GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Rain barrels for thermal mass?

creepybabyhulkhands | Posted in Energy Efficiency and Durability on

It’s hard to ask this question without getting a red-herring response about the principles of thermal dynamics or “have you tried ‘X’ instead..” I hope this may be a good place to ask.

After getting an $88,000+ quote to improve wall insulation on an existing house, I’ve been looking at other options for reducing climate control costs on my existing 3000 sq foot ranch home with a sweet southern exposure.

I’m considering using water for thermal mass. There Isn’t enough window coverage to use water for thermal mass indoors.  I am currently planning to use an array water barrels for rain harvesting against the southern wall anyway. Can anyone tell me if using stacked water barrels on the outside of the R-19 insulated wall is likely to have any impact on heating/cooling the home? 

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    I can't imagine what insulation package would cost $88K on a 3000' house other than a deep energy retrofit with thick rigid insulation over the entire exterior of the house. Can you elaborate on what was included in that bid?

    The kinds of energy savings you'd get from putting the thermal mass on the exterior of an R19 wall are pretty small and likely negative, especially if air can circulate freely between the walls and barrels. Putting the thermal mass on the interior side, which would buy you something, but in most cases not much. Putting the barrels indoors in front of south facing windows is a classic 1970s approach to passive solar, but without direct solar gains into the thermal mass just having the mass indoors only delivers a fraction of the performance.

    Even in a high mass wall with the thermal mass tight to the insulation layers the IRC requires at least half of the insulation to be on the exterior of the thermal mass to relax the R-values.

    In heating dominated climates the impact on heating/cooling with the barrels exterior to the insulation layer can even raise annual energy requirements slightly since it's blocking direct solar gains to the exterior siding of the house. In a cooling dominated climate it may be slightly positive, but it would have to be modeled for the house and site factors to know for sure. Either way the impact isn't much.

    FWIW: In the US $88K would buy more than 30,000 watts of PV solar (before any subsidies are applied), which for a typical 3000' house with 2x6/R19 walls and clear glass double panes is more than enough to hit Net Zero Energy. But that much wouldn't all fit on the house.

    1. creepybabyhulkhands | | #3

      Thanks for that thoughtful response. Speaking of the funky 70's, I've been exploring other options from the history books. Examples:
      -Trombe wall
      -Cooling ventilation tubes
      -Solar hot water to either wall registers or Hydronic flooring
      -Solar thermal collectors
      -Adding thermal mass to the wood stove
      As long as anything from the 70's has any effect, I thought it's worth using against my $300 per month energy bill. And you are right, despite being told that reducing use, insulating and sealing are the "veggies of energy efficiency" and PV solar is supposed to be the "dessert" $88K was shocking especially when PV is coming down. I inherited 3.5KW when I bought which is nice but not really moving the needle especially in our NW winters. I have acreage so 27 more KW may be more doable.

      That $88K was a soft quote but still a shock. I originally asked for a quote on simply adding insulation to the walls. The suggestions included using high end wall insulation (, aggressive air infiltration control and adding more insulation to the roof (R-55 now), plus the cost of replacing the hardiplank. The builder is famous for their net zero building.

      1. Expert Member
        Dana Dorsett | | #6

        Adding a staggered studwall full of dense packed cellulose on the interior side of the original studwall (also being dense packed), and adding 2" of exterior EPS under new siding is far more than an IMPROVEMENT to the existing wall insulation, and is of course going to be more expensive than typical insulation improvements. In much of the NW that wall stackup might even be used in a PassiveHouse.

        It's more than just insulation. It's adding new drywall, an interior side OSB sheathing layer, and 2-3" of EPS under new rainscreened siding on the exterior, roughly tripling the wall performance. That much change in wall thickness requires re-installing/re-working all of the window mounting & trim details, exterior & interior.

        Leaving the original drywall in place, dense packing the existing walls and adding 2" of cheap reclaimed roofing polyiso on the exterior would roughly double the performance, beating current IRC code minimums. That won't be super-cheap, but it would only be a fraction of $88K. Some amount of window detail work would likely be required on the exterior, but not the interior.

        Adding just 1" of foil faced polyiso and dense packing the cavities pf a 2x6 wall would meet or beat code min for all of the PNW, but for the NE corner of WA, the tip of the ID panhandle, and the higher elevation parts of ID (the climate zone 6B counties) bumping that to 2" would be necessary for dew point control at the sheathing layer. In many cases 1" can be installed with very little re-working of the window details.

        Even if perfectly air tight a 2x6/R19 wall with 16" o.c. studs only comes in at R13-R14 "whole wall" after factoring in the thermal bridging. A couple inches of foil faced polyiso with a shiny facer at the rainscreen gap doubles that to about R25-R26 whole wall. Dense packing over the R19s adds another R1 or so (while tightening it up considerably) so it's at R27 with air films and siding etc all added in. Bumping the polyiso to 3" brings it to about R30 whole wall.

        The staggered stud dense packed interior wall in the proposal adds about R10 to the R15-ish dense packed wall, the continuous 2" EPS and the interior OSB layer adds about R9. So the proposed stackup from H & H is roughly R35- better, but not twice as good, and perhaps not enough to rationalize the up-charge over the lifecycle of the building. See Table 2 on p.10 of this document:

        R35 whole-wall might have made lifecycle sense in new construction for the US climate zone 6 parts of ID/WA at the price of PV and efficiency of heat pumps back in 2009 when that was written, but would probably have been overkill for climate zone 4. But PV pricing has dropped by over half since then, and incremental efficiency gains have improved the efficiency of cold climate heat pumps by about 15-20% since then. In a retrofit bumping up at least one row and maybe two is more likely to hit the current financial rationality sweet-spot, unless going further is being subsidized. The R27 whole wall 2" foam-over is probably going to make sense, but if using reclaimed roofing foam adding 3" with a second layer of half-inch foil-faced on the rainscreen side might cost about the same, and would hit the R33-R35 whole-wall range.

        1. Brendan Albano | | #7

          You wrote, "In much of the NW that wall stackup might even be used in a PassiveHouse."

          In fact, it was used in a Passive House!

          But that's a very specific retrofit with very specific goals and requirements. No wonder creepybabyhulkhands is getting high quotes!

  2. Expert Member
    Zephyr7 | | #2

    Thermal mass serves to slow down thermal cycles by storing energy, it doesn’t provide any insulating value. Without a pic or drawing it’s difficult to say, but on the south side I’d expect the barrels would help a little with cooling during the day (casting a shadow, and absorbing some heat before it reaches the wall), and result in the structure cooling off more slowly at night. Any effect is likely to be pretty small though.

    I don’t think the barrels will do much for you to reduce heating loads in the winter. They might help a little at night, but normally you’d put the thermal mass inside the insulated structure, not outside, if you want heat retention.

    If you want to maximize heating, use black barrels. To maximize cooling, use white barrels. White barrels are more likely to degrade from UV exposure though so they probably won’t last as long (assuming polyethylene barrels).


    1. creepybabyhulkhands | | #4

      My original plan was place barrels against the wall but hadn't planned to bring them in the envelope. I was trying to decide whether to go with white barrels/black barrels. Rumor has it is that 110 gallons (minimum) is a lot of weight to swivel around every season... so no switching around. SInce we get all four seasons it's hard to pick which one to go with.

      It might be more helpful and cheaper to build a trellis, bahama shutters, or an awning during the summer at least

      1. Expert Member
        Zephyr7 | | #8

        If you have the climate for it, a pergola with a climbing vine can make for a very nice, and very green (literally :-) cooling option.


  3. Josh Durston | | #5
    1. Trevor Lambert | | #9

      It is interesting. Not sure how practical it is. That water is going to get pretty disgusting if left alone for very long. It's going to require maintenance, similar to what you'd have to do for a hot tub. He didn't build in any way of handling that maintenance, that I could see.

      He reduced the temperature swing by 30%, from 16degF to 11degF. That's an improvement, but it's still not exactly great. Given the hassle of all the control systems, monitoring, water maintenance, and how much space they take up, I think there are better solutions out there.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |