GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Is using phase change insulation with ductless minisplits the perfect marriage?

Patrick F | Posted in Green Products and Materials on

I’m interested in using phase change material with a ductless minisplit heatpump – they seem like the perfect combination. (Also, the phase-change insulation discussion on this site are pushing a decade old and I’m hoping for a refresh.) There seems to be several people selling phase change insulation materials now including – and – claiming a 25-35% HVAC reduction. I am specifically wondering if I used phase change material with a ductless minisplit would that allow me to reduce the size of the outdoor unit and number of indoor units I needed to install to have a consistent indoor temperatures throughout the the house. As I understand it, a minisplit ‘wants’ to run constantly at a narrow temp range, so it could be set around the phase change temps to take advantage of off-peak electricity – pre-cooling the phase change at night at off-peak rates and not have to work as hard in the daytime.

Some specifics to my situation – I am building a 2,000 sq ft house with R-19 walls and R-30+ ceiling. Our peak (day) electricity is 4x more expensive than our off-peak (night) rates. We are in Northern California (USDA Zone 10A) with good diurnal temps swings. The temperature range I’ve attached is a bit misleading with plenty of 100+ F days and few <30F days a year, but the general daily temperature range is accurate with only a few hot evening a year. I would appreciate your thoughts on the combination generally and how it would work for me specifically. Regards, Patrick

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. GBA Editor
    Martin Holladay | | #1

    I think you are under a misapprehension. Phase change materials are not appropriate for any space that requires "consistent indoor temperatures," as you put it. Phase change materials only work if you allow the indoor temperature to swing within a wide range, below and above the temperature at which phase change occurs. The classic example is a passive solar house with limited heating.

    Your case might be such a case -- if you are willing (during the summer) to lower your indoor thermostat level at night, and if you are willing to allow indoor temperatures to float higher during the day. This will only make sense, as you correctly note, if your local utility offers time-of-use billing.

    Most homeowners prefer consistent indoor temperatures. If your indoor temperatures are consistent, you won't get any benefit from phase-change materials.

  2. Patrick F | | #2

    Thanks for the quick reply - when I said consistent - I meant even throughout the house - could one centrally located head provide relatively even temperatures including next to the exterior walls. I don't picture huge temperature swings - BIOPCM has a Q23 (73F) material. That would allow me to cool the house to 71-72F at night and let it rise to 74-75F in the daytime with the 73F phase change material doing most of the work keeping the house cool during the day and kicking in the A/C when the material phase changed. Inversely, I would heat the house to 74F in the evening and turn on the heat at 72F (or a little less) during the day. I would think a 2-4 degree temp swing would be considered pretty consistent.

  3. Expert Member
    Dana Dorsett | | #3

    Plant hardiness zones aren't very useful when discussing thermal performance of houses. D.O.E. zones can be, but in CA the state climate zoning maps in Title 24 may be even more relevant, since CA building codes are tied to the Title 24 zones. Los Gatos CA is in US climate zone 3C, California Building Climate Zone 4.

    Martin is correct that without a temperature swing the "apparent thermal mass" of phase change materials doesn't really matter, since the change of phase that takes in or releases energy requires a temperature change. Mini-splits have ultra-narrow temperature swings, and would have to be manually bumped up/down a bit to take any advantage of the phase change material.

    In California under Title 24 building codes new home construction is required to be Zero Net Energy starting in 2020, and from a resale value point of view building anything less than that is imprudent right now.

    You might be able to get there with 2x6/R19 and R30 fluff in the attic if it's super air-tight, but it'll probably mean a large single pitch shed roof completely covered with solar panels, which isn't necessarily the best bang/buck. It's much more manageable if going with 2x6/ R20 and 1" of exterior insulating foam sheathing (or 1.0-1.5" ZIP-R sheathing) and an energy heel truss roof that can accommodate 15" of cellulose on the attic floor. R19s only perform at R18 when compressed to 5.5" in a stud bay, and they are such low density that air movement and convection is barely impeded. R20s are mid-density, far more air-retardent, and perform at R20 @ 5.5" thickness. A 2x6 / R20 wall without exterior insulating foam just barely meets the IRC 2015 code minimum for DOE climate zone 3C.

    A good starting point for making it easier to hit a Title 24 compliant Zero Net Energy performance with a PV array that fits on a "normal" looking house would be to use the "whole-assembly R" numbers in Table-2, p.10 of this document:

    Note, a 2x6/R20 wall is no more than R17 "whole-wall" after factoring in the thermal bridging, and is often closer to R14. A 2x6 R/19 wall is no more than R15, often a bit shy of R13. Adding an inch of rigid polyiso brings either reliably within range of R20 whole-wall. If advanced framing techniques and 24" o.c. spacing is used you can get there with 3/4" of rigid EPS on the exterior of a 2x6 /R20 wall. The learning curve for installing something like ZIP-R is fast, but there may be local seismic resilience requirements in Los Gatos that require a different fastener schedule than the manufacturer (Huber) prescribes. Standard sheathing works fine too, and while installing an inch of exterior foam isn't rocket science, it's probably an unusual assembly in your area. (It's IRC code minimum in some colder US locations.)

    Even though time of use off-peak rates are currently at night, at the rate solar is going onto the grid in California that may flip to the mid-day hours (or dynamic pricing may become an option) by 2025. The house is certainly going to outlive the current residential electric rate structures. It's better to design the house for lower peak & average loads than to design it to optimize an ephemeral electric rate structure.

    From a cost/benefit point of view it's probably worth spending the phase-change materials on a higher-R assembly and/or more rooftop panel, or paying somebody to optimize the window & roof overhang specifications for each cardinal direction and other details using BeOpt or similar. Tweaking the house design & window specs for optimal wintertime mid-day solar gain and deep rejection of afternoon solar gain will do a heluva lot more for peak loads, energy use, and maximal comfort than installing a lot of thermal mass (phase change or other) inside the house.

    Right-sized mini-splits can operate at very high efficiency if sized correctly for the peak loads and will be cheap to operate no matter what rate structures apply. Zone 3C is still a somewhat heating dominated climate (during the winter most of the mini-split power would be used at night, currently off-peak), but most of the sensible cooling gains will still happen during the daylight hours, which correlate reasonably well with solar output hours. The net subsidized cost of rooftop PV is comparable to or cheaper than the average first tier rates in most of CA on a lifecycle basis, and with rooftop solar pre-cooling during the high output mid-day hours and backing off during the evening peak is nicer to the grid, and SHOULD be cheaper if dynamic pricing or a more rational on/off peak rate structure gets implemented in the PV-tsunami world of mid 2020s California.

    A reasonably optimized house built to the Table 2 specs (or better) in that BA-1005 document is likely to use more annual energy on heating hot water than heating or cooling the house. Things such as heat pump water heaters &/or drainwater heat recovery heat exchangers on the primary shower are usually lifecycle cost-rational, and easy to implement. Drainwater heat recovery heat exchangers require at least 5' of vertical drain to be "worth it", so a second floor shower on a 2-story slab-on-grade house, or a 1-story with a full basement (less common these days than it used to be) would be needed. It doesn't work for every house design, but if it can be accommodated, plan for it. Heat pump water heaters are somewhat noisy, and while they reduce the cooling load and dehumidify, if you don't have a basement putting it in an attached garage can work in your climate (losing the cooling benefit, but the noise factor gets much reduced.)

  4. GBA Editor
    Martin Holladay | | #4

    If you are the kind of homeowner who likes to tinker, and you can figure out a way to keep the temperatures in the ranges required for the phase change to work, go ahead with your plan.The majority of homeowners, however, won't see any financial or energy benefit to the significant cost associated with phase change materials.

    In my opinion, phase change materials have always been an ingenious invention in search of an application. The application hasn't appeared yet, but a few suppliers continue to trumpet these materials.

  5. Patrick F | | #5

    Thank you for the correcting me on the zones and the detailed reply. We have 2x6s at 16" OC framing. It is a modern farmhouse with the exterior plywood sheathing as both seismic structure and the exterior of the house with battens added. I will forward your reply to my contractor and review my options with him.

    We do have an unfinished basement. For domestic hot water, I was originally looking at a integrated batch hot water collector like this, but I agree that a drain water heat recovery such as is probably better. The copperheart (I think was $2400) and claims to reduce hot water demand by 60% vs drainback at $900 and a 30% hot water reduction, but because we usually take showers in the morning, I think the drainback may be the better bet. I was planning on putting the heat pump water heater into a 'cool room' (or Larder) and act as the room's air conditioner - knowing that it will lower the COP slightly.

    Thank you, again. Patrick

  6. GBA Editor
    Martin Holladay | | #6

    Patrick and Jon,
    You'll almost always get a better yield from your investment dollars by investing in air sealing measures and extra insulation rather than thermal mass.

    Here is a link to an article with more information: All About Thermal Mass.

  7. Expert Member
    Dana Dorsett | | #7

    If you're investing in a batch solar water heater (which can be a very good investment in your part of CA) the economics of heat pump water heaters and drainwater heat recovery goes way down. Some very rough numbers: Properly sized the batch water heater will cover more than a third the hot water energy use in winter, over half in summer, something like 40% of annual hot water energy use (a more conservative estimate than the 60% claim.) A heat pump water heater would then only be saving half the energy use on the remaining 60%, so the hot water energy use would then have been reduced by about 70%, or about 30% of what a straight-ahead standard electric tank would be using. (Again, half is a conservative estimate for the heat pump water heater.

    Adding a drainwater heat recovery unit would then only be saving at best 20-25% of the remaining 30%, or 6-8% of what a standard electric tank would be using. At that point drainwater heat recovering is not cost effective on against rooftop solar + heat pump water heater, or even grid electricity without the PV. With a batch heater + heat pump water heater already in place drainwater heat recovery or MIGHT pay off up front if the returned capacity allows one to down-size the heat pump water heater to extend the "apparent size" of the tank for showering capacity. That is probably not the case for a family of 3-4, but maybe for a bigger household.

    The peak temperature of the batch heater is going to be in the late afternoon on most days, but it'll still be warm enough for AM showers. The overnight standby losses aren't nearly as big in your area as they are in colder locations. In a seismic zone there will probably be some extra requirements for support structure and tie-downs/restraints to allow that much water weight up on the roof.

    Air sealing the sheathing to the framing inside every stud bay and the seams of every piece of doubled-up framing, between the bottom plates & subfloor, etc is a cheap performance enhancement, turning the sheathing into a robust primary air barrier. In most areas using a polyurethane caulk would be an appropriate material to use, since it has excellent adhesion & longevity. But in seismic zones it is sometimes disallowed, since it behaves as a fairly strong adhesive, which stiffens up the wall assembly too much, making the wall more likely to walk off the foundation during a quake. The alternative is to use a tried & true acoustic sealant, but insulation contractors despise the stuff since it's pretty messy sticks to everything, and stays soft forever. With a powered caulking gun and a case or two of caulk it goes pretty quickly- it's not a huge labor cost-adder.

    In Los Gatos night time ventilation schemes for cooling tight well insulated houses can work pretty well, as long as excessive afternoon-evening solar gains are designed out to keep the late PM peak load well damped. Even in July-August the typical night time lows are below 60F, and the latent loads in that area are usually negative (=the outdoor air is almost always at a low enough absolute humidity for human health & comfort.)

  8. jj1 | | #8

    Patrick and Dana, FYI the latest generation of heat pump HW heaters provides significantly better efficiency. I just installed a Rheem Prestige Platinum 50 gallon heat pump HW heater in my house in Southampton NY, it features an Energy Factor of 3.5. Retail price was $1199 at the local (orange) retailer (model # XE50T10HD50U0), plus our local utility has a 50% rebate (perhaps California may have a relevant rebate program?). Customer reviews to date imply significantly improved reliability for this admittedly new model; also Rheem (#1 US market share for residential HW heaters) offers a 10 year warranty. If vertical space is available, a drainwater heat recovery device might still make sense if it allows a periodic houseful of guests to shower without purchase of a larger model HW heater in addition to significant long term electricity savings. GFX is selling its S3-60 model featuring 57% efficiency for $672 (free shipping currently) at

  9. Patrick F | | #9

    Jon R., We have an EV (electric vehicle) rate. It is not tiered, just base on time of day and is about $0.11/kWh in the evening and $.44/kWh in the daytime and around $0.28 /kWh in the partial peak mornings and evenings. However, I hope to go solar in the next 2-3 years which may have a very different rate structure.

    Martin, We do have a diurnal temp swing, but it is not a high desert temp swing. I will look at that more closely.

    What I understand you saying is that for my location an integrated solar batch heater may be a better investment than a drain water heat recovery system. A heat pump water heater still makes sense on the lower energy use needed and I should only add a drain water heat recovery system if I can get close to 'free' by reducing the size of the water heater. Thanks again.


  10. GBA Editor
    Martin Holladay | | #10

    You wrote, "We do have a diurnal temp swing, but it is not a high desert temp swing."

    You may have misunderstood my comments. When I wrote that phase change materials won't work unless you are willing to accept a wide ranging temperature, I was talking about your indoor temperatures, not outdoor temperatures.

    All locations in the U.S. have diurnal temperature swings (variations in temperature between day and night).

    Here's what I'm saying: If you keep your thermostat at 70 degrees, you won't get any benefit from phase change materials. You have to let your indoor temperature drop below the comfort level, and then rise above the comfort level, to get any benefit.

  11. Jon_R | | #11

    4x the cost for peak electricity is well outside the norms - consider simple thermal mass, using more of it to account for more of it being needed for the same temperature swing as a phase change material. Maybe even automate the decisions and process involved with creating air flow at night (to cool down the internal thermal mass), managing limits, humidity, etc.

  12. Jon_R | | #12

    Note that what temperature range you consider comfortable is up to you. For example, many people consider cooler at night optimal anyway.

    Also note that drywall, furnishings, etc alone provide a noticeable thermal mass effect - at no additional cost.

  13. Expert Member
    Dana Dorsett | | #13

    With drainwater heat recovery units fatter and longer is always better. A 3" x 60" has about the same recovery efficiency as a 4" x 48", and substantially less than a 4" x 60". If you're going that route, buy the tallest and fattest that fits.

    Renwability's PowerPipe series usually has a lower pressure drop than most, and will support higher flow. Ecodrain's V1000 series may be even better, but I haven't seen the data. Renewability use to sometimes throw up a "loss leader" unit at a heavily discounted price on Amazon, but something may have changed in their distribution agreements with Home Depot that precludes the undercutting on price. (I don't know, just a WAG.)

    In the past EcoInnovation's ThermoDrain series has been competitively priced when buying direct from the manufacturer.

    231 rue Ste Marie
    Saint Louis de Gonzague, QC, CANADA
    Tel: 888.881.7693
    Fax: 888.899.1135
    Tel INTERNATIONAL: +1 4503775900

    EMAIL: info(@)

    Comparing GFX Technology's recovery efficiency claimes to Canadian vendors' number is a bit like apples & pears. Natural Resources Canada (NRCAN) has a standardized test protocol, and maintains a list of third-party tested data for various models, but GFX Techonlogy products have yet to be listed. A slinky-coiled 3" x 60" like the GFX Technology S3-60 probably won't quite hit 57% under the NRCAN standard protocol but the best-in-class 3x60 Ecocrain V1000-3-60 runs ~60%. Most 3x60s are in the ~50% recovery range under NRCAN testing. A 4" x 60" will usually hit the mid to high 50s, as will the shorter best-in-class 4" x 48" Ecodrain V1000-4-48 . All else being equal, assume the GFX Technology S3-60 to be ~50% efficiency in an an apples-to-apples test.

    NRCAN listed models can be compared online here:

    A spreadsheet of all NRCAN listed units can be downloaded here:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |