ICF Cape Cod design worries: the roof deck nightmare
Last summer/fall my wife and I started designing a retirement home and after posting my worries on moisture to this blog, I received some great advice that gave much “cause for pause” to study more and also to provide the more detail members needed to better understand the design and what I was trying to accomplish. Since then, and as members of both GreenBuildingAdvisor.com and FineHomeBuilding.com, I’ve studied everything I could get my hands for 9-months from these two websites to help me with my Zone 4 Potomac River/Chesapeake Bay region home. Last week we broke ground and started footers so I’m trying to get my mind around some finer details.
I rely especially on the numerous Martin Holladay and Dr. Joe Lstiburek articles (hockey puck humor and all, I love his humor and examples), along with gleaning elements from the Fine Home Building (FHB) “The Passive House Build” 4-part series, the GBA Energy Star drawing details, the GBA “Pretty-good-house” (R-10, 20, 40, 60 formula), RESNET, HERS and such. Having said that, my wife and I chose to break one of Martin Holladay’s top rules for beginners and others, especially the simple roof line rule … because we loved the Cape Cod look … so we do have those nasty dormers (2 on front and 1 on rear), though we are trying hard to mitigate this poor choice by close attention to sprayfoam and air sealing. Our Insulated Concrete Form (IntegraSpec ICF) Cape has about 1700-1800 sq-ft on both 1st floor with MBR and a walkout basement floors. The 900 sq-ft 2nd floor includes 2 BR & 1 Bath which also has an adjacent room for the 2nd stair case via an exterior-grade firedoor directly to the Garage; it also has access to the room truss room above the 2-car garage as a storage room at the top of those same stairs.
Our home & lot is such that we have a long south facing front of house, 7degrees-45minutes from perfect south and a roof pitch of 36 degrees in hopes that we find a pot of gold to purchase some amount of PV array (a major financial stretch for us). Our plan is to use the VRF ducted and non-ducted splits from the likes of Dakin, Mitsubishi, LG, Fujitsu, or… along with a Venmar or someone’s ERV to ventilate and filter air, then finally perhaps an Ultra-Aire XT105 dehumidifier to help control our humid season. We’ve maintained our basic ICF approach for the full basement and main floor and now have specified a very detail Air and Vapor barriers in our drawing package. So now I figure I have arrived at a point where I am becoming dangerous and expensive and a bit confused about the finer details; … correction, I’m starting to get wicked confused in some of these details.
Here is the first of my worries, for fear of scaring you with all my nightmares at once and overwhelming you with living in my head, haha. I’ll post a couple more following this.
The Roof Deck Nightmare:
While we have changed our insulation approach to one where we use Spray Foam against the bottom of the roof deck and encapsulating the top cord of the truss so that all Fire Code accesses and any ventilation equipment are contained inside this barrier; I am still confused if I should run one of my ERV ventilation intake and exhaust tube up there to capture and remove moisture in this tight house give the OCSF layer ability to retain moisture? For fear of the Goldilocks principle and “believing” we need this barrier to be both an air barrier and vapor barrier, I believe I learned from you correctly that I needed “about” 3.0-inches of Closed-Cell Spray Foam (CCSF) as the air/vapor barrier. We plan to apply an additional balance of only 6” of Open-Cell Spray Foam (OCSF), because R-60 cost is making me question the Pretty-good-house R-60 “spec” for my Roof; the combo of 3” CCSF and 6” OCSF only gets me to about R-40, aarg? I am not sure how to cost effectively get to R-60 out of my roof, should I spec 5.7 more inches of OCSF or maybe Rigid foam above my roof deck? Do I really need 12” of OCSF in addition to the 3” of CCSF… Gee, some say after certain point spray foam does not really provide the specified R-Value, is this true? Also, for this semi-conditioned attic which will NOT be used to store anything except insulation, wiring and plumbing … should I run one of my ERV dirty air intakes (& maybe a fresh air supply)? Do I need to apply fire retard coating; I recall reading that I must NOT run ventilation to such unoccupied attic space?
Part of my roof concern is how I design to drive out heat gain BEFORE it ever hits my insulation; but also … is this even cost effective? Am I chasing pennies with cubic dollars? I suspect everyone worries about this, but a clear strategy and cost effective approach seems missing? I seriously worry about this whole cool roof concept and articles because when I examine the so-called cool roof shingles all I see is bragging over a tiny 10-degree attic temp drop and no reference to roof deck temp reductions … I was expecting much more. Where do I see the specific strategies for 100degree roof deck temp reductions some articles mention and what should that translate into as attic changes? So am I wrong, that these so-called energy star shingles are not the answer. I purchased and read everything in the FHB “Siding, Roofing, and Trim” book for pros by pros book but no strategy emerged there either. Oh yeah I am so confused by now. This had me looking at Standing Seam metal (ouch major expense) some venting ideas like the HunterPanels.com “Cool-Vent” vented nailbase insulated panels … wow is that expensive too ($57+ per 4×8 panel) which is in addition to the basic roof deck. Another idea was from Martin Holladay’s May 30, 2014 Musings, the http://DCIProducts.com/html/smartvent.htm Smart Vent which when used with their 16” OC baffle created a 2” airflow space under the roofdeck that Spray foam can be over and encapsulate along with their Smart Vent and Smart Ridge system.
Or … Maybe I should just build a roof over a roof with a 1-2” air-gap and a waterproof, but highly vapor permeable, underlayment from DuPont or someone on the first under roof that had the sprayfoam applied to its bottomside; and then some kind of radiant foil facing the sun on the 2nd upper roof that has the shingles applied. It might allow any moisture drying of the topside of the roof with the air flow between the layers and also get rid of some of the radiant heat trying to get to my sprayfoam insulation. Maybe Zip System in combination has a solution?
Aaarg, you see I am lost at this point with my new and dangerous “knowledge/information” because I’m trying to find and understand what is economically wise. Help 🙂
I have provided attachments however the final drawings because the .pdf file was 2.6MB and larger than 2MB limit. Below you will find the kessler31314am.pdf that shows the general layout of home and some detail toward the end of the file showing sprayfoam detail and ICF Foam alignment while the 2nd file show some other sprayfoam and alignment effort to create an air sealed and vapor sealed Gable end of Cape that may not be part of the dwg package.
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