Kaushal, this is a technical point, the reason from an energy standpoint they want little leakage is the assumption that the heating or cooling is coming from the grid or by burning fuel oil when passive homes don't need to be air tight to be comfortable and are healthier by not being air tight by using natural thermal inputs of heat or cold and store those in thermal-mass.
I think the concept is too simple for people, the home itself is thermal-mass that from historical development became Western box framing based on balloon framing's drawbacks, then there is stone, adobe, cement block or concrete-&-steel all of which conduct thermal energy from the outside world to interiors are varying rates and most depend on a source of heat distributed throughout via ducting or units in rooms.
Thermodynamically you want to insulate the OUTSIDE of the wall. This kills conduction regardless of wall construction and it's not the standard so who made the standards?
Well passive solar became autonomous design with engineering textbooks from about 2000 in the UK and elsewhere based on multi-family buildings with all modern appliances, power & water needs supplied by the home.
Here in the USA with pressure from fossil fuels we are stuck with ancient thinking and to patch it up and still keep the cash flowing to the fossil fuel industry little to no progress has been made to true alternatives and thermal design principles that supply a healthier, more comfortable home to live in and need no external source of energy.
As I've stated, if you don't collect and store the daily thermal inputs, you pay for energy big time and have a home that's unhealthy if it's air tight with all the outgassing of the modern materials, I personally can't sleep without the window open to fresh air regardless of temperature and don't enjoy sitting in rooms without some flowing through, it may be a reaction to CO2 levels and plastic smells but probably just too much backpacking ... ymmv.
Thanks for your response Tom. I would like to say that an energy efficient design of the sort that we intend is not the conventional American home that shamefully is an energy hog. We are designing a house that will give us thermal autonomy, i.e. thermal comfort without having to mechanically heat or cool our house (let's say to a degree of 90% of the time). In our house, we will not have anything that burns fuel oil or off gasses nasty chemicals. We are concerned about our health along with thermal comfort. For fresh air, we will of course have windows when we feel like it. But most of the time we will have a dedicated whole house ventilation system (a HRV) that continuously brings in fresh air and expels the stale.
Excellent Kaushal, all I can do at them moment is advocate using the roof as the main collector & piped water the main thermal-mass, then the wall construction using insulation on the outside under the siding so the look of the home is not altered as the easiest, least expensive thermal tricks. After doing the cabin I'll be able to have metrics for others to use.
You are getting great info on this website, I like it a lot and learn much here.
I think in all this you are getting bogged down in the law of diminishing returns
We are talking about a small house, in a sub 3000 heating degree day climate. If we took my own house ~ 2200 sf of conditioned space, and plunked it down in Berkley, I figure it would use about 12-13Million btu’s for heating and hot water in your climate. If I used electric resistance heat at 0.15$ kwh(wild guess) it would cost 570$ year. Commit to a minisplit for heating at a cop of 3.2, In your climate + heat pump water heater and you are talking about a total cost basis for heating of 200$ per year. The difference between this and passive house saves a maximum of 200$ per year, at what cost?
Point is, you don’t need to get to passive house levels to be comfortable, or save energy in a mild climate like Berkley .
By all means refit the house, change glazing- add outsulation, add solar DHW, tighten the shell, add a HRV. All this makes sense. Heck add a 2-3 panel solar air heater. Don’t sweat getting under 1.5 ach50 or pushing the extremes of insulation because it is past the cost and comfort justification point.
Consider adding PV once you get the heat load down to a reasonable value. You are in the range that PV is likely to be cheaper than the incremental cost of further energy reduction.
The bottom line is you have to sharpen your pencil and do the math carefully before committing to a passive house level retrofit, in a mild climate.
I take issue with your reasoning. Since Passive House is a performance metric, the climate is irrelevant in terms of marginal return analysis. Except for the air tightness, which is set at the same level in all climates, everything else is climate and project dependent. Therefore, in a mild climate, you save less, but you need less insulation. In a harsher climate, the opposite is true. One can certainly pass the point of diminishing returns on any project, and part of what is so great about Passive House analysis and energy modeling is that it identifies these points with clarity. In regard to air tightness, this is one of the most cost-effective performance measures, and a leaky house is bad in any climate where one can't sleep outside year 'round, since it hurts the performance most at the absolute worst time, i.e. when it is coldest outside because the stack effect is strongest then. Air leaks in buildings compromise all other efficiency measures to an alarming degree, to the point that air tightness is usually the low-hanging fruit on a retrofit. Discussion of PV vs. efficiency is an entirely different discussion. While I agree that supplying surplus renewable energy to the grid in summer is a good thing, it does not an efficient house make, IM(H)O. A PH joke for you: "Q - What do you call an inefficient building with a PV array on the roof? A - An inefficient building with a PV array on the roof!"
That is why I called out the air tightness specifically. Why stop at 0.6 ACH50? Why not 0.006 ACH50?
I will lay out my rationale for 1.5 being a good number.
Think about it this way, the OP has a 1000 SF ranch. From a surface area to SF perspective it is about worst case scenario for a house. Add to that an artificial metric of 0.6 ach50 and you have a big hurdle- that is somewhat unnecessary IMHO.
A 1000 sf house will have ~8000 cubic ft of volume 1 ach50 crudely translates to 0.05 achn or 400 cubic ft per hour so per year, this is 3000 hdd*24*0.018*400=518KBTU = 151 KWH = 22$ of resistance heat per year= 9$ from a minisplit.
I agree that air tightness is a big deal- up to a point. I deal with houses that are 6-15 ach50. Reducing them down to 2-3 makes a worthwhile change in load.
As far as I am concerned 1.5 ach50 is a good practical number in a small house. I am not arguing for code min houses- far from it. What 1.5 does get you is a real range hood, not a forehead greaser, and it gets a bath vent in addition to the HRV- in a small house.
I think the climate is relevant, because the OP does not want fossil fuel so you are probably going with a heat pump- the warmer the climate the higher the COP(again generally)
I always figure that when the HVAC+ DHW bill is ½ the phone bill, you can stop.
I agree that the shell is always more important than PV. BUT, at some point the shell is good enough for comfort and low HVAC costs- then your next best expenditure is renewables.
0.006 ACH50 would be better, but not feasibly attainable. 1.5 vs 0.6 ACH50 makes a big difference when you're aiming for Passive House levels of performance. I am not experienced enough to expound on the cost curve of air tightness, but I know that Passive House practitioners generally view 0.6 ACH50 as "easy," and 0.3 is more where they end up, on new builds. On our retrofits, we have come in between 0.2 and 0.6. "How much easier/cheaper is it to get 1.5 ACH50 vs 0.6?" is the first question, IM(H)O, not how much it costs supply fossil fuel energy to make up the difference. (BTW, heat pumps use fossil fuel, at the power plant, at about 30% efficiency, unfortunately!) Is the Passive House air tightness number "magical?" Is Passive House performance "magical?" I can't say, I just know it's very good, and there are people doing it all over the place, very much to the satisfaction of their clients!
"Leaky" is a relative term to thermal transfer rates and air is one of the least effective mediums, fluids and solids transfer heat far more efficiently than air.
So if you keep air transfer, leakage constant and upgrade to exterior insulation that reduces heat-transfer to 1/3 of before via the walls and nothing else, most homes will not need much to stay comfortable.
If you then add in the thermal-mass effect of the existing wall now separated from conduction to the outside that adds to thermal inertia also helping to maintain comfort zone with the least heat.
Then, if a home doesn't collect heat-cold and store it for the daily cycle obviously it will need many times more energy than one that does.
So, consider that in thermal modeling calculations.
True, a vessel leaking water at the same rate would lose more energy, but since the "vessel" in question is a building, it is filled with conditioned air, not water, and leaks are insidious.
Solar gains help to a point, but tend to be minimal at precisely the time they are most needed, when it is coldest, because that is the definition of winter, i.e. lack of sunshine. A building that can efficiently utilize its internal heat gains in winter and minimize its losses (i.e. a Passive House), when coupled with solar gains (direct gains most effective) has proven to be the least dependent on additional heating energy. This is all part of the modeling, and a 20+ year track record of success. Thermal mass is helpful as well, but mainly to mediate temperature swings, which a Passive House seeks to avoid. In summer, careful shading and night cooling (where possible) coupled with an efficient envelope does a great deal toward providing indoor comfort.
The passive solar vs. super-insulated debate is an old one, dating back to the 1970s, with Passive House in the efficiency camp. Passive House does utilize passive solar (and solar thermal for DHW, often) but it leans heavily on reducing losses and "heat-recycling" of internal heat gains via HRV/ERV.
Right, in all cases you have to reduce the wall heat-transfer rate to upgrade almost any thermal remodel and that's my point, by insulating the OUTSIDE you cut the conduction out of the equation as a direct path of heat-transfer from what is essentially an interior heat collector via the sheetrock and studs to a radiator, the sheathing, and that's typically in contact with the siding so conducts directly to the outdoors.
Then if you calk the joints on the board it helps create a better seal. Then if you add furring for condensation the conduction for the siding is greatly reduced from wind and overheating in direct sunshine.
Everything from the sheathing in becomes thermal-mass to the interior by doing this.
That's a rather inexpensive and effective upgrade w/o touching the interior and the residents can live in the rooms being upgraded. From modeling this it's worth about R45 for a R15 wall adding 1-1/2" insulation board.
Then, my modeling shows that hemp-mortar, the shiv or outer part of the plant mixed with hydrated lime, has better qualities for this idea likely from adding thermal-mass to the overall travel of heat; it's fireproof where that may matter, transfers moisture slowly but won't allow fungal growth and can be applied on top of sheathing or stucco using gunite equipment.
Using hemp-mortar the result is about R60 for 1-1/2" thick.
For winter solar gain in a city with small or tiny lots and setbacks, that's where using the roof for gain usually can work without messing with the roofing, as is, and if PVs are installed their underside can be used for the heat by framing them out and using that hot air ducted to where it can be stored or used directly. This isn't a passive system, you need a blower to move enough when it's producing to store for the rest of the day with sensors for automatic operation.
The point is that wall geometry matters a lot to heat-loss, normal stud walls are conductors and the sheathing a radiator, so, that's why insulating the outside is so important. This isn't a passive design concept, you still need a source of heat-cold to adjust comfort zone but the amount will be significantly smaller.