Insulation and Mass of Papercrete

Kelly Hart, who is your host at greenhomebuilding.com, has been involved with papercrete from the early days of its popularization. He included interviews with papercrete pioneers Mike McCain, Eric Patterson and Sean Sands in the video he produced: A Sampler of Alternative Homes: Approaching Sustainable Architecture. He also chronicled his own use of this amazing material in his video: Building with Bags: How We Made Our Experimental Earthbag/Papercrete House. Kelly has built and used both an electric barrel mixer, and a McCain-designed tow mixer. His house is plastered inside and out with papercrete and can be seen here. He can speak from his experience with this novel stuff, and is frank about both its pros and it cons.

Questions and Answers

C: I was involved in the 2001 Betec symposium and workshop at El Rito, NM and presented the PC portion. The Dept of Energy wanted to know the R value of papercrete and I poured some samples to be tested at Oak Ridge Nat Lab. and the results came bach at 2.25 per inch of insulation. I also had a compression test done in a lab in Farmington NM and the results are that they tested out at 2780 psi.

Q: We are planning a radiant floor with a hot air supply. We are planning one main channel that will be fed by a solar design similar to M. Reynolds solar toilet, basically a solar oven. We are planning to use papercrete as our flooring with a vapor barrier of 6 mil and inch or two of sand and 6-7 inches of papercrete with tile embedded into papercrete. We are planning to make a heavy mix with more sand and cement. The channel will be 7-8 inches wide and corrugated metal will bridge perpendicular to help transfer heat laterally.

A: My gut feeling is that papercrete is not the best material to use for this purpose, even with quite a bit of sand and cement in it. Radiant floor systems rely on direct transfer of heat through a good thermal mass material, such as concrete or adobe; the use of papercrete would likely inhibit the flow and storage of the heat. So, unless I misunderstand your intentions, I wouldn't advise it.

Q: Do you perhaps know if thermal insulation tests have actually been done on papercrete? I often read of R-values of between 2 and 3 per inch - where do these values come from?

A: I have heard the same, and the source for that information is a bit hazy, but I remember Mike McCain, one of the original papercrete guys saying about 2.5 per inch, and I think he had run some sort of test to determine this. The composition of the papercrete would certainly make a difference in this value, depending on how much mineral material is mixed in; the pure paper mix would have the best insulation value.

Q: I have been reading abut the discussions regarding papercrete (with cement), and padobe (paper and soil). Has anyone created padobe blocks with a hydraulic press using a machine similar to a press used to create rammed earth blocks?

A: Yes this has been tried, but the results have not been great, since the compressed material loses much of its insulating qualities.

Comment: Back when I started out futzing around with PC, I had some brief contact with a retired engineer from Johns-Manville, who worked for years in their research lab, working with a wide variety of materials. He told me that he had one of his buddies, who was still working, run an r-value test on a PC sample that was provided... This sample had a density of around 25 lbs/CF - which is fairly light weight stuff - and it came out with an r-value of slightly less than 1 per inch... He stated that, based on his years of experimenting with various materials, papercrete blocks MAYBE had the potential - if manufactured under ideal conditions with a highly optimized process (with everything JUST RIGHT) - of an r-value of MAYBE as much as 2 per inch... He thought that achieving anything more than that was highly unlikely. My own belief is that the r-value is dependent on the constituents of the PC in question... sand, cement, paper pulp, air, and other optional stuff. I've seen samples with densities running all the way from 20 PCF to 90 PCF. The 20 PCF mix had no sand, little cement, and mostly dried paper pulp; it was weak, but very insulative. The 90 PCF mix had much sand and cement, but not too much paper pulp. It was almost like regular concrete - strong and more of a thermal mass material than an insulator. With an in-between material like PC, it's hard to say where the insulating behavior leaves off and the thermal mass behavior begins. Or maybe we get both at the same time.

Q: Has anyone used papercrete in between cob walls for insulation?

A: Not that I know of. This would be an interesting experiment to conduct. The trick would be to get everything to cure and dry out, since the papercrete tends to take forever to do this, and when embedded between cob walls, it would take even longer, as well as the cob itself.

Q: I live in an older house that has very little insulation. I do not have the money to insulate so I was thinking that papercrete might be a good alternative to cellulose. I was envisioning forms built and pieces cast then carried into the attic and laid between the joists. I wonder about the weight of it vs. blown insulation and if some cellulose should be blown in on top of it, since the r-value of pc is small(1-2.5) I was thinking a 12 inch block might work. My ceilings are plaster board.

A: While you could do what you describe with papercrete, it would likely be a lot of work to make the papercrete blocks and get them to fit right. Also, the weight of them might be more than the ceiling could hold. I would suggest that you shop around for a good price on the cellulose insulation and perhaps get it up there without hiring professionals to save money.

C: I was involved in the 2001 Betec Symposium and workshop at El Rito, NM and presented the pc portion. The Dept of Energy wanted to know the R-value of papercrete and I poured some samples to be tested at Oak Ridge Nat Lab. and the results came back at 2.25 per inch of insulation.

Q: Is it also possible for paper and cement to be used as hollow blocks?

A: I suppose that one could cast hollow blocks of papercrete, but they would not be as strong as solid ones, nor would the hollow part enhance the insulation value. Since solid blocks are rather lightweight as is, this may not be worth the trouble.

Q: Does the compaction of papercrete slurry affect the strength of the dried finished product?

A: Yes, the more compaction that is applied to the slurry, the denser and "stronger" will be the product...at the loss of insulative value.

Q: In terms of texture and possible coloring, spraying papaercrete seems like a super method of weatherizing mobiles, yes? Is there something in your experience and expertise that might be a better substance to weatherize old mobiles ? For example, 'light concrete' caught my eye.

A: This is a topic that has generated quite a bit of discussion over the years, and I have yet to hear of a really easy solution. People have done this with strawbales, but this requires extra foundation and roof work, and the details for finishing doors and windows can be tricky. A simple spray-on insulation sounds like a good solution, but again there are hazards to this. One problem with papercrete is that it needs to be kept dry to be effective as insulation, since it acts like a sponge and holds lots of water. This means that it needs to breathe and it needs a good roof over it. Also, both papercrete and lightweight concrete need to be fairly thick (several inches) to be worthwhile as insulators, and this again may require additional foundation and roof work. So it is not simple.

Q: I live in Canada. This summer I'm planning to build a 12 ft x 20ft house on skids with a loft. For the structure I'd like to use locally harvested rough cut timber and for insulation papercrete, applied like plaster.

A: I am concerned about your use of papercrete as a plaster to provide insulation. With papercrete you get no more than R-2 per inch, so it would take at least 1 foot of papercrete to provide the kind of insulation that you would need in your climate. My suggestion is (if you want to use papercrete) to make papercrete blocks in advance and allow them to thoroughly dry before using them. Then these can be used as infill in your wood framed building.

Q: I am looking for some information about how compressed papercrete compares to normal papercrete, in terms of insulation.

A: I would expect compressed papercrete to be less insulating than standard papercrete.

Q: What could you tell me about optimizing papercrete mixture with maximum R-value in mind?

A: You get the highest R-value from papercrete if it is made with pure paper fiber and just a little bit of Portland cement. When you add sand it makes the product more durable but less insulating.

Q: We ran an R-value test and it came back at 1.07 per inch, is this typical from what you have seen? FYI - We also got compressive strength numbers on several mix ratio's ranged from 90psi - 270psi was the highest.

A: R-value and compressive strength are inverse amounts usually, and depend on the mix, with pure PC having a higher R-value (up to 2/inch) and PC with sand added being less.

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