How to insulate a bedroom exterior wall that is also the attic to a porch?
In thinking about a retrofit insulation to my 72 year old, 2 storey, load bearing brick home in Baltimore, Maryland, I am not sure what to do about the porch roof, which is a straight line continuation of the house roof. This results in a triangular ‘attic’ to the porch, with a porch ceiling, roof and a ‘kneewall’ that separates it from a second floor bedroom. The ceiling to this bedroom is sloped, leaving a gap between it and the rafters that form the roof. In the attic, one can see through this gap into the void of the porch attic.
I would like to insulate this wall of the bedroom. To do so from the inside means losing some square footage, so I wondering if I could fill this 100 sq ft (~450 cubic ft) porch attic with cellulose. But I don’t know whether to consider this a “wall” or a “roof deck.”
As I understand the code, if what I am doing is insulating a bedroom wall, that happens to have with a porch-attic sized cavity filled with cellulose insulation and a sloped cladding that looks awfully like a roof, then I don’t need to worry about condensation. As I read Martin Holladay describe it, “If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don’t have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.” (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing)
However, if I consider the porch roof to be an attic roof, since it is continuous with the attic, then I am required to maintain a certain ratio of cellulose to foam insulation to prevent condensation, or R-15 for an R-38 total (http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-conditioned-attics?searchterm=cellulose+attic). This would make me need to put a whole lot of foam above the porch roofdeck to balance the cellulose that is up to 8 ft deep!
I can’t imagine I am the first to deal with this problem, so I would appreciate your suggestions or known solutions.
As an added detail, the fireplace is on that wall, extending into the porch, and the chimney goes up through this porch attic. Cellulose would thus be in contact with the chimney. Also, there would be heat loss from the bedroom into the chimney.
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Replies
Richard,
It's hard to evaluate the best way to insulate your kneewall without a site visit.
1. Weatherization workers sometimes do fill large triangular attic areas like the one you describe completely full of cellulose insulation. There are potential disadvantages, though, including the fact that the cellulose will be in contact with the roof sheathing, with no intervening ventilation channel. I would advise against this route.
2. The best approach is probably to create access holes in the porch ceiling and to install rigid foam on the exterior side of the brick kneewall.
Martin, thank you.
Why is it that a wall in this climate is not considered at risk of sorption (the sheathing will not accumulate moisture) no matter how much cellulose I put interior to it, but the roof deck in the same house is considered at risk for sorption with the same amount of cellulose insulation under the roof deck between the rafters? I need insulation above the roof deck but not external to the wall. Why the difference?
Richard,
As I pointed out in my recent blog on the topic, a wall in your climate is probably at risk for sorption -- that is, the wall sheathing could accumulate moisture during the winter -- if you install a lot of cellulose on the interior side of the sheathing.
The risk can be managed by using a sheathing product that is more durable than OSB, by installing a ventilated rainscreen gap, and by limiting air leakage through the wall.
Insulation on the exterior side of wall sheathing is almost always a good idea, especially if it is thick enough to limit moisture accumulation in the sheathing.
Martin,
You make it sound like I have not read your blogs or other postings. Your blog refers to a cold climate, and refers me for specifics to your other blog:
"However, the foam also reduces the ability of the sheathing to dry to the exterior, so it’s important to be sure that the foam is thick enough. Thick foam is better than thin foam (see "Calculating the Minimum Thickness of Rigid Foam Sheathing.")"
In that blog on "Calculating Minimum Thickness," as I indicated in my question, I read that my climate:
"If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem."
Richard,
I'm sorry that I wasn't paying close attention to your location when I posted my last answers.
You're right that in your climate, you will probably be OK, for the reason you state -- winter temperatures are milder than they are farther north.
Here's the thing, though: risk occurs on a spectrum. Even in your climate, good moisture management practices are advantageous. If you built a thick cellulose-insulated wall with air leaks, OSB sheathing, and stucco without a rainscreen -- and I'm not saying you would -- your OSB might be at risk. So it always makes sense to build a robust wall that dries easily and avoids moisture problems.
This whole issue -- OSB durability in cold climates -- is one where even the smartest building scientists don't have as much data as they'd like. When in doubt, it never hurts to be cautious.
Richard,
As some article was once titled "A roof is not just a slanted wall".
Walls have some exposure to rain but, because they are vertical, can gravity drain - and often have claddings that can breathe to some extent and dry when they take on small amounts of moisture.
Roofs are exposed to the full fury of the rain and wind and sun (radiant inward moisture drive) and are typically covered with waterproof layers that don't breathe very well. So, not only is a roof much more likely to experience a major leak, but less able to dry. That is one of the reasons a vented roof is far more durable than an unvented roof. Also, because warm moist air rises in a home and is likely to leak at the ceiling level, condensation on cold sheathing is far more likely at the roof than at the wall.
Martin states: "Insulation on the exterior side of wall sheathing is almost always a good idea, especially if it is thick enough to limit moisture accumulation in the sheathing."
I will continue to argue with this contention. With today's homes, that are built with good air tightness and with point-source moisture control and interior ventilation systems, condensation within walls is far less of a problem than not allowing a damp wall to dry out. Most major water problems in a heating climate are due to leakage of exterior wind-driven rain or from ice dams, not interior condensation. Hence exterior foam insulation, which dramatically limits drying to the exterior during the heating season and also keeps the wall cavities warm enough to grow mold and decay organisms, is the wrong approach to hygrothermal management.
For renovations and retrofits, exterior foam may be the only feasible solution, but care must be taken to avoid the unintended consequences of trapping moisture in a warm cavity. A roof is always better off if it has at least an air gap under the sheathing if not full ventilation.
Richard,
As Martin points out in the first answer, a site visit would really help with determining the correct strategy for this atypical configuration ("cellulose that is up to 8 ft deep?"). I checked with the GBA powers-that-be, and they said it would be okay for me to offer my services if it would be helpful to talk to a local pro about this. I'm just 15 min. down the BW parkway from you and could come take a look at what you've got going on. You can e-mail me at [email protected] if it's anything you want to pursue.