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Community and Q&A

Manual J for Passive Solar

bill328 | Posted in Energy Efficiency and Durability on

I am building my own 1663 sq ft home out of Faswall ICF blocks. R25 walls, R50 ceiling, R10 floor. The house will be a south facing, passive solar home with an exposed slab on grade floor with hydronic in-floor heat. 81211 zip code in climate zone 6. Normally people don’t have or use AC here – just heat.

Manual J calculation is stating a 50,000 BTU heating load but I am not convinced that the Manual J methodology holds up when you are building a tightly sealed, passive solar home.

Does anyone know anything about this? Is there a modifier I could apply that would account for the passive solar heating impact? I am trying to avoid over sizing my Chiltrix system.

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  1. silkwj | | #1

    I would strongly recommend avoiding in-slab radiant in a passive solar home. By heating the slab you're preventing its use as thermal mass. When the slab runs overnight to keep the house warm, the next morning that slab is fully "charged" and the solar gain has nowhere to go, and your indoor temp could spike 15-20F+. I've actually been in a place like this when it happened. Passive solar works best with low-mass, fast-acting heating systems. The floor is the "passive" part (a good piece of it at least) of "passive solar." You don't want one without the other.

    As for the Manual J question, I would say it's just not suitable at all. In fact that's basically what it says on page 8 in the Manual. A passive house tool like PHPP or WUFI-Passive might be more appropriate.

    1. Expert Member
      NICK KEENAN | | #11

      When radiant floor heat first got big about 25 years ago you would often hear people say that it needed to be in a concrete floor for the "thermal mass." People would brag about a floor that took half a day to get up to temperature, as if that was a good thing.

      This was based on a complete misunderstanding of what was going on, it was wrong then and is wrong now. First, the most comfort from a heating system comes if it is as responsive as possible; the ideal system would achieve the desired temperature instantly at all times. What in floor radiant needs is a very even floor temperature, and the way to achieve that is to embed the piping in something with high thermal conductivity so that the heat is spread evenly. Concrete helps radiant floors not because it has "thermal mass" but because it has a very low R-value. Aluminum plates work too.

      The reason that you need an even temperature is that unlike a radiator, you're touching that radiant floor, so it can't be too hot in any one spot. So the heat needs to be spread out. Back 25 years ago this was a huge deal, insulation standards weren't nearly what they are today and it wasn't uncommon to have radiant floors that needed to pump out 30 or 40 BTU/sf. In order to get that kind of output without any one spot on the floor being objectionably hot the entire floor had to be pretty warm.

      If you insulate the **** out of the house you can run the water at a lower temperature and your overall comfort will be much greater.

  2. this_page_left_blank | | #2

    50kBTU seems high for a manual J on those details. I would question the person who performed it. Having said that, the insulation levels you cite don't seem adequate for a passive solar home in zone 6. Also, in my opinion the passive heating aspect shouldn't have any bearing on the size of the heating system. You need to be able to heat your home under the worst conditions, and that includes when you inevitably get several days in a row with minimal sun.

    I agree with Cramer in regards to in floor heating, but for a different reason. It's expensive compared to other options, and all of the advantages ascribed to it are mythical. It doesn't increase comfort, it doesn't reduce energy usage and it doesn't feel warm on your feet. In a super insulated house, the floor is only going to be a couple of degrees warmer than the desired air temperature, which is not enough to even take the chill off the concrete, let alone feel warm.

    The last thing I'll point out is that ICF is just about the least green building method you could choose.

  3. walta100 | | #3

    I am far from a passive house expert but I thought to be certified passive house you had to pay a third party to computer model your house with their proprietary software that would give you your required heat loads.

    When I see Passive house insulation specs zone 6 it looks something like R40 sub slab, R60 walls R100 roof combined with the most expensive window and doors imaginable. My problem with the passive house programs is cost is never a factor in the model.

    If you are not paying for passive certification I say stop using the word passive. Your insulation numbers sound closer to a pretty good house numbers to my ear.

    Consider modeling your house with BEopt and adjusting your choices to build the house with the lowest cost to build, finance and operate.

    From what I can tell the insulation installed in most passive houses will never save enough energy to recover the cost of the insulation.

    Try building a BEopt model where solar works I could not do it in my zip code. Turning my house in all directions changed the full year models numbers very little.


  4. bill328 | | #4

    #1. we are not seeking a "Passiv Haus" certification but our house will rely on "Passive Solar" using thermal mass to regulate and amplify heating. This is why we have an exposed slab floor and high thermal mass walls.
    #2. The ICF we are using is Faswall not a standard Nudura form. It's made from 85% recycled content. Understandably they must be filled with concrete in the end but we all must make tradeoffs between cost, availability of materials in the mountains and building codes.

    Thanks to all for taking time to respond.

    1. Expert Member
      NICK KEENAN | | #7

      The idea that "thermal mass" can "regulate and amplify heating" has been pretty well discredited. First, it's based on the mistaken assumption that the mass of an object determines its heat capacity, that heavier objects hold more heat. Concrete, which is usually suggested for "thermal mass," actually has a low specific heat, lower than just about any construction material other than steel. Regular construction materials like wood and drywall have higher specific heats than concrete, if you want to increase the heat capacity of your house you'd be better off using more of them.

      But there's a reason you don't put the heat capacity of the house into a Manual J. In order for heat to flow, there has to be a temperature difference. And the amount of heat flow is directly proportional to the temperature difference. In order for significant heat to flow from the air inside your house to the structure the air has to get quite a bit hotter during the day, and in order for significant heat to flow out of the structure and into the air during the night the air has to be significantly colder than the structure.

      Let me just throw out some numbers. Let's say the 50K BTU/hr is right, and there are 8 hours a day when the sun warms the house and is storing up energy. To get through the other 16 hours you need to bank 800K BTU. Let's say your house weighs 50,000 lbs and has an average specific heat of 0.5. To bank 800K BTU the temperature of the structure has to swing 32 degrees F over the course of the day. That's outside the acceptable comfort range for most people.

      I think you're going to find that maximizing the insulation rather than the weight of a house gives the lowest overall energy use.

      1. silkwj | | #9

        Agreed on the "mass" issue, it's a bit of a misnomer, but look at volumetric heat capacity - we don't build by weight, we build by volume. Concrete has a heat capacity around 30 Btu/cuftF, wood is 1/3 that. Steel is around 50. The only common material higher than any of those is water at around 60 Btu/cuftF. I agree TM is often a bit overblown in its effects ("effective R-value", anyone?) but it can and does work when things are balanced right.

        1. Expert Member
          NICK KEENAN | | #10

          Here's the thing: if you are using "active" climate control -- heating and cooling -- you don't want "thermal mass" or high heat capacity because it makes your building less responsive to the HVAC system. Which ultimately means a less comfortable building. And at the end of the day, the reason we have HVAC is for occupant comfort.

          Let's do a thought experiment. Imagine two houses side by side. One has a design heating load of 10K BTU/hr. The other one has a design load of 50K BTU/hr, and 25 tons of concrete on the first floor. Which one is going to be more comfortable? No contest, the first one.

  5. Expert Member
    RICHARD EVANS | | #5


    I'm not sure I understand what you are asking.

    The Manual J calc is meant to size your equipment based upon the 99% design temp for your area. This design temp likely occurs at night in the winter months. How much solar heat gain you receive is irrelevant for a manual J, unless you are asking about the effects of thermal mass. (There may be some, but ICFs squander this opportunity because of the interior foam.)

    Are you trying to determine how much supplemental heat will be needed for the year? If so, then taking solar heat gain into consideration is important. This is different than a manual J calc. Most people use energy modeling software for this but I prefer to do it by hand.

    You can use the website for help, if that is your goal:

  6. Expert Member
    RICHARD EVANS | | #6

    Bill, I just looked up "Faswall".

    This is very, very different than a conventional ICF. Your Manual J may actually be correct as I highly doubt that these walls have any meaningful R-value at all.

    Given your sunny location, perhaps thermal mass will help over time but this has little to do with a Manual J calculation. If its cloudy for a couple of days and you then have a cold, clear night- then you may very well need 50,000 btu's to bring the house up to 68F. Assuming this can happen within 99% of the time, then this is what the Manual J attempts to solve.

    I am always highly suspicious of companies that tout the benefits of thermal mass but low R-Value. But, I don't know much about building where you live.

  7. Expert Member
    NICK KEENAN | | #8

    To the question of oversizing the Chiltrix system, in general air-to-water heat pumps don't suffer the performance hit that minisplits and conventional heat pumps do when oversized. In fact, oversizing allows you to run more moderate temperatures in the piping which is typically more efficient.

    Oversizing becomes a drawback when you end up buying more equipment than you need, however.

  8. onslow | | #12


    I am west of "Bewnee" about 100 miles so I can't say what your winters are like. I have visited in the summer and I would warn that trying to be passive with perhaps large south facing windows may not work out as planned. Summer will be baking and winter nights followed by cloudy days would turn large window areas into refrigerated wall sections. If you can plan to add portico type protection of large glass areas during the summer, then I would recommend it.

    The 50,000 btu figure seems quite far off unless you have very high window area. I have nearly twice the heated area and calculated a low 30k figure for a -10 low. R30+ wall, R50+ ceiling, R15 under slab and outside of foundation. The Faswall system does seem to have an advantage in light of the wildfire risk here in Colorado. My own outsulation approach puts me at risk for radiant load damage. The 20-30' clear zone I have created will hopefully never be tested.

    However, it may be useful to look at Faswall as being much like a stud wall given the wood and cement mix. Durisol/Nexcem literature for a very similar product, seems to indicate about R2 per inch for bulk material. A quick check of Faswall did not produce a direct link to an R value for the bulk material. Perhaps it is higher per inch.

    While partial cavity volume can receive insulation, the webs will essentially act as through paths for thermal energy the same way as studs do. The cement fill that ties everything together is extra mass, but others have discussed that thoroughly already. I would look to the edges of your slab and the wall set on it as a potentially large thermal loss point. Putting foam on the outside edge of the slab to limit losses would work, but that does defeat the insect resistance feature at a critical point.

    Probably a bit late to consider:

    the cost of electrical work if you are not planning on furring out for standard drywall.

    the type of exterior coatings and flashing details needed to protect the block from absorbing water. It is still a cement based product.

    how will you set windows and doors, again with flashing and water protection details.

    air sealing details between ceiling plane and walls if venting the attic.

    I will disagree with DC about thermal mass to an extent. The interior mass of my house is all of the wood structure, plus the tile and 5/8 drywall. We did not want the noise of AC and the arid conditions here (along with the 30-40 diurnal temperature swings) have allowed us to be very comfortable. Opening at night to chill down the interior mass then closing up has proven successful so far. Ask me again in 5 years if the long term heat wave predicted materializes. That said, thermal mass in a more humid environment would not be as comfortable without a means of reducing the humidity in the air. Cool and damp is not fun.

    I ended up with cove heaters (yes, expensive to run) due to essentially unavailable mini-splits at the time of build. The mass inside the envelope also functions as a kind of flywheel, which only gets noticed if the power goes off. Fortunately not very often.

  9. Expert Member
    NICK KEENAN | | #13

    To the original question:

    You can model the solar impact on a house, it just takes some work. In economics, where they do a lot of modeling, they have a saying: "Every model is wrong, some are useful," which is abbreviated EMIWSAU. Keep that in mind.

    The Manual J models the heat loss and gain on the coldest and hottest days, but it has utility for modeling on other days. The underlying assumption is that heat flow is directly proportional to the difference between inside and outside temperature (EMIWSAU). So if your model says your heating need is 50K BTU with an outside temperature of 0F and an inside temperature of 72F, it says that with an outside temperature of 36F your heating need would be half that, or 25K BTU.

    What you want to model are not the extreme days, but the average days. For your location, you need to find the average high and low temperature for each month. You also need to find the average insolation for each month. Set up a spreadsheet, where for each month you model the 24 hours of an average day. Assume that the high temperature occurs at 2pm and the low at 6am, and that the temperature changes on a straight line between them. This gives hourly temperatures. For each hour calculate the heat loss, using the number from your manual J and the outside temperature. For each hour calculate the solar heat gain, assuming that the entire solar gain of the day occurs evenly from 9AM to 3PM, and using the area of your windows and an assumption about their rate of capture (10% would not be unreasonable).

    If the heat loss does not equal the heat gain you have to figure out the temperature change of the interior of the house. Estimate the weight of the house, and the average specific heat of the materials to estimate the heat capacity. Dividing the heat delta by the heat capacity of the house gives the temperature change. That plus the current temperature gives the temperature for the next hour.

    Presumably there will be a temperature that is too hot, where you start drawing shades or opening windows. Adjust your model so that if the temperature of the house goes over that temperature no more solar heat is absorbed. Also there will be a temperature below which you will turn on the heat. Adjust the model so that below that temperature heat is added to make up any deficit and the temperature falls no further.

    Finally, adjust the starting temperature of the day until the starting temperature is the same as the ending temperature. Do this for all twelve of the months. Multiply the typical day for each month by the number of days in that month to get an energy profile for the entire year.

    Your Manual J should have total area of walls, windows and roof, and assumptions about the insulation qualities of each. By adjusting those assumptions you can see how different constructions impact annual energy cost.

    While this is a lot of work, I encourage you to do the work to see how it all plays out; it's far less work than building a house. I went through this exercise about 15 years ago. What I found was that in my climate (Washington, DC) windows produce a lot of solar gain in the summer, spring and fall, but in the winter -- when heat is really needed -- not so much. In fact in the winter windows are net losers. The construction in my model that resulted in the lowest net annual energy usage was a windowless box with thick insulation in the walls and roof.

    Obviously that's construction doesn't create a very livable house. But after decades of chasing solar thermal I've given up on it. I now believe the most efficient house is tight and well insulated with few windows, all-electric utilities, and the biggest possible photo-electric array on the roof.

  10. Expert Member
    Akos | | #14

    Passive solar does not reduce the size of your mechanical system, just how much it runs during the heating season. It is very common to have multiple days without much sun in the winter, no amount of thermal mass will get you through a week of no sun, you still need heat during those days.

    My own home has enough southern facing windows and interior mass to get by without much heat on sunny days in the winter. If a snow storm moves in, with a couple of cloudy days in a row, the heat does run full tilt.

  11. BirchwoodBill | | #15

    The number seems high for that square footage. What did you use for the design temperature and the infiltration? Some of tools out there are for sizing boilers. Are you planning on using the Chiltrix for heat and DHW? If you are using radiant heat you Mays to consider panels with aluminum sheets. For -13f design temp, I keep our radiant floors between 85 and 103f.

  12. Expert Member
    NICK KEENAN | | #16

    Just to close the loop, I want to share the analysis I did that turned me off on solar thermal. The tl;dr version is that a well-insulated house uses less energy than one with windows situated to absorb solar heat.

    First step was to get average heating degree-days by month, which I got here:

    For Washington, DC I used:
    september 19
    october 202
    november 467
    december 755
    january 906
    February 741
    march 562
    april 269
    may 72

    From HDD, you can get average monthly temperature by taking HDD and dividing by 30 (it makes the math easier to assume all months have 30 days) and subtracting from 65. Here's what I got:

    september 64
    october 58
    november 49
    december 40
    january 35
    February 40
    march 46
    april 56
    may 63

    The units of R-Value are such that if you take the temperature difference and divide by the R-Value, you get BTU/hr per square foot. Multiply that by 24 to get BTU/day/SF. Here's what I got for an R-38 wall:
    september 5
    october 9
    november 15
    december 21
    january 24
    February 21
    march 17
    april 11
    may 6

    Similarly, here's what I got for an R6.7 (U=0.15) window, which is a very good window:
    september 29
    october 52
    november 85
    december 121
    january 140
    February 119
    march 97
    april 60
    may 35

    If you add up all the monthly numbers and multiply by 30 you get the annual BTU/SF. For the R-38 wall it's 3870 BTU/sf and for the U0.15 window it's 22,061 BTU/sf. So the window allows almost six times the heat loss as the wall, which makes sense since its R-Value is almost six times lower.

    Now that window lets in sun too, and with that sun comes some heat. How much? I got insolation figures from NREL. (Unfortunately this was a while ago and the link I saved is now dead). Here are the figures for Washington, DC, in kWh/day/m2:

    january 1.96
    February 2.8
    march 3.71
    april 4.55
    may 5.54
    june 6.03
    july 5.77
    august 5.34
    september 4.48
    october 3.4
    november 2.37
    december 1.81

    To convert these to BTU/day/sf, I have to do two conversions. First I have to convert from SI units into BTU's and feet. Second, I have to decide how much of that insolation actually converts into usable heat. There's a lot of factors -- angle of the sun, orientation of the window, SGC of the window, shading -- but I just settled on 10% for simplicity. So here is average daily solar gain, by month, in BTU/day/sf:
    september 142
    october 108
    november 75
    december 57
    january 62
    February 89
    march 118
    april 144
    may 176

    In all but four months -- November through February -- the solar gain is more than the heating load, that window does better than the insulated wall. However, those four months represent over 70% of the heating load for the year, and are the time of year when the sun is the weakest. The shortfall for those four months between what insolation provides and what is needed comes to 5,404 BTU/SF per year. That is what the heating system is going to have to provide. Recall that the heat load for the R38 wall was 3,870 BTU/SF per year. So even with the benefit of solar, that window uses about 50% more energy for heating than it would if it was replaced with a well-insulated wall. Yes, for eight months of the year the window provides all the heat you need, but for the other four months it's a liability.

    Now this is in Washington, DC, which is a mild climate. The coldest month of the year is January, when the average temperature is 35F (and average daytime high is 42F). As someone who grew up in New England, for me that would be a nice early spring day. In harsher climates -- where the winters are colder and the winter sun is dimmer -- the math works out even worse.

    There is a way to get all the heat you need from the sun: put a solar electric array on your roof. If your utility offers net metering, then they'll even take care of managing capacity for you.

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