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Adding Attic Insulation - Dealing with Existing Vapor Barrier


biking_brian's Avatar
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12-30-08, 08:22 PM   #1  
Adding Attic Insulation - Dealing with Existing Vapor Barrier

My single story house in the mountains of Southern California was built in the 1950s. Half of my unfinished attic is built with 2x4 joists, the other half with 2x6 joists. There are 3" thick rock wool batts in the 2x4 section. In the 2x6 section there are two 3" thick rock wool batts on top of each other.

R-38 is recommended in the attic - obviously, what I've got up there now isn't even close. So I bought a bunch of unfaced R-30 rolls, figuring I would just roll it on top of the old stuff, crosswise to the joists so I could cover them too.

Well, after I moved all 30+ of those rolls up into the attic, I noticed that the existing insulation has the vapor barrier is on the side facing the attic. In the 2x6 section, each of the two batts in the joist cavity has its vapor barrier facing upward. Some of the vapor barrier paper in the 2x4 section is a little beat up, probably due to past crawling around for remodeling. The vapor barrier paper in the 2x6 section is in pretty good shape.

None of the vapor barriers have been stapled to the joists, so I tried flipping one of the batts over so that the vapor barrier would be on the bottom. The paper is so fragile that it just disintegrates when you touch it, let along move it. Otherwise, the existing insulation is in decent shape. The climate here is dry enough year-round that I doubt there's been moisture trapped in the insulation.

I'd still like to leave the existing insulation alone and put the new unfaced R-30 on top. The question is what to do about a vapor barrier. Should I leave well enough alone and simply roll the new stuff on top. Or I staple a plastic vapor barrier on the joists and roll the new stuff on that?

 
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12-31-08, 05:47 AM   #2  
Hi Brian, when in doubt, leave it out. It sounds like what is there wouldn't qualify as a vb anyway, so leave it as is and put your unfaced over the top.

Almost. before you add the new insulation, it is important and saves on energy bills to seal any penetrations from the conditioned space into the attic. Metal flashing around the chimney, fire rated foam for vent pipes and electrical boxes and wiring. If you have any recessed lights in the ceiling to the attic that aren't IC rated and air sealed, they should be replaced, as insulation must be held back and they leak warm air into the attic.

Air sealing is an extra step that is a pain, but it is very important and has a good payback.

Make sure you don't block the eves/soffit area so you maintain your ventilation and you should be all set.

HNY
Bud

 
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12-31-08, 06:48 AM   #3  
I'm in agreement with Bud9051: a very important aspect of the attic floor insulation is to achieve a really good air seal at all penetrations from the conditioned spaces below, which is not something the people insulating 30 years ago were usually much concerned about. In addition you already have a reversed vapor retarder, if you encounter subfreezing conditions in the attic the dewpoint is going to be somewhere within the insulation, with liquid water condensing out within the insulation at that point.

An absolute minimum, if you are going to keep the existing insulation I would take a knife and slash the existing vapor retarder, and then insulate over that.

However I think you'd be better off removing the rock wool, installing a vapor retarder immediately above the conditioned space, and then re-insulating above that - I'm a big fan of the Icynene type materials as though they're more expensive initially if properly installed they inherently produce an extremely effective vapor retarder, to the point where it becomes a vapor barrier.

 
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12-31-08, 09:05 AM   #4  
Yes, there are a lot of gaps up there where air can flow. Good points about the air sealing - I was planning on doing that first even though I didn't mention it in my post. That will be relatively easy to do before I add new insulation. I'm glad the recessed lights I added were air-tight IC. The big culprit is the B-vent penetration with a huge gap around it. I already took some sheet metal to close most of the gap and got some firestopping caulk (expensive!) to seal between the sheet metal and the B-vent. I'll do the same for the water heater flue. Chimney is external, no problem there. I already bought fireblocking foam for the wire holes and the plumbing vent stacks. I'll caulk around the electrical boxes too. Thanks for the tips!

 
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12-31-08, 09:57 AM   #5  
Typically, when b-vent penetrates a ceiling you can't use a field fabricated fire stop, you have to use a stop UL listed for that component and application, which typically means you have to use a fire stop supplied by the manufacturer of that particular brand of b-vent. The manufacturer's name will be stamped every section of b-vent, you can give them a call and ask about the correct fire stop, at the same time ask to whether the use of insulation to seal the gap between the fire stop in the b-vent is allowed - typically, it's not.

 
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12-31-08, 11:23 AM   #6  
Brian - What'd you do with the sheet metal and caulk around the b-vents?
Would you say that you "flashed" them?

My plan is to replace my antique wall furnaces with a modern heat pump fairly soon, but apparently you can't DIY a heat pump and I'm at the mercy of the contractor. Whatever I do concerning my b-vents could be temporary in nature, but right now, they're what I've got and because they're oval, I had to special order sections to get them the proper height above my roof.

I'm not quickly finding an oval firestop at the online <A HREF="http://www.ventingpipe.com">merchant</A>, where I've ordered in the past (per Michael's caution) and none of my local stores stock oval, so if you don't mind me asking; What'd you do with that big gap around vent pipe?


(Disclaimer: My intention was to just box around them and leave an uninsulated area for a while, but everyone says to seal every hole and these represent pretty big, airflow-enabling gaps in my ceiling)

Thanks

 
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12-31-08, 04:10 PM   #7  
Yes, I "flashed" them, there's some good photos on pages 1.8 and 1.9 of this file:

http://www.energystar.gov/ia/partner...e_May_2008.pdf

It was quick and easy enough to get some sheet metal, trim it to cover most of the gap, and then caulk the remaining gap - so (especially if your install is temporary) I suggest doing that rather than trying to find an oval firestop.

 
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12-31-08, 05:12 PM   #8  
Thanks - I actually have plenty of the metal flashing from where I've been doing my roof and unless it's hardened, I have some of the special caulk from where I patched some cracks in my fireboxes. IOW: I should be good to go and the pictures help a lot. (I don't know that I would've thought of the tabs, all by myself)

Thanks and good luck with your project;
As for myself, I'm headed back to the attic...

 
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12-31-08, 05:29 PM   #9  
Posted By: Michael Thomas Typically, when b-vent penetrates a ceiling you can't use a field fabricated fire stop, you have to use a stop UL listed for that component and application, which typically means you have to use a fire stop supplied by the manufacturer of that particular brand of b-vent. The manufacturer's name will be stamped every section of b-vent, you can give them a call and ask about the correct fire stop, at the same time ask to whether the use of insulation to seal the gap between the fire stop in the b-vent is allowed - typically, it's not.
Hmmm, I didn't read this until after I wrote my reply above. Oh my, that B-vent was probably added when the floor furnace was installed - which was probably in the late 1960s when natural gas service came to town. So I'm not sure if that manufacturer would be in business or not. Also, the framing around the penetration was a bit unusual - I don't remember the details, but I believe it was near a top plate. So my recollection is that only something that was field fabricated would work.

 
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01-01-09, 05:01 AM   #10  
The building codes require that the b-vent be installed per the manufacturer's instructions, the b-vent manufacturers instructions require a UL listed fire stop, a field fabricated fire stop is not UL listed - as I understand it you can create your own insulation dam, but not your own fire stop.

This is from the Simpson design and installation guide, but every manufacturer says something similar:


Fig. 1

BTW, When people think about fire stops and supports for vent pipes most are thinking almost entirely in terms of fire stopping, but the manufacturers of the listed stops are also thinking about controlling excessive thermal bridging, especially when you have the b-vent passing through and unconditioned space like an attic. The reason b-vent is required in unconditioned spaces is to prevent excessive condensation of the exhaust gases that pass through it, if you have excessive thermal bridging whatever the b-vent is attached to acts as a heat sink, and you can get excessive condensation at this location.

This isn't just a, theoretical concern, for example last week I did a moisture intrusion inspection where the IR camera detected a cool spot on the second floor ceiling , but were moisture meter detected no excessive moisture at the ceiling.

Up in the attic we found a section of vent pipe which was attached to a metal chimney chase extending about above the roof.


Fig 2

The entire assembly was cooling down to a point where exhaust gases were condensing within the vent, running down to an elbow, exiting there and soaking the insulation below - the reason the occupants were not seeing liquid water on the ceiling was because of a foil vapor barrier on top of the drywall, however there was a little lake of water up there under the insulation adding moisture the attic.


Fig. 3

This was a pretty extreme example, but a good illustration of the principle.


Last edited by Michael Thomas; 01-01-09 at 05:22 AM.
 
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01-01-09, 09:23 AM   #11  
What you're saying about the code requiring the B-vent to be installed per manufacturers instructions and that the manufacturers require a UL listed firestop makes sense - but the advise in that design guide I quoted seems to be in conflict. Perhaps for existing installations where there is no existing firestop, the manufacturers instructions at the time didn't require a UL listed firestop, so I'd be OK per code with a field fabricated firestop? That may be something for me to check with the AHJ.

Regardless of whether or not a field fab stop is allowed, you have a good point about thermal bridging. Since a DIYer like myself can't easily cut a perfect oval in sheet metal, I'll always have a small gap between the stop and the B-vent that needs to be filled with the firestopping caulk. I would guess that the thermal conductivity of the caulk would be less than that of the sheet metal, perhaps making thermal bridging less of a concern. Furthermore, I would think that you could also try to keep the amount of sheet metal used to a minimum to provide less of a heat sink.

 
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01-02-09, 07:05 AM   #12  
(hopefully it's not too late to ask a ? about attic insulation)


BACKGROUND: we're buying a 1950 contempory-house. It currently has ~3" of attic-insulation installed with the paper-side facing UP... (this paper side is rather dry & brittle & cracks easily when moved). You access the attic by lifting a 2X4 bathroom closet-top.

QUESTION: Rather than struggling with removing all the old insulation thru this small-hatch... IS THIS A GOOD-PLAN: "flip" the old-insulation & install un-faced on top? (I'll also plan to chaulk & fill "air-gaps" I come across)

 
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