Insulation questions 100 year old house
Last Post 20 Oct 2014 06:09 PM by Dana1. 6 Replies.
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15 Oct 2014 11:55 PM
Gut reno on a house in East Nashville. Stripped to studs. Walls are full dimension 2x4 with diagonally placed 1x6 yellow pine with brick veneer( 1" air gap between sheathing and brick) Large overhangs. Brick has been painted with 4 coats of paint. Thinking of flash and batt with OC foam to seal the 1x6 joints, and Roxul for sound attenuation.( fire house is a half block away). I was planning on a vapor barrier on the inside under the gyp. Is this a reasonable stack up? Second question - attic was finished off, is stripped and will be redone. Problem is that rafters are full dimension 2x4. Collar ties have been reset at 10' height, and there will be knee walls, so I'm actually beefing up the overall strength by cutting down the open span. But trying to figure how to get at least some marginal insulation in the roof. Thinking of creating an air gap under the sheathing by running 3/4" inside the sides of the bays, and adding 3/4" Foilbacked Iso to gain some reflectivity. Then rip down 2x4 and add to the edge of the rafter to make up what I took with the previous assembly. Again use Roxul to finish off. Obviously, not ideal from an R value, but better than the total lack of insulation originally there. Would use Membrane to retard moisture and allow drying to the inside. Lastly on the knee walls, was again planning on running Foilbacked ISO, this time on the exterior of the studs, picture frame with Dow foam, and finish with Roxul. Again use Membrane. Thoughts
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15 Oct 2014 11:58 PM
Some clarifications as I couldn't edit when reviewing: the 1x6s are the sheathing.
And 3/4 x 3/4 furring strips would be placed at the sides of the rafter bays to create the air gap with the ISO board
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16 Oct 2014 02:15 PM
Posted By Nashvegas on 15 Oct 2014 11:55 PM
Gut reno on a house in East Nashville. Stripped to studs. Walls are full dimension 2x4 with diagonally placed 1x6 yellow pine with brick veneer( 1" air gap between sheathing and brick) Large overhangs. Brick has been painted with 4 coats of paint. Thinking of flash and batt with OC foam to seal the 1x6 joints, and Roxul for sound attenuation.( fire house is a half block away). I was planning on a vapor barrier on the inside under the gyp. Is this a reasonable stack up? Second question - attic was finished off, is stripped and will be redone. Problem is that rafters are full dimension 2x4. Collar ties have been reset at 10' height, and there will be knee walls, so I'm actually beefing up the overall strength by cutting down the open span. But trying to figure how to get at least some marginal insulation in the roof. Thinking of creating an air gap under the sheathing by running 3/4" inside the sides of the bays, and adding 3/4" Foilbacked Iso to gain some reflectivity. Then rip down 2x4 and add to the edge of the rafter to make up what I took with the previous assembly. Again use Roxul to finish off. Obviously, not ideal from an R value, but better than the total lack of insulation originally there. Would use Membrane to retard moisture and allow drying to the inside. Lastly on the knee walls, was again planning on running Foilbacked ISO, this time on the exterior of the studs, picture frame with Dow foam, and finish with Roxul. Again use Membrane. Thoughts

That's a TERRIBLE wall stack up for a brick veneer in TN!  With an interior side vapor barrier you would be guaranteed to have chronic condensation inside the studwall cavities during the cooling season.

A flash'n'batt with 1" of CLOSED CELL foam would put a ~1 perm vapor retarder between the sheathing & interior, which would be sufficiently vapor retardent to allow the interior side to remain fairly vapor open, allowing the studs to dry toward the interior. With ~1" of closed cell foam and 3" of fiber in the full-dimension 2x4 cavity you would have more than enough foam-R for wintertime dew point control at the foam/fiber boundary.   The sheathing would still have some capacity to dry toward the interior, but it's drying direction would be primarily into the cavity.

Batts are not the best choice for old full-dimension framing. Even if the stud spacing is a standard 16" o.c. the batts are designed for a nominal cavity width of 14.5", not the 14.0" you would actually have.  Compressing it into a 3" depth instead of 4" isn't a problem (that increases the R/inch), but the narrower cavity width causes buckling & void issues. It's FAR better to use blown insulation in those cavities (either cellulose at 2.8lbs minimum density, or new-school fiberglass at 1.8lbs min. density.)  In a full-gut situation this is best done blowing in mesh, and rolling it flat to the studs before hanging the drywall.

Even an all open-cell foam solution (4") would be a problem during the cooling season, since it's too vapor open at 4" to be fully safe, and you may end up with failing interior paint or mold in the drywall facers during the cooling season. But if you put an vapor barrier in there you would end up with soggy open cell foam, and potential rot on the studs.

On the vented insulated roof deck, code demands a minimum of 1" of air space between the roof deck and insulation, but 2" is even better. It also has to have full soffit to ridge ventilation to really work.   Here again you can do OK going unvented with a flash'n'fill approach using closed cell polyurethane.   The amount of foam-R needed depends on both climate zone and the total R.  Nashville is in US climate zone 4A:

http://www.energyvanguard.com/Portals/88935/images/iecc-climate-zone-map-energy-code-warm-moist-line-800.jpg



Per IRC 2012 chapter 8, if you were building to the full IRC 2012 R49 code min for zone 4A you would need R15 for dew-point control at the foam/fiber interface, and you would skip the interior vapor retarders.  But you're not going to be insulating to anywhere near R49. What is important is the RATIO of foam-R to total-R.  R15/R49 is about 30% foam with about 70% fiber.  If you're only installing R15 of fiber, an inch of closed cell foam is enough.  If you're bumping that to R25 of fiber, 1.5" of closed cell would be in order.  Even at 2" the vapor retardency of the foam would still be high enough for the roof deck to dry at a very reasonable seasonal rate, and even 1" would be sufficient to limit the wintertime moisture loading issues to something quite tame.

If you rip down some 1" x 2" rigid polyiso strips for the rafter edges to adjust  rather than ripping down wood, you would cut the thermal bridging by more than half, which is a huge performance improvement, but you DON'T want to put foil faced iso over the whole interior side of an unvented roof assembly since it would create a moisture trap- the vapor permeance of the foil facers are WAY too low. You could do that with 1" unfaced EPS though, since at that thickness it has a vapor retardency no tighter than latex paint, and won't create a moisture trap.

On the kneewalls, it's far easier to air-seal the house if you just continued down the rafter line with whatever you do for the cathedralized ceiling section.  If you insulated the kneewalls and mini-attic floor instead, you would have to meticulously build air dams for every joist bay running under the wall, and air seal each with can-foam or something, as well as any electrical/plumbing penetrations coming through the mini-attic floor.  Doing it at the roof deck ends up being less work, fewer square feet of insulation and higher performance.  On a retrofit, air-sealing each of the joist bays usually ends up being a fools errand (speaking as one who has played that fool multiple times. )


I'm not a big fan of closed cell foam due to the fact that it's blown with HFC245fa, which has very high global warming potential (about 1000x CO2) But when used in thinner layers like this for dew point and vapor retardency control it's still "worth it".   Similarly XPS is blown with an even worse stuff (HFC134a, at ~1400x CO2), but both polyiso and EPS are blown with comparatively benign pentane (~7x CO2).

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16 Oct 2014 03:19 PM
Dana
Thanks for the reply. You were exactly the person I wanted to respond. A couple comments: my idea on the walls was that even with the brick, since it's painted there should be far less moisture through it and with the 1" air gap there would not be moisture trapped in the walls. The stud bays would dry to the outside. Your thoughts? Or I guess I know you thought but the fallacy in what I'm thinking?
Regarding rafter bays- I can foam against without an air gap? That is what you're indicating? And I like the idea of foam extensions on the studs, but does that really cut transmission in half? You still have the sides of the rafters picking up heat/cold. I see on the exterior how it works, but on the interior?

Regarding the knee walls I hear what you are saying. the soffit is well ventilated, so I'll need to close all of that yes? I think over all it would solve several issues nicely. Basically I'll have an unvented roof correct?

One last question totally off this thread. Was out in New Mexico. Zone 6. Design point -25F. Builders out there are runnin 2x6 with tyvek and filling bays with open cell foam no interior moisture barrier. Won't moisture pass through and condense On the inside of the sheathing?
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16 Oct 2014 06:28 PM
Posted By Nashvegas on 16 Oct 2014 03:19 PM
Dana
Thanks for the reply. You were exactly the person I wanted to respond. A couple comments: my idea on the walls was that even with the brick, since it's painted there should be far less moisture through it and with the 1" air gap there would not be moisture trapped in the walls. The stud bays would dry to the outside. Your thoughts? Or I guess I know you thought but the fallacy in what I'm thinking?
Regarding rafter bays- I can foam against without an air gap? That is what you're indicating? And I like the idea of foam extensions on the studs, but does that really cut transmission in half? You still have the sides of the rafters picking up heat/cold. I see on the exterior how it works, but on the interior?

Regarding the knee walls I hear what you are saying. the soffit is well ventilated, so I'll need to close all of that yes? I think over all it would solve several issues nicely. Basically I'll have an unvented roof correct?

One last question totally off this thread. Was out in New Mexico. Zone 6. Design point -25F. Builders out there are runnin 2x6 with tyvek and filling bays with open cell foam no interior moisture barrier. Won't moisture pass through and condense On the inside of the sheathing?

The moisture in the brick is as much from adsorption from the air in the vent cavity as anything, which it takes on as it cools overnight- it's not merely a matter of rain/dew wetting from the exterior. The cavity air is essentially outdoor air, which has a fairly high moisture content during the summer in your location. Even if the paint has effectively zero vapor permeance, this would still be an issue (possibly even more so, since it limits the ability of the brick to dry toward the exterior.)

Air conditioned homes with true interior vapor barriers (like 6 mil polyethylene or foil) can end up with condensation issues even without brick/stucco exteriors in a TN climate.  Vapor barriers cause as many (or more) problems as they solve in your climate.

Yes, you can foam directly against the roof deck without an air gap and go completely unvented, as long as you keep the R-ratio sufficienty up to snuff for whatever fiber insulation you are installing there.

If you go with an unvented roof deck approach, yes, you seal up ALL of the venting into the mini-attic spaces.

The foam strips work no matter which side of the assembly they are installed.  A dry full-dimension 2x4 made of southern yellow pine has an R-value along the 4" axis of about R5.  A 1" strip of polyiso has an R-value of about R6.  The path from the sides of the studs through the fiber insulation to any part of the sheet rock is still at least R3.7 (R4 if rock wool).  So while the higher conductivity of the wood makes for not very smooth iso-therm lines in the cross section, there is still at LEAST R9-R10 on even the very lowest-R paths, and the gain/loss thorugh the stud really is cut by about half.  It doesn't much affect the mid-cavity performance, but the "whole assembly" performance is enhanced nearly as much as if you had put a full sheet of foam there.

Are you sure the location in NM was a climate zone 6? All counties in NM are zone 5 or lower, but I suppose at 10,000' you might see a zone 6 climate.  The 99% outside design temps in NM are very low compared to zone 5A locations, due to the extreme dryness & clarity of the air, but it is still zone 5.

Standard latex interior paint has a vapor permeance of 3-5 perms- it's a vapor retarder, just not a super powerful vapor retarder. In  NM climate the extreme dryness of the wintertime air would allow them a bit more leeway.  If the structural sheathing is fiberboard it's pretty easy to get away with it- if OSB it's a bit harder.  If they were using cut in bracing instead of sheathing and spraying foam directly on the Tyvek (which I understand some contractors do in the SW) the flow-through aspect works even better. The drying rate through 25+ perm Tyvek is an order of magnitude higher than through OSB.  The summertime humidity there is very low too. They would have to air condition down to 45F or so to create a summertime moisture problem in those walls, since that's where their average summertime dew points live. In TN you have many days where the dew points bump well into the 70sF.


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17 Oct 2014 07:43 AM
Thanks Again Dana. I understand the issue with the brick and cavities better now. Regarding the NM question, I may have my zone incorrect. Maps for Mountainous areas are not very accurate. It is at 9000' And the builders are building with OSB on the sheathing. AC is typically not used as summertime temps rarely get into the 80's for more than a few days, and the diurnal swing is large so windows get opened in the evenings and closed in the mornings. As most of the homes are vacation only, perhaps the issue isn't so critical, but if being used full time, I would think the use of humidifiers to keep the levels up to reasonable levels could make this an issue?. One builder did say when they used fiberglass and polyE barrier in years past, they ended up with dripping cavities. I ask the question as we plan on moving to there in about a year and beginning a project there.
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20 Oct 2014 06:09 PM
Posted By Nashvegas on 17 Oct 2014 07:43 AM
Thanks Again Dana. I understand the issue with the brick and cavities better now. Regarding the NM question, I may have my zone incorrect. Maps for Mountainous areas are not very accurate. It is at 9000' And the builders are building with OSB on the sheathing. AC is typically not used as summertime temps rarely get into the 80's for more than a few days, and the diurnal swing is large so windows get opened in the evenings and closed in the mornings. As most of the homes are vacation only, perhaps the issue isn't so critical, but if being used full time, I would think the use of humidifiers to keep the levels up to reasonable levels could make this an issue?. One builder did say when they used fiberglass and polyE barrier in years past, they ended up with dripping cavities. I ask the question as we plan on moving to there in about a year and beginning a project there.
Running humifidifiers indeed raises the risks, especially when "...reasonable levels..." can mean 50% RH or higher. (30%-35%  would be the recommended range for that stackup in winter.)
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