Contractors - insulation question for upstairs

Blaze

Blaze

The Jeeper Reaper
Joined
Aug 9, 2005
Location
Wake Forest, NC
I'm in the process of finishing off the upstairs in my house, hopefully I'll pass the electrical inspection tomorrow and then I can start insulating.

I'm in Wake County, and have been looking at the NC Residential Building codes and just want to be sure I am buying the right insulation.

R15 for the walls, even the exterior walls. The ceiling needs to be R30c or R38. Floors were done when the house was built, so I'm hoping they don't make me worry about that because it is under the subflooring already.

This correct?
 
somebody may correct me but I don't think it's required for floors between living spaces anyway. They're pre-existing and grandfathered in anyhow.
 
Sounds like you are finishing an attic or otherwise unfinished space upstairs? That would be why there is insulation in the floor/ceiling assembly. Not necessary between finished floors, but since it is there it is no problem.

If you have the option, I would highly recommend insulating the roof, as opposed to the ceiling of the top floor with attic above. That, paired with soffit vents and a ridge vent, is much more energy efficient. If you don't want to/have to mess with what you already have, there is nothing really wrong with insulating the ceiling and leaving the attic unconditioned... just make sure you have a good vent fan up there with a thermostat.

As for code, it says what it says and the inspectors are usually pretty good at answering questions.
 
Yep, unfinished walkup.

I plan on insulating the roof for energy efficiency rather than just the ceiling, too. It really isn't that much more insulation to do it that way.
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Ah. So basically this was originally the "attic" and all the "attic insulation" is actual now in your floor? And the "walls' are actually the rood and now really the ceiling?
To maximize your insulation power on those diagonals you might want to look into the spray foam insulation, much higher R per inch in those cavities.
 
Insulation codes vary. Call the county or any local insulation company and they will tell you what you need to know.
Just FYI it is cheaper to have a company put it in than buying it at any box store
 
RatLabGuy said:
Ah. So basically this was originally the "attic" and all the "attic insulation" is actual now in your floor? And the "walls' are actually the rood and now really the ceiling?
To maximize your insulation power on those diagonals you might want to look into the spray foam insulation, much higher R per inch in those cavities.
Click to expand...

Yes, correct.

I'll call a few places to get a quote on installing insulation before doing it myself.
 
Insulation requirements are specified by IBC. You're in climate zone 4. Table N1102.1 specifies the requirements. It's either R30 continuous or R38 non-continuous. They consider it to be continuous if the full, uncompressed thickness of the insulation extends over the top plate at the eaves. If that isn't possible (usually isn't due to available space), then you must install R38 everywhere else. For reasons I'll discuss below, you might want to stick with R30. It might not be code, but it's also not as thick.

Cyd and I talked about your insulation location, and decided a couple of different things. For starters, your house isn't designed to have insulation installed against the roof sheathing. That should be avoided to prevent moisture build-up that could prematurely deteriorate the roof sheathing and shingles. If your roof has soffit vents and a ridge vent, you should be sure to have enough clear air space against the sheathing to allow for free air flow. There are actually calculations you can do to determine what this clearance should be, but I would shoot for at least 4" overall. If you have less at the eaves, that's fine. They actually make foam spacers for this purpose that slide down against the roof sheathing and leave a clear path for air to travel from the ridge vent. Given how little space in the rafters that you have to work with, the proper way to do it would be to frame out ceiling joists below the rafters or pad down to make the joist cavity thicker. You may not want to do this for head height reasons, but it is the "right" way. Padding down also contributes to thermal bridging, whereas a separate ceiling joist can have insulation installed on top of it.

I would also consider beefing up the wall insulation to R19. R15 is the code requirement for a wall that is exposed to the exterior, but what you really have is a wall that's exposed to an attic space that will be super heated (110-115F is easy on a pleasant day), making the temperature differential across the wall much higher.

It looks like the space has its own mechanical unit, so I wouldn't worry about the insulation between the attic and the rooms below. It won't hurt anything, and may reduce noise transmission.

You can also build up the insulation from one or more smaller units. Get an R15 batt that fits in a 3-1/2" stud cavity, then install R19 batts on top of the studs in the opposite direction, etc.

There are also an assortment of spray applied products. Open-cell and closed-cell polyurethane foams, sprayed cellulose, etc. We don't think any of them are appropriate here, given the risk of moisture build-up on the roof sheathing mentioned above.
 
Shawn, can you explain where the concern about moisture buildup comes from? This to me looks like any other attic, sheathing on top of rafters.
If you spray closed cell foam on the underside of the sheathing to fill the spaces, doesn't that also serve as a moisture barrier?

Couldn't you convert this over to a non-vented style "hot roof" roof that is just 100% sealed?
 
Closed cell polyurethane is expensive and watertight. So it costs a lot to get it thick enough. The big fear long term is that you have a nail pop, dry-rotten pipe boot, or something that allows for a small leak that wets the rafters and sheathing but never goes any father. So it slowly rots without you knowing about it. They're also subject to thermal bridging at the joists, so you have to consider wrapping the entire joist in foam as well. (Wood is R1 per inch).

But, if you're willing to pay the upfront cost, if you can get a continuous barrier with no air gaps, and if you can stomach the risk of explosion during installation, it's a pretty good product that isn't subject to the wetting concerns in cold weather. (That last paragraph in my previous post could have been better written.)

Open cell is not a vapor barrier, so it's subject to the same wetting concerns as fiberglass.
 
http://www.buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/

This is a really nice discussion of the condensation issues with a good example of how to solve it. I don't think this is applicable to Blaze's situation, but if the house is due for shingles soon anyway, it would work. You could easily add an inch of XPS and 3/8" sheathing over the existing roof sheathing. Just need to make sure it all gets mechanically fastened back to the roof joists.
 
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