Polycarbonate for use heating thermal mass, will it work?

I am in the process of purchasing my land, and am trying to hash out the finer details of my home so I can start getting ready to build. I am going to be making an earthship inspired passive solar home, but with straw bales and cob, as opposed to a tire wall.

As far as budgets go, they don't really get any tighter than mine, so in looking for ways to keep costs down, I went looking for something less expensive than glass. What I found was polycarbonate sheeting. The double walled stuff sounds amazing! Great for greenhouses, diffuses light, and cuts down UV. That's Awesome! But will it allow the winter sun to heat the interior floors and cob walls of my home, helping me keep the place warm in the winter?

I am concerned that the diffused and lack of UV wont charge up the thermal mass well enough, or in a way comparable to glass. Google wasn't very helpful, so i turn my question to you, oh knowledgeable permaculturists!

Of course. You already answered your question. It will work, but just not as well as some of the more expensive alternatives. Just plan your build according to your efficiency needs, either downsizing your heating space, or increasing your mass and harvest. I like to point people this this site: builditsolar.com

I'm using my greenhouse to create heat and shade (seasonal adaptations); just purchased polycarbonate roofing panels (bronze) and also seek experienced info; found this Solar Heat Storage methods which may help inform your decisions ... my panels are not here yet, and I'm going to adjust the roof slope(to @ 54%) for optimal performance here. What I have learned so far in my research is that, polycarbonate panels are the ultimate roofing ... and I want to hear from experienced users also...

People are always replacing windows and it's hard to get rid of the old ones. Look for installers in your area, find one you can get old stuff from.
Even a crappy old glass window is better than a poly carb sheet that will be gone in 10 years.

keep an eye out for windows, sliding doors etc on craigslist. Used glass will still outlast any plastic.

Dan Broun wrote:
I am concerned that the diffused and lack of UV wont charge up the thermal mass well enough, or in a way comparable to glass.

That is the crux of the matter, all plastics absorb in the UV when compared to glass. And as they absorb that UV, it damages their molecular structure, meaning that after decades plastic disintegrates while glass endures.

The decision is probably less a technical one based on materials science than it is one based on expediency -- if you run into a good deal on cheap sheets of polycarbonate or bargain basement panes of glass, get it and then make it work.

For some of us, glass is not an option. My site receives huge, smashing blasts of wind at one corner of my yard and I'm pretty sure glass would break frequently. And, people are more frugal around here, not a lot of opportunity to find glass unused. So, I'll go with the polycarb panels and in ten years, there may be a better solution. I do appreciate the good reasons to use glass.

Just from experience, glass is only as good as the frame it is in. If you decide to go with used glass...contact a window place, they often haul out the old windows, and aluminium framed windows are superior. My recommend glass for your purpose, would be used sliding glass doors. Even the oldest full sheet sliding glass are double pane, to some extent. And very little in the way to clean.

this is where culture/social/business reality comes between us - there's not a lot of remodeling going on here in Salto... one commercial building with lovely large window walls is still looking for a tenant on a very busy thoroughfare, I first saw it almost a year ago. the housing market is relatively stagnant and things don't change much house-wise. I understand the re-purpose process. It just isn't applicable in this situation. Maybe in Montevideo where there's a lot more people and activity. People keep what others would assign as castaway materials in the US. I'm surrounded by people who have all sorts of super old wood standing along walls in their yards because they will either find a use for it, or somebody will buy it for the same value as new materials. Haven't seen any glass, yet... so, I already ordered the panels the other day....

Dan Broun wrote:But will it allow the winter sun to heat the interior floors and cob walls of my home, helping me keep the place warm in the winter?

Did you come across this detailed spec doc? page 8 : Solar Gain + and this has even more tech detail solar heat gain page 5 I'm getting the bronze because the heat here is punishing. A trade off on light v. comfort - I don't have to invest in an air conditioning or heating system in my living space with this set up.

Good point Pia. The usa is a high consumer waste land. Where do you live Dan?

I've been living in a cold climate, in houses heated only by passive solar for almost 20 years. We use removeable greenhouses on the sun-facing side of thick-walled adobe and rammed earth buildings.

We use glass in our windows, flexible film for our attached greenhouses, and polycarbonate in skylights and the lightwells over the stairways, because of the idea of someday, a pane of glass breaking and falling, slicing through the air down into someone's head ... well. With polycarbonate, it's very important to get good quality UV resistant stuff, and to read the sticker before you install it, making sure to install the UV resistant side towards the sun! Our carpenter doesn't read so we found some polycarbonate panels turned yellow within a year, and that must have been why.

The single-layer polycarbonate has been very long lasting, and stayed clear for ages. It's warmer to the hand in winter than glass is, but of course it's only single. The places where we installed double or five-wall polycarbonate, our best efforts with tape and silicon glue gun haven't been able to keep dust out, so it's much less transparent after a few months. Personally I much prefer single pane glazing, and use an insulating curtain in the winter, rather than double glazing of any kind, which inevitably sooner or later will not be clear because of dust inside. Maybe fancy German evacuated gas-filled double windows wouldn't get dusty inside, but I don't think the OP is looking that way.

I love those Earthship books, but I have one suggestion for you if you are basing your design on those. The sloping sun-facing glazing is wonderful in winter, and indeed I love the greenspaces it creates, and the heat it provides to our heavy thermal-massy earthen houses. But come spring time, it starts getting roasting hot in there in the daytime. From March through April we open both ends and it's bearable, but some of the plants suffer from the heat, and you certainly don't want to spend time in there in the midday. Sometime in late April or early May we remove the attached greenhouse, and because we've got a normal wall and vertical windows on the sun-facing side, then all summer the rooms are comfortable, not too hot. We use flexible UV resistant greenhouse plastic, so we just roll it up and tie it under the eaves for the summer so as not to make new nail holes each year. We fix the bottom and sides with earth in a trench and/or sandbags, to avoid piercing the plastic, or in some locations where necessary we'll use battens on the sides, but the plastic does rip within a few years then.

We don't use backup heat though I'll admit that on some January nights I'd like to. We've never even considered buying a fan for summer. So I'd say our passive solar heating system works great. Of course every climate and site has different potential. Overheating is no joke, and sloping glazing like earthships is much more likely to overheat than is vertical glazing.

I find that the thinner (4mm or 6mm) twin-wall panels tend to be a bit flexible- under the winds here they tend to flex and pop out of their frames! They're sturdy enough n that they can be chased down the garden and returned to their frames undamaged- but it is annoying.

The X-wall and multi-wall panels are much sturdier (but more expensive!)

Guys, I am so sorry. I apparently created this post in a near sleepless delirium after starting a new job, and forgot about it entirely until looking at my profile to reference something else. I just started a new job working nights, and the beginning took a bit of a toll on me. There are some fantastic responses here, I really appreciate the links, and the info.

I live in southern Missouri, basically in the Ozarks. I had forgotten to mention that only the top part of the greenhouse will be slanted, if at all, and I will be keeping it shaded in the summer, most likely with some home brew shutter system.

Thank you all so much for your responses, and excellent, detailed information, as well as personal experiences. This project will begin coming into fruition in the next few months, though slowly, and in stages. I will be filming it for to put up on youtube, and will be sure to keep a thread on it in the building forum so that anyone interested can watch, and of course, give constructive criticism.

I received my poly sheets (2) today - but, lol... certain other critical items not yet delivered... and, sigh, the sheets are transparent not bronze. So, I'll be using shade cloth in the summer. Soon as I cut and install, someday (this is latin america), I'll update progress.

Right on Pia! It's a start, anyway. But if the sheets are not what you ordered, can't you send them back?

wish it were that simple ...Salto is far from the source and there's a slew of cultural considerations and I already queried the supplier about other parts of my order... no breathable tape fpr the ends is an issue... (people mostly don't question things here, they simply accept and adapt) ... I think I can manage between shade cloth and vertical plantings to manage the sun ... thinking to also drop the slope on the roof from the original plan - because - there will be plenty of sun entering through the walls & that should allow plenty if sun exposure, I think.... so, been thinking about various ways to do vertical plantings... worst of it, this moment, is the non-square frame (not built by me, my worker did not use my design/specs)... ha! ... sides measure (in meters) 4.3 x 3.9 x 3.83 x 3.63 the sheets are 5.8 x 4.2 (so they will fit, just needs adjustments on top supports.

I started trying to adjust the roof supports today by adding more "beams" - and then I thought it may be easier to attach one sheet to two (side and center) support beams and then lay that on the longest side then cut and place the next sheet on... thinking to that by having the supports on the sheets before laying on top, it will be easier to get them on to the roof in one fell swoop... sigh...

This greenhouse building experience is exemplary of why I would love to have a skilled permie come live here rent free. I am not surrounded by permies or learned/skilled labor... and some things require two capable persons.

Dan Broun wrote:Right on Pia! It's a start, anyway. But if the sheets are not what you ordered, can't you send them back?

why use bronze sun cutting poly when this can be one? right?


via Vertical Grow for shade?

Ahhh, yeah that makes sense. In some ways I would love that, in others... not so much. I really like the idea of a living roof, but its out of my budget. Good luck finding a traveling permie, they should really have a classified section on this place, because a lot of what is discussed wouldnt qualify for woofing.

Pia Jensen wrote:why use bronze sun cutting poly when this can be one? right?


via Vertical Grow for shade?

Hi Pia, my name is Hernan form uruguay. I have tried to contact you but I could not find you. Could you please send me an email to liber.proyecto@gmail.com? I would like to talk about your comments. Thank you!!