How to insulate underside of a inset balcony?
Hello everyone, I’m in zone 5 Iowa
I’m designing my home and want to showcase the mile long views from a small second story “inset” balcony. I considered a flat roof and terrace, but decided the balcony would see less water if under the main roof and large eaves.
I know a inset balcony increases the exterior surface area and reduces interior space, but it’s small, and this is what I want, and I hope to get some recommendations on how to insulate the balcony floor properly.
What I’m wondering is if I use 24″ TJI’s for the second story floor system, is 24″ enough space or R value to not have to add exterior insulation and another deck? For example, sheath the balcony floor with AdvanTech, apply closed cell spray foam under (even though I dislike it) and blown-in to fill the remaining cavity? My thoughts is that the balcony isn’t really a flat roof, since it’s cavity is open the the entire length of the joist space allowing any condensation to dissipate to the interior of the home during extreme temperatures.. This method however does not address the 1/4″ per foot of pitch I should have on the balcony.
Or should I use 16″ TJI’s, and then raise the entire inside floor system by nailing 2×6’s on top of the TJI’s , so that my inside/outside floors remain even after Ive added tapered exterior insulation to the balcony?
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Replies
Mike, an inset balcony should be treated like any other low-slope roof, including being pitched for drainage. In zone 5 the IRC calls for R-30 in floors and R-49 in roofs. The balcony floor serves both purposes so you need at least R-49. Although some people and material suppliers advocate for unvented low-slope roofs without foam, unfortunately they are never safe, resilient assemblies--moisture can accumulate unless everything is perfect on day 1 and never changes. While you're right that the moisture might be able to dissipate to the interior, this is a situation where I spec foam, or a flash-and-fill system.
In a zone 5 roof you need at least 41% of the R-value to be in the foam layer in a flash-and-fill system. If you use HFO-blown foam, and you should if you care an iota about the environment (the blowing agents are orders of magnitude less bad than conventional HFC-blown foam) you can expect about R-6.5/in (regardless of what foam salespeople tell you) and fluffy insulation is usually around R-3.5/in. To fill a 24" cavity, you would want at least 7" of the HFO-blown closed cell foam (R-45.5) and no more than 17" of fluffy insulation (R-59.5) for a total of R-105, with safe ratios. That is not likely a cost-effective amount of insulation so you can reduce the total amount, but maintain at least the 41% at the foam layer.
Thanks Michael.
Yes 16" TJI's would be adequate, and for my small 7x10 balcony Id be tempted to fill the entire floor cavity with HFO blown foam.
Sort of off topic, but do you have a favorite flat roofing product you could recommend for this balcony? I have access to 8' wide rolled metal, so I could hand seam the edges, solder the corner have a one piece metal roof.
Thanks again.
Mike, I only have experience with soldered flat-seam copper or EPDM for low-slope roofs. EPDM will likely cost a lot less but will only last 15-30 years; copper should last much longer. Other metals that would work include stainless steel, zinc or zinc-coated copper ("Freedom Gray"). Do you have experience soldering copper flashing or roofing? It's similar to soldering pipes but there are special tools and techniques for a better-quality job. Other options include TPO, fluid-applied, torch-down bituminous, and other products; I just don't have experience with them.
16" of HFO-blown foam would be about R-100. Because it still has a relatively high carbon footprint, I would not go beyond R-50 or R-60; there are quickly diminishing returns above that range.
Ordinarily there is a step down of at least four inches and more like six or seven when you go through an exterior door. If the interior floor and exterior floor are at the same level it's really hard to weatherproof the door. This means that you have to use taller joists for the interior section than for the balcony.
To answer your question about roofing products, I have a roof deck done with Duradek: https://duradek.com/
It's sixteen years old and I'm still happy with it.
DCContrarian , does a balcony door need to be higher even if that door is 5' back under a roof '?
"Need to" as in code requires it? I don't know. I would think it's a good idea. It's hard to keep out wind-driven rain or snow when the floors are even.
The balcony 7" lower is a good idea. But the floor framing requires a beam I hadn't planned on.. I wanted a clear span of 30' but would need support those floor joists at 22' instead.
Having a step down is not an IRC requirement.
After watching the foam shrinkage video, that's the last straw for me..
I'll have to add a few inch's of Mineral Wool exterior insulation to the balcony floor, and raise the door, or entire second story floor, and keep Foam away from my home.. But its looking like a real challenge placing a balcony on the same floor system it's adjacent to. Because as DCContrarian mentioned, the balcony needs to be 7" lower.. Which would require a separate lower floor system.
Mike, that's usually what I've done for framing in this type of situation. I would still use closed cell foam; the video you linked to on another thread showed shrinkage from a different type of insulation. You might be fine with just mineral wool, if you also use a high quality variable permeance membrane at the interior. Houses tend to be resilient. But as a designer I have to plan for the worst, and closed cell foam provides the best insurance in this case.
I don't know how far you're going to inset, but I have a balcony (actually 2 balconies) that are inset 4' on Martha's Vineyard, Zone 5. Due to the maritime climate, we rarely have more than 10" in any snowfall. The roof decking with a liquid applied waterproofing is set about 4" below the sill level with sleepers and wood decking bringing the walking surface up level with the bottom of the sill. It's certainly a much nicer transition to walk out level, but I understand the concern with heavy snow buildup in some situations. Having the waterproofing 4" below the sill with wood decking handles liquid water well.
Thanks Hugh, your balcony's are exactly what I want.. But when you say "The roof decking is set about 4" below the sill level with sleepers and wood decking bringing the walking surface up level with the bottom of the sill."
I imagine this required the balconies to have their own separate lower flooring system?
Rather than thinking of it as a separate lower flooring system, it might help to think of the entire level having one system of joists, and the interior having a raised floor above that system and the balcony having a sloping floor above that system.
flooring system under decks is of reduced depth compared to adjacent floor system with less insulation than if I was building today, only 5-1/2 " R-19 batts....closed cell foam today would provide about R-38