What to do with individual condos: increase cavity insulation?
A current interior renovation of a two-floor condo unit with three exposed sides in climate zone 5A offers the opportunity to air seal and re-insulate to a much higher R IF prudent. Though I MIGHT BE TAKING SOME OF THE FOLLOWING OUT OF CONTEXT, there is support for interior R upgrades in condos… Dr. Bailes air sealed and insulated a condo bathroom (reference a), taking the wall assembly from R 9 to R 14. D. Dorsett suggested adding 1″ of polyiso behind wainscotting (ref b), which might add up to R 6. Etc.
BUT M. Holladay wrote in “All About Wall Rot,” “Scared? You should be” (c). Building Science Corp. and GBA prescribe the external R needed to make a wall rot-resistant relative to its interior R (d, e). In a re-analysis of these prescriptions, Martin wrote though in some cases it is safe to add less than the prescribed amount, “…time being, I’m sticking by my rules. Builders who follow my rules won’t get into trouble” (f). One of his reasons was a wall with exterior “thin foam” that was safe as built was less robust against moisture if future changes were made clueless of envelope issues (adding a humidfier etc).
In the current project, assume we upgrade the interior (the only) insulation from the current nominal R 11 to R 16 now. Multifamily buildings should undergo exterior energy retrofits. If this occurs after my family has sold and the current board retired, the association will key the safe minimum external R to the nominal R 11 of the batts ALL other units have, or worse, discount the internal R due to poor installation of those batts and add even less foam. In these scenarios, exterior foam that is proper thickness for all the other units will be THIN FOAM for this unit (please see attachment).
Imagine others do variations (deep insulated service cavities etc.) – by the time of an external retrofit there’s a mosaic of maybe eight condos with interior R 12 – 22 while 42 have an effective R 9. Interior R upgraders moved, board changes, and the exterior R is based on examination of one of the 42 non-upgraded condos. Result – eight with too-thin exterior foam scattered around causing mold problems or being OK but less robust and just waiting for that humidifier…
I’d love to insulate to the hilt. But given the above, maybe the best policy for ALL condo owners in zone 5a is to focus on Energy Star appliances etc. but to NOT add insulation. Am I wrong???
Thanks.
a) https://www.greenbuildingadvisor.com/article/how-i-fixed-my-leaky-underinsulated-exterior-wall
b) https://www.greenbuildingadvisor.com/question/insulation-and-air-conditioning
c) https://www.greenbuildingadvisor.com/article/all-about-wall-rot
d) https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
e) https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
f) https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies , https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
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Replies
You can spray the walls with 1.5" to 2" of closed cell spray foam. This will not only seal the air leaks but will prevent any condensation worries down the road. No matter how much insulation it goes on the outside, it will just make the wall preform even better.
A lower cost option is a well detailed and sealed interior air/vapour barrier. This would allow less exterior insulation:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
This is a standard approach in Ontario.
Akos,
Thank you for the info. I'm reading the link and a few other papers and will be back with questions.
OK, I think I get the spray-foam idea. Fig. 8 in https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies shows this and says that the spray-foam to balance-of-cavity-insulation must be from the same table that gives safe exterior foam-to-cavity-insulation ratios. They basically moved all the insulation function to the stud cavity.
But, I'm not very open to closed cell for a few reasons. There seems evidence that foam can pull away from framing and/or crack over time. EVEN if that's wrong for a normal application, it's more likely at this condo because it's 135 ft from a commuter/heavy freight line - the place literally rattles when the freight trains go by, probably vibrates imperceptibly when the commuter lines go by. Further, there is controversy over the impacts of its manufacture and its global warming potential/CO2e payback. Finally, I (and others) have allergies and I've read if the mixing isn't just right there can be lingering odors. So, if at all possible. I'd prefer to use less complicated materials en masse (or at least materials not applied wet en masse).
Maybe a design with well detailed and sealed interior barriers you suggested, made of conventional materials, would be the way to go if it can be shown to function well in the current retrofit AND if foam goes on the whole building later but is too thin for this unit's interior R.
Thanks.
Closed cell polyurethane is essentially Gorilla Glue- it may separate from the studs if the surfaces weren't clean enough, but even if it does it'll still stick to the sheathing. If the studs are reasonably clean the closed cell foam will add structural capacity to the assembly. If there's enough vibration from the rail traffic to break the bond with the foam from the sheathing it will have long since pulled the sheathing nails from the studs.
The GWP of closed cell foam varies dramatically with the blowing agent used. The industry standard HFC245fa has a 100 year GWP of about 1000 x CO2, whereas the new HFO1234ze blown foams have a 100 year GWP of about 7 x CO2. An inch of closed cell foam and 4.5" of cellulose in a 2x6 cavity has sufficient dew point margin for zone 5A.
But I'm still not a fan of closed cell foam between studs, even if properly mixed & applied, and blown with HFO blowing agents. For one, it's a waste of expensive foam due to the thermal briding of the framing adding FAR less "whole assembly" performance than the R-values would imply, and even the polymer has a real environmental impact.
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
In zone 5A simply air sealing the sheathing the framing the full perimeter with polyurethane caulk and using an all-cellulose cavity fill and either half-perm "vapor barrier" latex primer on the interior or a variable permeance 2-mil nylon (Certainteed MemBrain) or somewhat more vapor tight Intello Plus "smart" vapor retarder is perfectly safe & code-legal, and does not affect future exterior insulating choices at all. Cellulose has the opposite of a "carbon footprint"- by most carbon accounting methods it's sequestered carbon. It also has enough thermal mass to measurably (if only slightly) reduce energy use if dense-packed in a 2x6 cavity. In a zone 5A climate 3.2lbs per cubic foot is enough to keep it from settling over time, but 3.5lbs is more commonly used as the target density, which doesn't affect the cost in a meaningful way.
If there is back-ventilation on the siding (air between the cladding and sheathing, such as a brick veneer cavity or vinyl/aluminum siding) it would be safe to use cellulose or rock wool/fiberglass batts or 1.8lbs+ density blown fiberglass in the cavities even without smart vapor retarders or half-perm paint as long as it's reasonably air tight.
First off, Zone 5A isn't a very risky climate. With vented cladding (say, a brick veneer w/ cavity, or vinyl siding) you don't even need an interior side vapor retarder tighter than standard latex paint on wallboard to control moisture drives from the interior.
Even Zone 5A houses without vented cladding, in most cases dense-packed cellulose in the framing cavities is fine, redistributing and sharing the moisture burden with the structural framing and sheathing, even without resorting to low perm paints or other interior side vapor retarders. A major performance improvement in going from R11 batts to dense packed fiber is the air tightness, which in many cases has a bigger impact than the mere R-value.
As long as you don't make the interior TOO vapor-tight, it won't really matter what's added to the exterior at some later date. Alternatively, adding a true vapor barrier on the interior makes the amount and vapor retardency of exterior insulation less relevant, as long as the bulk water details are done correctly. There are many thousands of houses in zones 5-8 in Canada with exterior foam much thinner than would be needed for dew point control at the sheathing, that stand as existence proofs that 6 mil polyethylene on the interior can work if properly detailed. The poly vapor barrier approach is less resilient to bulk water incursions than a more vapor-open assembly though, and in zone 5A it's better to err on the vapor open side, and just concentrate on air-tightness.
Rather than trying to come up with generic prescriptives that would cover all/most condos in zone 5A, how about we cook up something relevant to YOUR building's wall stackup and location?
Dana,
“5A isn’t… very risky… even without vented cladding, in most cases dense-packed… fine… even without interior vapor retarders” – sounds like there’s enough flexibility that the now and possible future scenarios could be met.
Thanks for the offer to cook something up. I've read about deep energy retrofits for houses. But showing a condo board that a currently proposed wall is safe now AND with unkowns that might be added later is beyond me. Ultimately they’ll want an interior-to-cladding (no exterior R) design similar to walls that have fared well for years and that has fared well in buildings with too-thin foam to show it dries well in both scenarios (modeling would probably appease them in stead).
Location is Michigan City, Indiana (Zone 5a). It's not on Lake Michigan's beach but is only 1/2 mile away (so, windy). It's 135 ft from a commuter/freight rail (wrote the rest of this before your 2cd reply but sounds like we could go with cellulose and skip closed cell). While the association’s five buildings are at different angles to prevailing wind/sun, I hope we design a wall robust enough that future to-the-studs renovations in any of these buildings could use it. If that's unwise and design is better keyed to a specific building's, or even condo's, orientation, let me know.
The existing stackup could take time. It's 2x4 framing and I know the rest of the original stackup from blueprints. BUT, some building wrap and new cladding (Hardyboard? vented? not?) was added over the old siding ten-ish years ago. Before we continue this, I need to find out the make and model of the wrap, make and model of the new cladding, and if its vented or not no we know the vapor properties (or something) of each layer, right?
Thanks