Cellulose vs. Fiberglass Insulation in Vented Attic CZ3
randy_j
| Posted in General Questions on
A little background for my house nearing completion. 3000 sq/ft ranch over an unfinished basement. All HVAC and plumbing is located in the basement. My builder generally uses blown fiberglass for vented roofs such as mine. In my case I’ve specked R50. I’ve requested the possibility of using cellulose instead as it’s more environmentally friendly. The GC raised concerns about being in a hot humid climate (SC) . Looking for input from the experts. I did come across this post where Dana makes some excellent points in a dry zone 3. https://www.greenbuildingadvisor.com/question/densepack-and-loosepack-fiberglass-or-cellulose Regards Randy
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Replies
Randy,
I tried to find a few reputable sources on the equilibrium moisture content (EMC) of cellulose vs relative humidity (RH), and I came across this older paper that might be of some use.
https://web.ornl.gov/sci/buildings/conf-archive/1998%20B7%20papers/015_Rose.pdf
You'll want to look at sasmple 3, cellulose with no vapor barrier, as that's pretty close to what blown in the attic would represent. Their study was looking at walls, but I think the same conclusions apply.
For a 40% interior RH (temperature at 0F, winter conditions), their cellulose wall reached an EMC of about 13%, which is far below what would cause problems. Now that was for an interior wall, where your attic is essentially exterior, at somewhat elevated temperatures.
With a fixed moisture content in the air, assuming you have a vented attic, the attic space will always have a lower RH than the outside air around it. I don't think you'll have a problem with condensation, or any problems with mold, fwiw.
On the other hand, fiberglass and cellulose are so closely related thermally that there's not practically any difference between them, in terms of their performance. If you had to choose one today, I don't think you'd be unhappy about fiberglass.
WE see a great amount of data about how much moisture cellulose will
hold (so called "Buffering") in these articles but almost nothing on
the real and more important question. What if any change occurs in
the thermal resistance of the cellulose as the moisture content goes up?
What's the R-value at 39% as opposed to 50%, 65%, etc. ?
gstan,
I have seen multiple references that show a large decline in R value for cellulose as the moisture content increases. If you google it, you'll find them. I don't have any saved back for that purpose at the moment.
Read the posts to a few sites like this look for posts about people having problem with cellulose insulation. I only vaguely recall one and the OP never replied.
We don’t see complains about any of the fluffy insulation. Buy whatever fluffy insulation that gives you the most Rs value per dollar.
If celluloses R value did fall due normal amounts of moisture the fiberglass marketing departments would be very vocal in spreading the word.
Walta
I have browsed other sites but they have almost exclusively been for colder climate zones then mine. Perhaps I missed some so will keep digging.
Thanks
WALTA: I'm not so sure about the fiberglass people having anything to say
about declining R-value due to increasing moisture content - I have yet to see
any data on blown in fiberglass R-values when the moisture content gets
high - perhaps both of them are vulnerable under these conditions!
gstan,
Unlike cellulose fiberglass won't take on moisture from the air. So unless it get saturated by a leak, it should maintain it's R-value pretty well.
Blown fiberglass is more susceptible to wind-washing though.
MALCOLM: You may be correct but, It's my impression that fiberglass
won't absorb (cellulose will) but it will adsorb -
the question is how much and with what effect on R-value.
Can't find any hard data on either of them - are the manufactures
hiding it or have there never been any studies?
gstan,
Fiberglass will adsorb, or become saturated, but only in circumstances which represent a problem in the wall or roof. You don't find damp fibreglass in assembles that are functioning as they should. Cellulose however will take on moisture in what we would think of as normal predicted circumstances, and attempt to equalize itself with the RH in the surrounding air - much as all the wood-based products around it do.
Thanks for the reply’s so far guys, cost is about the same. The GC/installer said he would do it, I’ve just been un able to find a good solid answer that says cellulose or fiberglass is the right choice in my climate. Perhaps there isn’t a right answer in the end.
I forgot to mention the roof was sheathed with a radiant barrier on the back side (attic side) as that’s pretty common practice down here. In the thread linked above Dana Dorset states:
“ In the attic, the higher opacity of cellulose to infra-red, along with it's much higher air retardency at open blown density, and the lower thermal diffusivity (a thermal mass effect) make cellulose hand's down the better performer at any R-value.
Fiberglass is partially translucent to infra red radiation, and when there is a 130F+ roof deck radiating down the temperature an inch into the fiberglass can be higher than the attic's air temperature(!), so you're insulating at a higher temp with an inch less of insulation.”
This really has me wanting to use cellulose but am obviously concerned with out humid summers, not so much for the cellulose itself as it’s treated with borax but more the wood that’s it’s touching. Will it hold enough moisture to cause problems to the ceiling wood members. I did strap this ceiling north east style so I could make a good transition from the external zip R6 to the Sheetrock to isolate my building envelope.
Thanks
randy_j: Sorry we got so far away from your question - my fault I think.
To get back on topic, cellulose is probably the better choice for vented
attics because of its much higher resistance to convection as well as
radiation. Loss of R-value (per inch) due to moisture retention being
unknown for both cellulose and fiberglass it seems logical to pick
cellulose (just add more if you have the space and it worries you) -
I have actually seen fiberglass batts with cellulose blown over them -
don't know that I would recommend it but can't think of any real
downside either. GOOD LUCK!
Cellulose is more resistant to convection and more able to help "seal" a leaky surface if you haven't done a good job of air sealing before insulating, since cellulose is more resistant to airflow in general.
In my own view, I go with loose fill cellulose for most jobs, but use loose fill fiberglass where sagging ceilings is a concern. Fiberglass is lighter weight per unit R value compared to cellulose. Aside from the additional physical weight of cellolose, I'm not aware of any other downsides to the product. I did look into the potential for moisture accumulation (I'm in an area that has very high summertime humidity levels, but still CZ5, so colder on average then your CZ), and I didn't really find any data as to R value variations with moisture content, or any problems with the material related to high average humidity levels.
Bill
Installers like blown glass, but it has nothing to do with what is "better". They like it because the newer product is light, doesn't itch, and has very low dust. The last insulation installer I used for spray foam no longer offers cellulose as an option, although they had previously.
Cellulose is my preference 100% for the reasons Bill mentioned. It's also a far better noise dampener for the same reasons (density and reduced air movement) if you live on a busy street. If you compare STC wall ratings using fibreglass vs cellulose, you'll see about a 5db difference on a typical stud wall, which would "sound" to your ears 30-35% quieter. That difference would be considerably larger for attics where we're adding R60-80.
Moisture drive in a hot attic would be to the attic space so unless you have a roof leak, I would not worry at all about RH.
I just had my attic air sealed and had blown in fiberglass. I'm in central Texas, hot and very humid. The attic upgrade included removal of all cellulose insulation, extensive air sealing of all penetrations, installation of radiant barrier, and finally R-49 of blown cellulose. This is a 45 yo house and had several places where we had to re-attach the sheetrock to the joists. in one place the gap was 3 inches. My installer no longer installs cellulose due to what he believes is a problem clumping of cellulose as the very long humid summers (and springs and falls) make the cellulose less effective over time. I will say that as the ice storm we just lived through, we lost power for 3 days in freezing and near freezing temperatures and my house never got below 60.