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I’m finalizing the design for an attic conversion on our 1 1/2 story builder bungalow in Portland, OR (built 1908). I will first note the planned approach and then will list some questions and would appreciate any feedback you may offer.
Existing Conditions: House has simple rectangular plan, 25’x36′ overall footprint. First (main) floor level has large living/dining room, kitchen on one side (east), two bedrooms, one bath and stairs to attic on other side (west). First floor walls are leaky: uninsulated, 2×4 @ 16s, with 1″ sheathing boards and bevel cedar siding (no felt).
Attic floor is 2×6 @ 16s, sheathed w/ T&G fir, and cellulose insulation from the old days. Roof is a simple gable, framed with 2×4 at 32″ o.c. that surprisingly seem to be holding up well (no visible deflection). Skip sheathing, cedar shingles, then asphalt shingles that are on their last leg. When we bought the house the attic was “finished” but dingy (and no insulation at roof). I tore out the plaster and lath, exposing the simple beauty of the roof, but now must figure out what to do with it. We have two little kids now and need the space! Head room is pretty good, pitch is about 7:12 with ridge at about 10′ above floor.
Basement is actually a glorified crawlspace, dugout to provide head room in center, dirt floor, with 10mil VB on ground. Foundation stem walls are 6″ wide with no spread footing, and about 4′-5′ high terminating about 18″ below grade.
Inefficient gas furnace in basement that we are trying not to use much as we don’t trust the IAQ that emanates. Trying to heat with our wood stove only. Difficult as you can imagine with no insulation.
The Plan: Convert attic to three bedrooms, one bathroom. Leave attic floor as is. Add beam across living room ceiling to pick up 2×6 joists reducing span to 10’6″ (span now is 15′ and a bit bouncy) with posts strategically spaced to work in plan taken down to footings in basement. Tear off roofing, leave skip sheathing. Remove roof framing near mid-section of house, enough to frame in two shed dormers (12′ wide) place on each side of the gable and meeting at the gable. Local code allows keeping the existing roof framing as is, but new framing must be to code. Will frame dormer roofs with 2x8s @ 16″ with a structural ridge at center, matching the existing roof ridge, and with loads brought down to basement by numerous post and beam.
Plan to insulate with Icynene to achieve R-15 at existing 2×4 and R-38 at new 2×8 to meet city code for attic conversions. Been thinking about other approaches using rigid foam over the skip sheathing, or may need to supplement the Icynene anyway to make the code req’ts. Would like to go with non-vented roof assembly, but have no idea yet how the building department will look on it. Plan to insulate entirely at the roof plane, right down to sill plate at top of floor to leave warm side attics. Install gyp. board over all foamed cavities, seal all perimeters and penetrations, paint with vapor retarder paint if city (or building science) requires it. Walls in dormers will be foamed too.
Plan to use corrugated metal on the roof (BHP galvalume type product) with exposed fasteners. Overall assembly would be 1) framing, 2)sheathing (skip at 2×4 roof, 1/2″ plywood at 2×8 roof), 3) underlayment (felt at 2×4 roof, ice and water membrane at 2×8 roof, as slope is only 2:12), 4) 2x purlins across rafters at 24″ o.c., 5)1-1/2″ rigid foam between purlins(if necessary and/or beneficial), 6)felt underlayment, and 7) metal roof panels. No venting.
Bathroom will get 80cfm exhaust fan. Bathroom downstairs has 80cfm exhaust, kitchen range exhaust if 600cfm.
Note also no plans to insulate or seal the first floor walls.
Install 4-6 vents in basement walls. Cover with another 10mil poly VB (existing VB is all messed up).
Questions:
1. Anyone see an achilles heel in this approach?
2. Anyone have experience with performance of unvented roofs in Portland or sim. climate (mixed-humid)?
3. Is the 32″ framing going to make it? Should we tear off the 2×4 and skips and reframe the whole thing with 2×10 and ridge? (Keeping in mind that budget is very limited).
4. Will an airtight attic lead to IAQ problems esp. considering our wood stove heat source?
5. Does the VB in basement need to be sealed/taped to the stem walls?
Any and all input would be appreciated! I have a sense there are many of you out there who have dealt with this type of retrofit.
Mike
Replies
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I'm finalizing the design for an attic conversion on our 1 1/2 story builder bungalow in Portland, OR (built 1908). I will first note the planned approach and then will list some questions and would appreciate any feedback you may offer.
Existing Conditions: House has simple rectangular plan, 25'x36' overall footprint. First (main) floor level has large living/dining room, kitchen on one side (east), two bedrooms, one bath and stairs to attic on other side (west). First floor walls are leaky: uninsulated, 2x4 @ 16s, with 1" sheathing boards and bevel cedar siding (no felt).
Attic floor is 2x6 @ 16s, sheathed w/ T&G fir, and cellulose insulation from the old days. Roof is a simple gable, framed with 2x4 at 32" o.c. that surprisingly seem to be holding up well (no visible deflection). Skip sheathing, cedar shingles, then asphalt shingles that are on their last leg. When we bought the house the attic was "finished" but dingy (and no insulation at roof). I tore out the plaster and lath, exposing the simple beauty of the roof, but now must figure out what to do with it. We have two little kids now and need the space! Head room is pretty good, pitch is about 7:12 with ridge at about 10' above floor.
Basement is actually a glorified crawlspace, dugout to provide head room in center, dirt floor, with 10mil VB on ground. Foundation stem walls are 6" wide with no spread footing, and about 4'-5' high terminating about 18" below grade.
Inefficient gas furnace in basement that we are trying not to use much as we don't trust the IAQ that emanates. Trying to heat with our wood stove only. Difficult as you can imagine with no insulation.
The Plan: Convert attic to three bedrooms, one bathroom. Leave attic floor as is. Add beam across living room ceiling to pick up 2x6 joists reducing span to 10'6" (span now is 15' and a bit bouncy) with posts strategically spaced to work in plan taken down to footings in basement. Tear off roofing, leave skip sheathing. Remove roof framing near mid-section of house, enough to frame in two shed dormers (12' wide) place on each side of the gable and meeting at the gable. Local code allows keeping the existing roof framing as is, but new framing must be to code. Will frame dormer roofs with 2x8s @ 16" with a structural ridge at center, matching the existing roof ridge, and with loads brought down to basement by numerous post and beam.
Plan to insulate with Icynene to achieve R-15 at existing 2x4 and R-38 at new 2x8 to meet city code for attic conversions. Been thinking about other approaches using rigid foam over the skip sheathing, or may need to supplement the Icynene anyway to make the code req'ts. Would like to go with non-vented roof assembly, but have no idea yet how the building department will look on it. Plan to insulate entirely at the roof plane, right down to sill plate at top of floor to leave warm side attics. Install gyp. board over all foamed cavities, seal all perimeters and penetrations, paint with vapor retarder paint if city (or building science) requires it. Walls in dormers will be foamed too.
Plan to use corrugated metal on the roof (BHP galvalume type product) with exposed fasteners. Overall assembly would be 1) framing, 2)sheathing (skip at 2x4 roof, 1/2" plywood at 2x8 roof), 3) underlayment (felt at 2x4 roof, ice and water membrane at 2x8 roof, as slope is only 2:12), 4) 2x purlins across rafters at 24" o.c., 5)1-1/2" rigid foam between purlins(if necessary and/or beneficial), 6)felt underlayment, and 7) metal roof panels. No venting.
Bathroom will get 80cfm exhaust fan. Bathroom downstairs has 80cfm exhaust, kitchen range exhaust if 600cfm.
Note also no plans to insulate or seal the first floor walls.
Install 4-6 vents in basement walls. Cover with another 10mil poly VB (existing VB is all messed up).
Questions:
1. Anyone see an achilles heel in this approach?
2. Anyone have experience with performance of unvented roofs in Portland or sim. climate (mixed-humid)?
3. Is the 32" framing going to make it? Should we tear off the 2x4 and skips and reframe the whole thing with 2x10 and ridge? (Keeping in mind that budget is very limited).
4. Will an airtight attic lead to IAQ problems esp. considering our wood stove heat source?
5. Does the VB in basement need to be sealed/taped to the stem walls?
Any and all input would be appreciated! I have a sense there are many of you out there who have dealt with this type of retrofit.
Mike