Attic insulation — existing
I have a 1800 sq. ft. ranch home built in 1969. Climate zone 5. Existing attic floor/ceiling area has a layer of 3″ fiberglass insulation with vapor retarder facing the heated space. The second layer of insulation is a 3″ layer of fiberglass with vapor retarder facing the attic area.
Both layers have been installed inside the joist bays and stuffed very tightly at the top of wall/ceiling intersection-no soffit vents in place. The roof has box vents and gable vents in place for ventilation.
I want to add a layer of unfaced R-30 over the top of the existing joist bays to help control thermal bridging and heat loss through this minimum insulation layer already in place. i was also thinking of adding a layer of 1/2″ fanfold insulation and then the unfaced batts on top of the fanfold. any thoughts or assistance would be appreciated thanks
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First you need to remove the vapor barrier that's on top and then install un-faced over the top of whats there. If the old insulation has odor, remove it and start over. It's less costly to hire a professionally trained insulator to perform this task than it would be for you to make a mistake and start over. Your time is worth money to. Save it and hire a pro.
Here's a great piece of reading material to help you along... http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ba_airsealing_report.pdf
Bob,
I'm assuming that the facing on the fiberglass batts is kraft facing. Kraft facing is a "smart" vapor retarder with variable permeance. It rarely contributes to moisture problems, even when it it facing the wrong way. So if your existing attic insulation is dry, I don't think (unlike Richard) that there is any reason to remove the kraft facing on the top of your existing insulation.
The problem area in your attic is near the eaves, where you don't have enough clearance between the top plates of your walls and the underside of your roof sheathing for an adequate thickness of insulation. These areas of thin insulation are often responsible for ice dams.
If you have a history of ice dams, or if you want to do a better job than most insulation contractors, these areas with reduced clearance can be insulated with spray foam, using a two-component spray foam kit that you can buy at a lumber yard.
Before insulating these areas, make sure that you install ventilation baffles to connect the soffit with the air in the attic above your insulation. AccuVent is a brand of ventilation baffle that is fairly stiff.
Once this work is done, you can add R-30 insulation above the existing insulation layer if you want. The best insulation for this purpose is cellulose, since cellulose fills small gaps and reduces convection through the insulation. However, if for some reason you don't want to install cellulose, you can install unfaced fiberglass batts above your existing insulation.
Useful information on the work I described can be found in GBA's 10-part video series, How to Air-Seal an Attic.
Martin,
What your saying it's ok to have 2 layers of Kraft paper as described? One on the bottom facing the heated room and one over the top facing the existing fiberglass facing the cold attic (OREO Cookie scenario) and then add more fiberglass above the second vapor barrier?
Can you kindly provide something you may have in your library of awesome advice to support this theory?
I could not find anything to this nature published by any major fiberglass manufacturer. Wouldn't this be classified as a Grade 3 installation? I found these installation procedures, none of them make any representation of this Oreo cookie scenario.
Installation Guide.. http://www.certainteed.com/resources/30-49-109%20Insulation%20Guide.pdf
and http://www2.owenscorning.com/around/insulation/fallpromo/homeownersguidetoinsulating.pdf
https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/batt-insulation-still-making-me-batty
Richard,
If you search "kraft variable permeance" on this site, you can find some previous articles on this general topic.
Richard,
As I already explained, kraft paper is a smart vapor retarder. If it ever gets damp, its vapor permeance increases.
The vapor permeance of the kraft layer is almost irrelevant, however. If Bob installs R-30 insulation on top of the kraft layer, it will be kept warm -- warm enough to avoid any worries about condensation.
Clearly, if this kraft is a problem, the uppermost facing should be soaking wet. If the kraft facing is now dry, it will only be dryer -- not wetter -- after Bob adds more insulation on top.
The vapor permeance of asphalted kraft facers may be about 0.4-0.6 perms when dry, but as the paper adsorbs moisture it goes north of 5 perms long before beads of liquid moisture would become evident. It's not a moisture trap for moisture diffusion.
I suppose with big enough air-leak it could be a challenge, but even unfaced batts can have moisture issues with big air leaks.
Fan-fold XPS typically has 0.6-0.8 perm facers that are NOT variable-permeance. In a zone 5 location it's important that the insulation on the exterior of the fan-fold be at least 40% of the total R. It sounds like you have about R20-R22 of existing batts, and you're putting R30 over that, so you'll have more than 50% of the R on the top side, so sure, go ahead and use the fan-fold goods if you like. If you detail it as an air-barrier you'll limit the overall air flow (convective or infiltration), reducing the condensation issues.
As Martin points out, when you insulate over it, the krafts facers will remain warmer & drier, which will put them in their lower permeance condition, further limiting the rate of moisture diffusion through the assembly.
Thanks for the explanation Dana. I had a conversation with a fiberglass professional today and he to stated the second vapor barrier should be removed because it does not meet the published standard.
Martin, I was seeking manufacturer data, not an opinion. Thanks anyway.
"I was seeking manufacturer data, not an opinion."
Here's a 2- page flyer version, not the full-press science data, but with footnote source references to the test results cited:
http://www2.owenscorning.com/literature/pdfs/10017860%20Moisture%20Control%20Perform%20Kraft-Asphalt%20Facing%20Tech%20Bulletin.pdf
Quoting from the flyer starting at the last paragraph on page 1:
"Kraft-asphalt is one such
“smart” material. Testing per
E96 shows its permeance
increases significantly as relative
humidity increases, thus
improving its ability to allow
excess moisture flow. The
2005 edition of the ASHRAE
Handbook of Fundamentals
lists “Blanket thermal insulation
backup paper, asphalt coated”
as having a water vapor
permeance range from 0.4
to 4.2
2
. Independent testing,
under the auspices of the
National Institute for Science
& Technology, found a
similar range in water vapor
permeance
3. "
References at the bottom of the flyer:
1 ICC Intl. Residential Code (IRC) Section R318 (2006), “Moisture Vapor Retarders”, and ICC Intl. energy Conservation Code (IEC
C) Section 402.5,
“Moisture Control”.
2 American Society of Heating, Refrigerating and Air-Conditioning Engineers; 2005 Fundamentals Handbook; Chapter 25, Table 7B.
3 Burch, D.M., Thomas, W.C., and Fanney, A.H., “Water vapor permeability measurements of common building materials,” ASHRAE Tra
nsactions,
v 98(2), 1992, p 486-494
That's good 'nuff for the kind o' girlz I go with- YMMV ! :-)
If you need actual data you'll have to dig deeper, look up the source papers on it.