adhering Ice & Water membrane an acceptable and best approach? Under this approach the CDX plywood sheathing is sandwiched between two non-permeable products (the I&W underlayment and SPF closed-cell foam). The primary risk here is the CDX plywood gets wet and is unable to dry inward or outward due to the sandwiching between two non-permeable products. Moisture could arise from a leak developing post construction or the roof sheathing not being sufficiently “dried in” before application of the SPF closed-cell foam.
2. Related to question #1, are the following acceptable underlayment alternatives to #30 Felt?
a. Grace Ice & Water Shield (self-adhering membrane) ($0.91 s.f.)
b. Grace Tri-Flex Synthetic Underlayment (fastener applied) ($0.13 s.f.)
c. Henry I&W Blueskin RF200 (self- adhering membrane) ($0.72)
Each of these satisfy the building code and can tolerate UV exposure for periods ranging from 90 to 120 days.
Section R905.1.1. of the IRC requires ASTM D 226 Type II or ASTM D 4869 Type IV felt underlayment for wood shakes and shingles but allows as an alternative the use of a self-adhering polymer-modified bitumen underlayment complying with ASTMD 1970 installed in accordance with both the underlayment’s manufacturer’s and roof covering manufacturer’s instructions for the deck material, roof ventilation configuration and climate exposure for the roof covering to be installed.
In reading the ICC-ES Evaluation Reports for the above products under “conditions for use” it states “installation is limited to roofs with ventilated attic spaces, in accordance with the requirements of the applicable code”……and the CSSB dictates the use of #30 felt for both roof and sidewall underlayment and has not addressed new products on the market.
3. For the side-walls, are the following acceptable underlayment alternatives to #30 Felt?
a. Henry Blueskin VP100 (self-adhering membrane) ($0.61 per s.f.)
b. Grace Vycor enV-S (self-adhering membrane) ($0.83 per s.f.)
Although more a more expensive product cost than #30 felt paper, these are self-adhering membranes (lower installation costs) that are vapor permeable and provide a stronger air barrier.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
User...832,
Does the CSSB have specs for closed-woven valleys? They seem potentially pretty problematic. Unlike that on architectural shingles, the weave with cedar shakes is a cosmetic one, and still relies on the hidden flashing to shed the water in a particularly vulnerable location.
Malcom,
I haven't seen any CSSB specs for closed-woven valleys and I do know they are more labor intensive. I agree it is cosmetic and something our architect likes. The only guidance I've seen with respect to installation is an article in JLC that I have attached.
Thanks,
Paul
User-6970832,
First of all, can you tell us your name? (I'm Martin.)
Lots of issues here. I'm not going to tackle them all. I'm going to focus on just one issue: the water-resistive barrier (WRB) that you chose for your walls.
Why do you want to use #30 asphalt felt for your WRB? Some brands of #30 asphalt felt have a vapor permeance that is very low -- as low as 0.5 perm when dry. Is that really what you want? These days, most builders prefer a vapor-permeable WRB that permits rapid outward drying.
Hi Martin, my name is Paul Condrin.
I'm open to using another type of WRB but was steered toward using the #30 Felt as the Cedar Shake and Shingle Bureau recommends it their Exterior and Interior Wall Manual that states "underlayment shall be No. 30 felt conforming to ASTM D226 Type II or ASTM D4869 Type IV." Granted they make this recommendation without taking in consideration how the wall is insulated. My understanding is the ASTM rated felt has a permeance of only 5 perms when dry but a higher rating of nearly 60 when wet. Thank you for your help.
Tangential comment: I built in a neighborhood in which the neighborhood Architectural Control Committee required cedar shake roofs (for appearance). Most of us used cedar lap siding. One home went up in flames very rapidly in a spectacular fire, totaling the home and car (luckily no one hurt, no other homes affected directly). Afterwards the city banned cedar from all roofs and re-roofing. If you live in an area subject to wildfires I'd reconsider all cedar. You might also plan your landscaping and maintenance to minimize fire risk. With your attention to detail you already may have thought this through.
In your climate with cold wood sheathing, I would favor interior perms (measured dry) lower than the exterior perms (measured wet). For walls or roofs (the moisture principles are largely the same).
I'd favor continuous exterior insulation over thermally bridged, expensive and perhaps environmentally harmful spray foam.
In comparing your roofing underlayment alternatives, seems as though you are balancing 4 things:
1. protection against bulk water (self-sealing nature of penetrations for membranes)
2. drying potential (more vapor permeable underlayments)
3. UV resistance
4. resistance to wind damage prior to cladding
I think the Grace EnV-S is probably the strongest candidate in balancing these 4 issues.
Peter