BIBS in walls and cathedral ceilings
Climate zone 5, new construction – ranch home mostly with cathedral ceilings. I have a few questions…
First – I am northwest of Detroit – Within climate zone 5 but close to 6. Should we plan to make our roof and walls safer than the minimum percentage of exterior to interior insulation/? if so, by how much?
Roof – low sloped, unvented cathedral ceiling assembly with rigid foam insulation R20-25 exterior to the sheathing
1- Is Owens Corning L77 BIBS system acceptable for the air permeable fluffy insulation under the sheathing? Our insulation contractor thinks this system does the best job of maintaining contact with the undersurface of the roof sheathing. Are there ‘better’ alternatives?
Wall – 2×6 studs with min rigid foam insulation R7.5 exterior to sheathing
2- Is this BIBS system good for the insulation inside the wall sheathing – between the studs? Are there ‘better’ alternatives?
3- We are planning brick veneer – should we avoid OSB sheathing and use plywood since brick is a reservoir cladding? We are planning to have an air space between the brick and the insulation. Is there a better substitute with R-Value? I like the idea of using the foam as the sheathing but I’m getting push back from the insulation contractor. I haven’t talked to the framer yet.
BTW – we would avoid putting too much insulation inside the sheathing to keep at least 27% and 41% rigid insulation outside the sheathing for the walls and roof, respectively.
Thanks in advance for any responses,
Alan
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Replies
HI Alan -
1. is CT BIBS "ok?" is there a "better" alternative? As an air-permeable cavity insulation, this is a perfectly fine insulation. As to whether there is a better one, that depends on what you hold as more important: cost, ease of installation, environmental footprint, etc.
2. BIBS for walls vs roofs/attics - i still like BIBs, walls or attics... But just for comparison purposes, check out: https://www.greenbuildingadvisor.com/article/getting-to-know-spider-insulation and https://www.greenbuildingadvisor.com/article/cellulose-insulation.
3. OSB with brick veneer? That dedicated free-draining space between the brick veneer is critical to moisture performance. Keep it clear of mortar droppings. The vapor permeability of OSB, as a lower-end Class III vapor retarder is just fine for this assembly. And you might consider Huber ZIP-R to get you the continuous rigid on the exterior and a rigid weather and air barrier with the taped ZIP...
What about "erring" a bit on the proportions of continuous exterior: air permeable cavity insulation? Well, on the one hand, consider that this ratio is based on a a certain average interior relative humidity for the winter; if you keep your home a bit drier in the winter than "average" then you get more grace on the ratio. On the other hand, with global warming your climate is likely to be a bit warmer than what history tells us, but maybe wetter as well?
You can't go wrong by shifting the ratios a bit to favor the rigid, but that proposition is generally associated with greater cost and difficulty of installation. Clear as mud, huh?
Peter
Yup, clear as mud...
Thank you for the suggestion of Zip sheathing.
Alan