Fiberglass or Cellulose?
I am going to build a house in zone 3B, hot and dry, Southern New Mexico. I am leaning toward using fiberglass insulation even though I know cellulose is from recycled materials and has a lower embodied energy. My reasoning follows. Please point out anything I am missing or wrong about.
There shouldn’t be a great deal of difference in cost between the blown fiberglass and cellulose materials. Moisture vapor is so minimal in zone 3B it is almost not a design consideration. We get 5-12″ of rain per year, avg about 6-8″ over the last decade. Large rain events over 2″ are rare and commonly years in between. Like West TX our desert washes don’t have trees and most of Arizona’s hot desert does… It is very dry here but that explains why a lot of these old poorly built houses are still standing.
Over the ceiling (vented roof) I want loosepack fiberglass because it will allow smaller rain events (from a roof leak) to reach the sheetrock rather than being absorbed and redistributed in cellulose. I know this may sound a strange but 20-40 years from now when nobody is paying attention and the roof begins to leak (or sooner if a panel blows up in a storm) a sheetrock wet spot on the ceiling will provide a cheap early warning system.
Loose fill fiberglass on an r-value/weight basis is about twice as efficient as cellulose so there will be less weight on the ceiling sheetrock.
Finally, from what I have read fiberglass will not compact as much (even excluding the first 3 months) over decades of time as cellulose and will retain its r-value better.
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The exterior walls, will be double stud 2×4 aligned inner and outer at 16″ o.c.. (to simplify framing and insulating). This will create dense-fiber packing bays 16″x”9-10″ which are not much larger than 2×6 @ 24″oc. I intend to pack the walls myself using a lower powered machine borrowed from a ‘box store’. I have already discussed this with several knowledgeable people.
https://www.greenbuildingadvisor.com/question/dense-packing-cellulose-in-a-wider-wall-with-a-low-powered-blowing-machine
It seems like densepack fiberglass should be easier to pack than cellulose when using a lower powered machine since it weighs about half as much per r-value eg, the airstream will be carrying only half as much weight at a given r-value application rate. But, fiberglass fibers could be heavier/cross section and not float as well in the airstream or the springiness and/or shape of the individual fiberglass fibers could inhibit packing a great deal more than cellulose fibers. There could be other things too affecting the ease of packing.
I would sure like to hear from anybody who is knowledgeable about how easily fiberglass fibers densepack in comparison to cellulose fibers?
All thoughts and insights will be appreciated. Thanks!
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Cellulose R values as published in the data sheets assume it's compacted, so it doesn't really degrade. I hadn't really thought much about the weight difference before, but that's probably the biggest issue aside from water retention between the two materials. It's true that fiberglass can "dry out" better than cellulose if it gets wet, but I've never liked the "let a leak come through" argument for ANY type of insulation. I've heard people use that as a reason to not use spray foam too -- "I want to see a roof leak". Just maintain the roof and don't have the leak in the first place, which is the best option all around.
Note that cellulose is better at resisting air movement if you have any air leaks too.
I can't really offer much as to differences between the two in regards to dense packing. I've never made any comparisons there myself.
Bill
Hi Zephyr7,
You said, "This is actually not a bad idea at all, but I don't see RFIDs being able to work for this. I've worked with those before, and they barely have enough power to do what they do, which is basically to send out ("send" being a strong word here) a serial number. These tags are only powered when they're queried too, which precludes any type of alarming."
Here is a link: https://www.atlasrfidstore.com/rfid-insider/active-rfid-vs-passive-rfid
It says, "Passive RFID tags do not all operate at the same frequency. There are three main frequencies within which passive RFID tags operate. The frequency range, along with other factors, strongly determines the read range, attachment materials, and application options.
125 - 134 KHz – Low Frequency (LF) – An extremely long wavelength with usually a short read range of about 1 - 10 centimeters. This frequency is typically used with animal tracking because it is not affected much by water or metal.
13.56 MHz – High Frequency (HF) & Near-Field Communication (NFC) – A medium wavelength with a typical read range of about 1 centimeter up to 1 meter. This frequency is used with data transmissions, access control applications, DVD kiosks, and passport security – applications that do not require a long read range.
865 - 960 MHz – Ultra High Frequency (UHF) – A short, high-energy wavelength of about a one meter which translates to long read range. Passive UHF tags can be read from an average distance of about 5 - 6 meters, but larger UHF tags can achieve up to 30+ meters of read range in ideal conditions. This frequency is typically used with race timing, IT asset tracking, file tracking, and laundry management as all these applications typically need more than a meter of read range.
The technology is here, note the range for NFC passive, that would work with a phone, even without a battery. I am pretty sure that this could be engineered and sold cheaply.
Even if the devices had to have a small watch battery, those are cheap too, and I suspect the powered system could last 10 years... and could be replaced the next time you painted, poked though a 1" or smaller hole into the sheetrock. Or even pull it out of the wall, replace the watch battery and slide it back it. This works, it just needs somebody to implement it. I am not an EE, I have plenty of money, and I prefer pursuing my own interests, but I did get a full ride to graduate engineering school at Illinois to work on statistical design. I do try to keep up with reality, with what is going on, and with what is possible.
Hi Bill,
You said, "I can't really offer much as to differences between the two", and I agree. If there was only the one choice I am sure I would be fine with it. But since I have to choose, I do like the idea of a leak getting down into the sheetrock quickly rather than turning the cellulose into a sponge I am unaware of.
SierraWayfarer, I think it's probably a mistake to use fiberglass for the roof insulation. Besides being more "green" cellulose will also shield you from heat coming from the attic much better than fiberglass. There's a lot of anecdotal evidence that this is true. Several people over the years on this forum have complained about very hot houses in summer in warmer climates with fiberglass in the attic. The consensus view is that non dense packed cellulose stops air movement much better than the equivalent fiberglass.
Eric,
Yeah, the non-dense pack might allow for a lot more air circulation. I will look into that but I bet you are right. I can't wait to call Corning and not get a straight answer...
Thanks!
Hi Eric,
I will go with the loose pack cellulose in the attic. I can't deal with truss heel heights almost 1/2 again higher caused by using loose pack fiberglass instead of loose pack cellulose. Further, the concern on the web with loose pack attic fiber glass and convective heat loss is still pervasive and I didn't find Johns Manville or Owens Corning fighting it very hard and their mouthpiece organization's argument's strike me as disingenuous.
I am still concerned about moisture buildup in loose pack cellulose that would cause mold growth in the nearly year around warm areas along the bottom cords of trusses and at the plates if the roof developed undetected leaks.
I will try to use a sensor system for the attic cellulose. An idea that immediately comes to mind is using a hay bale moisture probe ($250) and a few plastic capped dimples in the ceiling sheetrock. I could insert the moisture probe through the sheetrock dimples into the bottom of the cellulose after extended rain periods.
I will use dense pack fiberglass in the walls and hope the sheetrock will show water stains that will inform me of wall leaks.
Thanks for your 'lookout' on loose pack fiberglass in the attic.
I'm glad you're going that route. I imagine New Mexico can get pretty hot. That's interesting what you're doing for moisture detection. Keep us informed how it goes.
It would have always been an "experiment" to try to dense pack cellulose with a low power blower for an 8" wall. It might have worked but it also might not have. I appreciate the fact that you even considered it. Most people wouldn't have.
HI Eric,
New Mexico is weird, the Land of Enchanment is not too much of an exaggeration. Hot and cold. The desert, everywhere, drops temperatures rapidly at sundown because of the low humidity. The average daily temperature change in Southern New Mexico and West Texas is greater than any other region of the nation and pretty much the world.
I love it here but, it is different... For instance: a typical city dweller would be ASTONISHED (not an exaggeration, i was) by the stars he sees in our mountains at the right spot (a bowl) and when weather conditions are right. A not uncommon weather condition or location. The next four nights will be perfect for star gazing according to the weather I read this morning.
I have been googling on loose pack fiberglass and reduced r-values and found this by Martin Holliday. It mentions problems at very cold temperatures but nothing about hot temperature reductions in r-value. It straightened me out on loose pack fiberglass having a lower r-value than cellulose. I would need very tall heels on the trusses at the walls. Definitely a concern! https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Blown%20Insulation%20for%20Attics.pdf
Wanting an earlier detection system in the event of a leak is a valid concern, especially if it's a small leak that takes a long time to detect. But if you have a vapor retarder in the ceiling it will still take a long time to see any leaks, and they won't show up where the problem is.
I've done up to R-140 in roofs with cellulose and have not seen any problems with drywall sagging.
Even heavily compressed cellulose still insulates at least as well as fiberglass. (Loose-blown fiberglass can be anywhere from R-2.2/in to R-3.5/in. Cellulose is R-3.5-3.8/in.)
Hi Michael,
An early roof/wall leak moisture detection system. Like a hay-bale moisture probe. Bunch of cute little 'artistic dimples' in the sheetrock that I could pop a cap loose on and insert the moisture probe, after a period of rain. I have no idea if anything like that exist.
Moisture is the biggest bugaboo in all structues. Inexpensive moisture sensors thoughout walls and ceilings would make sense.
At first, your idea sounds funny, like having a pop-up indicator that your turkey is done cooking. But it's actually pretty smart. As our friends at Green Architects Lounge like to say, it could be a six-digit idea! If you want to email me I can connect you with a company I know that is always looking for innovative ideas for building materials and will compensate fairly. (michael at michaelmaines.com.)
Hi Michael,
Feel free to any idea I post. I would be flattered.
Give them your idea, its better than mine, and it is already mostly engineered. The pop out on a turkey is caused by its internal pressure (of a gas) reaching a certain level that corresponds to the 'done temperature' of the turkey breast and then the pressure blows through a seal and pushes out the plastic rod. All we need is for that seal to degrade at a certain moisture level rather than pressure level. It would be a good senior design project for a college material engineering school, finding that material that degrades at the right moisture level.
The best way that I have thought of (and looked for but didn't find) would be with a RF (radio frequency) tag attached to a ceramic moisture censor. The same sort of tag that is attached to clothing to keep shop lifters from stealing, that they take off the clothing at checkout. The censor would be powered instantaneously (and only then) by a local RF signal/charge sent from a nearby querying smart phone or pc. We could walk around the house and check the moisture level of our insulation with our phone or schedule our pc to do it automatically. We already have the technology. Ceramic moisture sensors and the RF tags are cheap. They only need to be married up.
I am surprised that some company hasn't already started marketing this. It would be a multi-billion dollar market worldwide, in every wall and ceiling. Add a heat sensor and you have a very early fire warning system. IOT, the internet of things, is coming. With the heat sensor a homeowner warned by a shrilling computer might be able to stop the fire in its infancy. The code should and probably will require something like this fairly so0n. They should be pretty cheap, a couple of bucks each, or less. Again, a good senior engineering design project, this time for EE students.
Till then, I will do it the hard way. I don't mind, anything that gets me off my ass in a reasonable fashion is a very good thing! Though it is irritating that this doesn't already exist, to serve me! That IS the job of the world, to serve me! Ha ha! Just so you are certain, that poor humor was directed at myself.
This is actually not a bad idea at all, but I don't see RFIDs being able to work for this. I've worked with those before, and they barely have enough power to do what they do, which is basically to send out ("send" being a strong word here) a serial number. These tags are only powered when they're queried too, which precludes any type of alarming.
A monitored system would be much more useful, something that could alarm when a problem is detected. That means a system that can either automatically scan multiple, addressible (so you know where they are) sensors, or a cable type of system that can sense along a cable.
Here is the interesting part: the cable type systems already exist, but are very expensive. They are somewhat commonly used in datacenters. The addressible type system may be more interesting here, because I've already developed such a system and have a functional design and prototype parts. My system was developed to sense, monitor, and alarm for temperature though -- so that we could do heat maps to see thermal gradients in a datacenter facility -- but I did allow for moisture and humidity sensing in the modules. It uses a simple two-wire bus that can be thousands of feet long (I tested up to about 4,000 feet), and can address 32 sensors per bus, a number which is easily expandable with a software change.
If there is some real interest, I could make a few small changes to the design to optimize it for moisture sensing purposes by removing the temperature sensing. The temperature sensing and humidity sensing elements are actually the most expensive parts on the sensor boards, so a system optimized to sense only moisture would be cheaper to implement. The sensor boards are pretty small, about 1.25" square, designed to fit in small plastic boxes but that could be changed.
Sooo... Is it worth investing the time to make this production level for moisture sensing in walls and attics?
I added a pic of a different, but similar sensor I designed some years ago. This one used a more complex network/wiring scheme, but it's the same physical size board.
Bill
Another consideration for the walls: it takes skill and practice to install dense pack cellulose in a wall, particularly a deep wall. Finding a contractor with those skills is easier in some areas than others. So finding out the skills available in your area is a good first step. If the best installation contractor you can find prefers working with fiberglass, let them do their thing.
Another consideration in general: for reasons I don't really understand, mice seem to like fiberglass better than cellulose. I'm not sure whether that's an issue in your area.
Hi Charlie, thanks for your thoughts.
I think I can do a densepack provided I can get even a marginally suitable machine. We discussed that quite a bit in the link I provided at the top of this post.
Mice seem to be more of a cooler climate problem (coming in for the winter) but there is always a possibility.
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.
According to the charts and graphs in this bit of marketing fluff from a German fiberboard insulation manufacturer, at a given R-value the thermal diffusivity of fiberglass is about 4x that of cellulose, (though the diffusivity of cellulose is about 2x that of fiberboard) which is enough to make a real difference in a hot dry climate:
https://gutex.de/en/product-range/product-properties/insulation-in-summer/
With R38+ cellulose in the attic the peak ceiling temp lags the peak roof/attic temperature by hours, lowering the peak and average cooling load while increasing comfort levels.
In walls the diffusivity and IR opacity effects aren't as important as in attics- there is less thickness & mass to work with, an the peak temperatures of the siding are lower than than those of roofs. But the higher resistance to air flow still matters. In a zone 3B location 3lbs per cubic foot is sufficient to prevent settling over time, and even 2.5lbs could take decades. Even though 1.0lbs density fiberglass won't settle over time (R13 in a 2x4 wall, the same as cellulose) , it's more of an air filter than an air retarder. To hit the same or better air retardency as 3lbs cellulose with blown fiberglass takes ~1.8lbs density. At 1.8ls it will hit R15- slightly higher than 3lbs cellulose, but not necessarily a higher performer during the cooling season after thermal mass effects are factored in.
I'm not convinced it will be any easier to hit 1.8 lbs density with fiberglass than 3lbs with cellulose with a single stage rental blower, but I haven't really tried. (I can hit ~3lbs cellulose with single stage blower pretty consistently.) It should be dead-easy to hit 1.0lbs with fiberglass or 2.5lbs with cellulose if R-value is your primary goal here. But at those densities the lower density cellulose is FAR more air retardent.
The borate fire retardents in cellulose is also pretty effective at mitigating against termites and other wood boring insects (not that you have any of those species in NM :-) )
Since this is a house not yet built, detailing the wall sheathing as the primary air barrier, and using cellulose BATTS would deliver most of the benefits of blown cellulose when installed at high quality level.
BTW: Since 2012 for zone 3B the IRC calls out R13 + R5 continuous insulation if going with 2x4 framing, alternatively R20 in 2x6 framing. The 2x6/R20 approach is usually cost effective on energy costs over just a few decades, even if local codes haven't been bumped that high yet. The 2x4/R13 + R5 c.i. approach has a longer "pay off" but will often result in a somewhat more moisture resilient house.
Hi Dana, thanks for your insights. I started to respond immediately but your posts are generally so information dense, especially with terminology, that it takes me a while to study my way through them.
YOU EXPLAINED:
The much greater translucency of fiberglass to infrared in comparison to cellulose explains in part of why fiberglass underperforms in hot attics. I see that. Then the thermal diffusivity of fiberglass (sort of like glass conducting heat much better than wood which is obvious if you have ever pulled the end of a burning wood ember from a fire as oppose to trying to pull out a glass bottle, ouch!) being about 4x greater than cellulose further explains why cellulose greatly outperforms fiberglass in hot attics. These facts and the fact that loose pack fiberglass is much more prone to convective heat loses in winter have definitely decided me for cellulose in the attic.
I am concerned with cellulose holding moisture along roof truss bottom cords in houses with roof leaks, especially slow leaks that go a long time without being discovered. The temperature along the bottom cords of trusses covered in cellulose insulation will be ideal for mold. I am not sure the borate in the cellulose will stop the mold in the truss, especially between the ceiling and the truss where the borate wont reach. I believe I can implement a cost effective moisture sensor system for attic cellulose insulation.
On dense packing the walls with fiberglass and a single stage blower: I am pretty handy and I will get it packed as tightly as possible. If I have to add loose fill to the tops of walls in later years I will. Fiberglass in the wall will allow water leaks to drain to the bottom and stain the wallboard and inform the owner of a problem. I think the walls will be aligned double 2x4s at 10" o.c. so they will have a pretty high r-value for this climate regardless of how well I am able to pack them.
I will probably add a small amount of thermal bridging in the walls for increased out of plane shear (maybe some plywood scrap). I am trying to build with the future in mind. Double stud walls without rot will provide lots of options for the future. I think we should try to build longer lasting houses. I think building to facilitate future rebuilds is part of building green. Fiberglass in the wall will allow water leaks to reach the wall board and inform the owner with stains in the wallboard.
Thanks again Dana for explaining why fiberglass under performs in hot attics.