Wall, roof, and ceiling assembly for Climate Zone 6: Code vs. practicality and functional goals
Looking for advice and constructive advice. I am considering a new renovation for a cottage in northern Michigan. Climate Zone 6 in the Code books. Generally R20 in the walls, R49 in the ceiling. Heating only no AC. I am considering the following plan:
Exterior Walls from outside to interior: 3/8 Cedar Shingles over 30# felt, over 3/4 CDX sheathing on 2X4 studs. Cavity filled with dense packed Cellulose. 1X 3 Horizontal strapping with 1 inch foil face Poly-iso foam board. Seal all seams with Foil faced tape and seal electrical and other penetrations to fully airtight.5/8 drywall is final finish. The goal is R20 with some thermal bridging mitigation (1″ foam) and vapor perm barrier at the initial interior interface, foil side of foam. Materials with increasing permeability to the exterior. Assembly intended to dry to the outside.
Ceiling, from Interior to well vented attic: 5/8 drywall-layer1 Foil faced 1 inch Iso-foam board-1 X 3 strapping-layer 2 foil faced 1 inch Iso-foam board-2 X 8 joists-dense pack cellulose-covered by attic floor rough in 1 X 4 boards. Again all seams sealed with foil tape. NO ceiling penetrations like lights or other. Partitions installed after insulation from underneath.
Again, increasing permeability to the outside( upside vented attic). Intended to minimize heated air migration and allow for drying to the external air in the attic. I am relying of more than “textbook” thermal performance of the double layer foam with airspace between. I expect this should be a result of additional radiant thermal performance and the better than normal air movement barrier. Any suggestions? Could add additional inches of Cellulose on top side of joists. 2 X4 strapping. However if air migration is well mitigated, taking convection and lost warm air out of the calculation, then what is the justification for high R? Only conduction and radiation remain to transfer thermal energy and there is no “g” in those equations.
Roof: cold design deck: Metal on 12/12 pitch with 2/4 open Purlins over the rafters. No continuous sheathing or Water&ice covering. Metal on this pitch shouldn’t need it. Enhanced ventilation into the attic space with open air to four directions. In other words, wind from anywhere to maintain the roof material at ambient. Ventilated to disperse any stray water vapor that might find its way to this space.
Since this is “Green Building Advice” I am trying to avoid the “Spray foam everything” proposed elsewhere, using recycled paper in the cellulose, and minimizing non-renewable materials in favor of less processed lumber. The exterior surface area is fairly small so some 1 inch Poly Iso foam with foil face is a compromise.Plywood rather than OSB, or 2 inch Styrene foam sheathing. Plain old 30# felt in place of Tyvek.Cedar shingles, not Vinyl siding. perm increases when wet, and can wick liquid water away.
Assembly drying to the interior or the exterior, or both? I have read many discussion regarding the drying to interior concept but I fail to understand how a wall could be expected to dry “into” an interior with significantly greater water vapor pressure. If the goal of very airtight living space is achieved, where will the water vapor emitted by the occupants and their activities go? Breathing, cooking, bathing and all else. Now must the design depend on active powered De-humidification?
Thank you for replies!
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Replies
Mike,
I'll just address your last question. Walls can be designed to dry to either the interior or the exterior, but that simply means that moisture that takes its way into the walls follows this path to dry, not that either are designed to remove the moist air from inside the house. Designing a wall that would dissipate that moisture through the structure isn't a good idea. Most well designed homes now rely on mechanical ventilation, or if they don't care about energy efficiency, opening a window.
Mike,
If you want to install a continuous layer of rigid foam on your walls -- and I think that's a good idea -- it's almost always better to install the foam on the exterior rather than the interior, for three reasons:
1. Exterior foam does a better job of addressing rim joists than interior foam.
2. Exterior foam does a better job of addressing partition intersections than interior foam.
3. Exterior foam has far fewer penetrations (especially electrical boxes) than interior foam, and therefore does a much better job of reducing air leakage.
For more information on exterior rigid foam, see these two articles:
Calculating the Minimum Thickness of Rigid Foam Sheathing
How to Install Rigid Foam Sheathing
To insulate a vented, unconditioned attic, the best approach is simply to install deep cellulose on your attic floor, unless you are unable to get the required depth of insulation near the eaves. Your description of rigid foam and strapping on your ceiling is impossible to decipher. If you want to install interior rigid foam on your ceiling, here's what you do: Install rigid foam on the underside of the joists -- either one, two or three layers -- and then install your 1x3 or 1x4 strapping. Finally, fasten the drywall to the strapping.
Martin,
Forgive my possibly misleading statements. I was commenting on the expectation for a wall that has absorbed water, from any source, to dry towards the interior, particularly if interior humidity is high. I suspect that this is less reliable than the traditional goal "dry to the exterior" , at least in colder climates like the upper Midwest. To be sure Southern Louisiana, is a different story. I have no intent of mitigating, or controlling interior humidity by vapor migration through walls or other. I only intended to highlight a plan for a deliberate path for vapor to evolve from the assembly. Old school plan is for lowest perm vapor barrier on the interior, and increasing permeability toward the exterior. Interior living space humidity and temperature are always controversial. Mechanical ventilation and control is expected, but I wish to avoid dependence on this for integrity of the structure. Rather have a passive control plan with a living space comfort control (HVAC) system as a nice to have add on.
Thanks,
Mike B
Mike,
You addressed your comments to me, but I wasn't the one who challenged your understanding of moisture flows. That was Malcolm.
But I'll try to answer your question.
Q. "If the goal of very airtight living space is achieved, where will the water vapor emitted by the occupants and their activities go?"
A. In most cases, the water vapor emitted by the occupants doesn't have to go anywhere. It can stay right where it is. In the winter, the interior air will always be warm and moist. The exterior air will always be cold and dry. That's the way things are supposed to be. All is right with this world.
If a house has very high indoor humidity during the winter -- that's rare, but possible -- it may be necessary to run the ventilation equipment (for example, a bathroom exhaust fan) for more minutes per day. Increasing the ventilation rate during the winter lowers the relative humidity of the indoor air.