Dense pack cellulose in imperfectly sealed walls?
Hi all,
I’ve just bought an old (circa 1900) house in Ulster County, NY (Climate Zone 6) and am getting ready to insulate the house. It’s already been gutted, so I have open stud bays to work with (rough sawn 2×4 walls, rough sawn 2×6 rafters). I’m leaning towards dense pack cellulose, so that I’ll have the option down the road of putting rigid foam on the outside of the sheathing when I inevitably need to re-side the house, but am a little worried about the fact that the sheathing and siding of the house are not very tight – I can see daylight in plenty of places. Would that cause a major problem down the road with moisture getting in? What about air sealing? I know dense pack is better than fiberglass, but it’s still not a proper air seal, is it?
I know 4″ of dense pack won’t pass code, but that shouldn’t be a problem, as I’m an existing non-conforming house. Also, I can’t afford to pay someone to come in and spray foam the whole house, so that isn’t really an option.
And finally, I’d like to do the work myself, if at all possible. I have experience dense packing walls, but don’t own a blower, so that might be difficult. All constructive advice and alternative options are very much appreciated. House is 1400 sf.
Thanks!
Sam
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Replies
Samuel,
If you plan to insulate your 2x6 rafters, you should read this article: How to Build an Insulated Cathedral Ceiling.
Doing some air sealing work before you insulate your stud bays with cellulose is a good idea. How much work to do depends on your budget and your energy level. One possibility is to caulk the wide gaps in your sheathing. Another possibility is to install one thin layer of rigid foam against the sheathing, using the cut-and-cobble method, followed by dense-packed cellulose in the remainder of each stud bay. For more on this approach, see Cut-and-Cobble Insulation.
A third possibility is to simply install dense-packed cellulose now, and to address the air leaks at the sheathing level when you get around to installing exterior rigid foam and new siding.
Samuel,
Moisture, which damages walls, comes from either the exterior through the intrusion of bulk wetting and vapour drive, or the interior, carried on air or to a lesser extent through materials.
As you say: cellulose retards air movement, but you still need an air barrier. Because you aren't planning to work on the exterior right away, you are limited to providing that barrier on the interior, either by installing a (smart) membrane, or by the way you detail the interior finishes to make them air tight.
How much of a problem the less than perfect exterior represents depends on a number of things. The climate, the amount of wetting the walls take, and their ability to dry. That you can see the sheathing probably means you don't have a WRB. Stapling up sheets of building paper in the stud bays before dense packing will go some way to alleviating the concerns.
Good luck!
Thanks guys! I'd read the Cathedral Ceiling post, but not the cut-and-cobble one. So, I'm leaning towards rigid foam on the inside of the rafters (to create vented rafter bays) and dense packed cellulose in the stud walls, with the intention of adding rigid foam on the outside down the line. I have two followup questions:
1. How concerned should I be with moisture penetrating my exterior wall assembly (clapboards and sheathing boards), and messing with the cellulose? Is cellulose susceptible to moisture/mold?
2. Assuming that moisture coming in through the outside wall assembly is bad and should be avoided, one solution might be to buy some spray foam kits and do 1" of spray foam in each stud bay and then dense pack the rest. However, my understanding is that if I have any foam on the inside of the sheathing, I would NOT want to put rigid foam on the outside, down the line. Am I correct in that understanding?
Thank you so much for the advice!!
Sam
Samuel,
Q. "How concerned should I be with moisture penetrating my exterior wall assembly (clapboards and sheathing boards), and messing with the cellulose? Is cellulose susceptible to moisture/mold?"
A. It's always good to be concerned. Ideally, your house has (a) wide roof overhangs, (b) limited exposure to wind-driven rain, (c) siding in good condition, (d) flashing at windows and other penetrations, (e) a water-resistive barrier (perhaps asphalt felt) integrated with the window flashing.
If the wall has (f) a ventilated rainscreen gap between the WRB and the siding, it would be even better. But that's hoping for too much.
So, those are the factors in your judgment call. The more problems, the greater the worry. The fewer problems, the less the worry.
If you suspect that your wall has problems, it is important to schedule your siding repairs (and installation of a new WRB) sooner rather than later.
Samuel,
If you decide to do a flash-and-fill job (spray foam plus cellulose), the minimum thickness of the closed-cell spray foam layer, assuming a 2x4 wall in Climate Zone 6, is 1 1/2 inch. For more information on this topic, see Flash-and-Batt Insulation.
If you do that, and then you come around in a year or two and install a layer of continuous rigid foam on the exterior side of your sheathing, I don't think you'll have any problems -- as long as you do a good job with exterior detailing and exterior flashing. Remember, a ventilated rainscreen gap provides a lot of protection to any wall.
Extremely helpful, thank you, Martin! Just to be clear - when I come back to do the rigid foam on the outside of the house, you're saying it would be good to put a ventilated rain screen on the outside of the rigid foam, correct?
Thanks,
Sam
A flash 'n fill on a 2x4 wall is quite a bit more expensive than a full-fill of open cell foam + 2-mil nylon (Certainteed MemBrain) which is somewhat more expensive than a full fill of open cell foam + half-perrm paint ("vapor barrier latex").
Even a 1" flash of 2lb closed cell foam has a much polymer as a full 4" of half-pound foam, and only adds about R1 to the "whole wall" performance after thermal bridging, a difference that can be made up for far more cheaply with a 1/4" of fan-fold XPS siding underlayment under the wallboard (in lieu of MemBrain or half-perm paint), or an additional 1/4" of foam thickness on the exterior. Non-perforated XPS siding underlayment usually has vapor retardent facers, and is typically at the very low end of class-III vapor retardency or the high end of Class-II vapor retardency, between 0.7- 2.0 perms, depending on the product. any of which would be sufficient protection from interior moisture drives for plank sheathing under clapboards, when combined with the inherent air tightness of 4" of open cell foam.
Both the XPS and half-perm paint interior side vapor retarders reduce the drying capacity toward the interior by quite a bit compared to the 2-mil nylon solution. A shot of 1.5" of 2lb foam typically comes in between 0.5-0.8 perms, which less than 1/5 the drying rate of 2-mil nylon + standard latex paint when the open cell foam has high humidity due to the sheathing releasing moisture with warmer spring temps. The 2-mil nylon is also in that range when the sheathing is cold, and holding onto it's moisture, but is over 10 perms when the open cell foam is at a higher moisture content, which allows it to dry toward the interior. This characteristic makes open cell foam + 2-mil nylon far more resilient than 1/4 fan fold XPS or half-perm paint, and more resilience than the 1.5" flash'n'fill.
You could also do dense pack cellulose + 2 mil nylon, but if there isn't any window flashing in this antique that would be more risky than a full-fill open-cell solution.