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Attic conversion to living space

Brad Hamm | Posted in Energy Efficiency and Durability on

We are getting mixed advice on how to properly insulate an attic roof which will become a living space. The peak of the room will be 7′ and with a 6.5 – 7/12 pitch terminating at the exterior bearing walls. There is a 3′ pony wall on both sides which makes the shorter dimension 14′, the long (ridge direction) is 30′ (trying to help you visualize here). Behind the pony walls are “dead spaces” with blown in insulation between the floor joists, which continue another 5 or 6′ to the ext bearing wall (the ones mentioned before). The rafters are 2x6s at 24″ oc and will remain.

Thank you for reading this far… The question is how to insulate the ceiling/roof and with what? I found your article (Martin) about closed vs open cell foam, and venting vs unvented roofs but am having a hard decision because of the shallowness of the rafters. And should the roof be insulated in the dead space to the bearing wall? Does there need to be an air gap all the way to the eave since that dead space between isn’t conditioned? I’d love to get away with using Batts for the ease of installation and repair.

Finally, we wanted to finish the ceiling with gyp board and I’m curious how to make it airtight to help reduce any potential moisture build up. Thank you for any advice. -B

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    A short wall intersecting a sloped ceiling is usually referred to as a "knee wall". A "pony wall" is a more generic term usually used in other contexts, often (but not exclusively) the short framed wall section atop a foundation wall that extends below grade.

    For the sloped finished roof to be vented it must have a 1" minimum depth continuous path for air to flow extending from the eaves all the way to a ridge. Most knee walls in attics are built with blocking between the rafters where it rests on top of the knee wall, impedes that path. But even if it were clear, with only 2x6 rafters the 1" minimum air gap requirement would limit you to a fairly pathetic R value, well below code minimum anywhere.

    The best solution would be to install cheap fiber insulation completely filling the rafter bays, and add sufficient rigid foam above the roof deck for dew point control at the roof deck. The ratio of foam-R to total-R necessary for dew point control varies by climate. Where are you located?

    If rigid foam above the roof deck is not an option, closed cell spray polyurethane on the underside of the roof sufficient for dew point control for filling the rest with fiber insulation works. At 2" of closed cell foam you would have R12, and it would leave you 3" of space for fiber. If you used R15 batts that would make R27 total, and a ratio of R12/R27= 44%, which would be sufficient for dew point any location in US climate zone 5 or lower, but in zone 6 you need the ratio to be at least 50%, which means you'd have to back off to lower denstity R11 or R12 batts or go with thicker foam (which is pretty pricy.)

    In most locations you could also do OK with a full 5.5" of open cell foam if you painted the ceiling with half perm paint aka "vapor barrier latex". The open cell foam is sufficiently air tight, but it's permeable to water vapor. You don't want to use polyethylene or foil vapor barriers, since that would prevent all, drying, which is risky. Half perm paint or 2-mil nylon (Certainteed MemBrain) would prevent excessive wintertime moisture build up, but would still allow the assembly to dry over the summer (in most climates- maybe not in zone 8).

  2. Brad Hamm | | #2

    Thank you Dana! So I forgot to mention that the there is an existing ridge vent. How does this effect your answer? We are in Cape Girardeau, MO which is 100 mi south of STL along the Mississippi River. I'm fine with the idea of using Closed Cell Foam for the R value it gives, but do we need to do anything to prep the plywood sheathing? We would like to avoid the rigid foam on the roof since the shingles are only 5 yrs old and you know, $$. Can you comment on "foil" insulation also? Can we be as crafty as applying foil to reflect radiation? Thanks for taking time to answer before. Best - B

  3. GBA Editor
    Martin Holladay | | #3

    Brad,
    First of all, you probably want to read the following article (which explains why it is always better to include the triangular attics behind the kneewalls in the conditioned space of your house, rather than to try to make them outside of the conditioned space of your house): “Two Ways to Insulate Attic Kneewalls.”

    Here is a link to an article that will list all of the different ways to insulate your roof assembly: How to Build an Insulated Cathedral Ceiling.

    Since you don't have much ceiling height, you probably want to use insulation with a high R-value per inch. That means closed-cell spray polyurethane foam. You may still want to include ventilation channels between the top of the foam insulation and the underside of the roof sheathing. To learn more about ventilation baffles, see Site-Built Ventilation Baffles for Roofs.

    Finally, I'll cut and paste some advice from one of my articles, All About Attics:

    "In some cases, converting your attic into a spare bedroom or home office may make sense. However, bad (or even dangerous) attic conversions are more common than attic conversions that are graceful and energy-efficient.

    "Some points to bear in mind:

    "• Some building codes require third-floor bedrooms to include two modes of egress — a requirement that is often interpreted to mean two separate stairways.
    "• Older homes often have 2x6 or 2x8 rafters that don’t provide enough depth for adequate roof insulation. Skimping on insulation to gain an inch or two of headroom is a flawed strategy; better approaches include popping a dormer or adding rigid foam insulation above the existing roof sheathing.

    "Your unfinished attic may appear spacious. Once it becomes clear that the space can't be finished unless an adequate thickness of insulation is installed, however, ceilings usually end up lower than homeowners expect. When a conflict arises between a homeowner’s desire for adequate headroom and an energy consultant’s advice on minimum R-values, the energy consultant usually loses. Unfortunately, the result is often a space that is both cramped and inefficient.

    "That said, it’s possible to convert an attic into usable space and still maintain the integrity of your home’s thermal envelope. Before you commit to an attic conversion, however, consult a reputable architect."

  4. Expert Member
    Dana Dorsett | | #4

    Assuming 24" on center spacing and no blocking, full fill of 5.25" of R6/inch closed cell foam in a 2x6 rafter bay yields a "whole assembly" of about R26.5, (U0.038), best case. Odds are it'll be less than that. The roof deck would not be able to dry toward the interior very quickly since the foam would be ~0.2 perms.

    If you did a full fill of 5.5" of R3.7/inch open cell foam you would have a whole-assembly R of R19.6, and would have plenty of drying capacity, but you would need to use a smart vapor retarder or half-perm latex to make it fully moisture safe, unless you put a couple inches of rigid polyiso insulation above the roof deck. If you installed the rigid foam above the roof deck you would be at a more reasonable R30-R31 whole-assembly R, or ~U0.032, the roof deck could dry toward the interior freely through open cell foam, the rate being limited only by standard latex paint (3-5 perms.)

    "...the shingles are only 5 yrs old and you know, $$..."

    The closed cell solution would cost on the order of $5-6 per square foot.

    The open cell foam + smart vapor retarder would cost about $2-2.50 per square foot.

    If you add the 2" polyiso that adds about $1-1.25 per square foot (less than 50 cents if you use reclaimed roofing foam), plus another 25-30 square foot for a half-inch OSB nailer deck over the foam, plus whatever the re-roofing costs.

    It's not insane to stage it, doing an open cell + smart vapor retarder solution for the initial build out, then plan to add the above deck foam when it's time to re-roof. In your climate there is no "payback" in energy cost savings on the ~$3 per square foot cost adder of an all closed cell solution within the lifecycle of an asphalt roofing shingle unless you anticipate extremely high energy costs. When it's time to re-roof there is a lifecycle benefit to adding up to 4" of roofing polyiso instead of the minimum 2" necessary for dew point control, (due to the very long lifecycle of the foam) especially if using reclaimed goods. With 4" of polyiso over 2x6/open cell foam you would also be fully compliant with (slightly above) the IRC 2015 code min performance. There is at least one large-scale foam reclaimer in St. Louis, as well as some smaller ones, eg:

    https://stlouis.craigslist.org/mad/5860589933.html

    Foil "insulation" is not insulation, and only provides a significant benefit when there are air gaps on both sides of the foil, and the bulk insulation levels are extremely low.

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