I read this on one of the other forums - "...if you spend more money for air conditioning than space heating, you shouldn't install any horizontal insulation under a slab on grade. Instead, just install vertical perimeter insulation at the slab edge -- assuming, of course, that you have a detail that is consistent with local recommendations for foiling termites."

With a house in Zone 3, let's say Southern California and the slab is finished as the flooring for the house: If radiant heating is employed throughout without an insulated slab, won't the heat generated in the slab migrate down into the ground instead of up into the house?

Heat flows to cold, so as long as the ground temperature is lower than the desired house temperature, heat will flow to the ground. Under slab insulation is smart in (at least) most of the country.

You should approach mass as a tradeoff across the four seasons and look for the compromise that causes the least damage. What's good in winter can hurt performance in summer, and I'm wondering if that's the case here. No insulation effectively adds the soil beneath your house to its mass -- a plus in the cooling season. It isn't necessarily the negative in the winter that you imagine because dry soil is a poor conductor of heat. Yes, you will be heating the soil initially but that heat isn't going anywhere quickly. You can split the difference by insulating under the slab for the outside three feet plus the perimeter edges to slow transfer to soil that will cool overnight. But those suggestions are only a guess by a guy who knows squat about San Diego soils or winters. While modeling software also involves guesswork, it's expert guesswork based on real data. Pick one and use it.

To plan HVAC at this stage is the tail wagging the dog, particularly in a passive solar house. Banks of windows ARE a major source of btus in the desert SW. But no one here knows the style of house you want or what your site allows. If you do need heat and comfort is your goal, radiant floors aren't the only to get there. Radiant ceilings, powered by a pv array on the roof, amount to decadence on a dime, operationally speaking.

Once you know enough about design a final trap awaits, one that snared me. Put a Kiss poster on the wall, as in Keep It Simple, Stupid.

Thanks for all your responses.

I do agree with Toddm that insulation helps in the winter and hurts in the summer.

Here is calc we did to see the payback time is if we insulated in Michigan with 50 degree ground temp and 6 months of heating.


r-value___ area___ u-value___ delta T___ btu/hour___ Monthly__ 6 months___ install cost___ payback(years)

___2____ 3700___ 0.5______25______ 46250_____ 102.26___ 613.56______ 724.61________ 1.2

___5____ 3700___ 0.2______25______ 18500_____ 40.9_____ 245.4_______ 1811.53_______ 7.4

___10___ 3700___ 0.1______25______ 9250______ 20.45____ 122.7_______ 3623.06_______ 29.5

___15___ 3700___ 0.06_____25______ 6166______ 13.63____ 81.78_______ 5434.59_______ 66.4

Using propane, I get numbers ~6 times larger. Eg, for 46250 btu/hr, I get > $600/mo. That makes even R15 look good. $.15/kwh electricity plus a heat pump isn't much less. On the other hand, soil is insulation and so it heats up and then isn't 50F any more. Or it might be close to the surface and remain less than 50F.

When running a financial analysis on something with a 50+ year lifecycle for the house it really calls for a net-present-value analysis over at LEAST a 30 year time frame. Try using the mortgage loan interest as the discount rate, and take a look at what 0.25%/annum and 1%/annum energy price inflation does to for the present value numbers.

Or, you might assume that energy costs will never be higher than the the levelized cost of small scale photovoltaic power at 2015 pricing and use that without energy price inflation. That comes in at about 15-18 cents/kwh assuming 3-4% discount rate and a 15% capacity factor and $3.50/watt installed price for the levelized cost calc. That's about $45/MMBTU. Used in a ductless heat pump it'll be less than $20/MMBTU so you might want to use that number as an upper bound. (PV will get cheaper, and heat pumps more efficient, but the heat pump will also have up-front and maintenance costs to factor into the lifecycle.) That's roughly the marginal cost of $2/gallon propane burned in a condensing furnace.

Something that won't show up in the NPV or simple payback numbers is the value of being able to put stuff on the floor without accumulating mold in summer, though it doesn't take more than ~R5 to achieve that with 50F deep soil temps.

Irregardless of the ratio of how much you spend on heating vs cooling. What insulation level are your local code officials requiring (if any) under a heated slab?

We can't perdict the future so using present costs is all we have to help determine how much we should insulate. I wasn't using a 50+ year lifecycle, it was how long it would take to pay back the cost of the insulation from it's energy savings.

What other method might you suggest?

In my opinion, this way is much better then a SWAG.....

newbostonconst: Basing the financial analysis on current natural gas pricing would be a mistake, since that's a highly volatile market. The fracking boom has temporarily over-produced natural gas, but it's more expensive to produce as the traditional sources peter out, and fracked shale wells are short-lived, and are generally money losers unless the well also produces liquids. At todays oil prices even many/most of those with a decent liquids fractions are losing money. At the very least use the 10 year price history average for natural gas, not this week's spot price.

We CAN predict that future energy costs will not exceed the present day lifecycle cost of PV solar by very much, and those costs are trending down at a rapid clip.

It's also important to look at what happens using modest energy price inflation/deflation assumptions. There is hedge value in higher R against future price increases. This is particularly true for fossil-fired heating sources. Fossil fuels are commodities, commodities with high price volatility but also getting more expensive to extract. The mid to long term trend is going to be up, the current dip in oil/ gas/ coal pricing notwithstanding, as the cheap & easy stuff dries up at an increasing rate. By contrast, PV and wind power are technology, technology that is seeing cost reductions and incremental performance improvements as production volumes increase. As more of this technology gets deployed it puts downward pressure on energy pricing broadly, but electricity pricing specifically. In many markets the marginal cost of heat pumps are already cost competitive with condensing natural gas, even at the current historical low wholesale pricing for gas (a price that realistically has to go up, since fracked wells with dry gas are money losers in the current market.)

Two points, Dana: Money loser or no, a dry shale gas well represents major sunk costs, almost zero production expense and thus considerable incentive for strapped e&p companies to opt for cashflow regardless of price. The last gas bubble, circa 1982, lasted 20 years. Shale gas isn't strictly comparable because producers must keep fracking, and there's less pressure to produce before Bubba down the road does. Fact is Marcellus production in 1H 2015 was up up 6 percent.

Second, solar economics depend not only on govt subsidies but also on aid from nonsolar utility customers that is under fire. http://www.nytimes.com/2015/04/19/business/energy-environment/solar-power-battle-puts-hawaii-at-forefront-of-worldwide-changes.html (At scale, net metering involves costs all utility customers pay; An AZ utility just slapped a $50/mo surcharge on solar customers. What's more, NJ's once lucrative market in solar credits, a direct transfer from nonsolar to solar customers, also went on hiatus and came back decidedly less lucrative.)

This isn't to say that solar is a bad bet or that low fossil fuel prices will stay low. But risking $3k on a new condensing gas furnace is 6 times safer than $20k on a 10kw solar array plus heat pump, particularly if the latter will be demonstrably cheaper when the furnace gives up the ghost.

When making insulation decisions, I'd put more stake in average price for the previous 15 years (adjusted for inflation) than I would in today's unusually low energy prices. Of course it often get overshadowed by "we won't live here that long" which favors code minimums

PV solar decisions: don't count on being able to sell to the utility at retail prices That's a short term subsidy.

Mobile America is not a knock on pv. This study says pv as a home improvement in
SoCal was fully recovered in resale and then some. http://www.nber.org/papers/w17200

Posted By toddm on 08 Jan 2016 06:28 PM
Two points, Dana: Money loser or no, a dry shale gas well represents major sunk costs, almost zero production expense and thus considerable incentive for strapped e&p companies to opt for cashflow regardless of price. The last gas bubble, circa 1982, lasted 20 years. Shale gas isn't strictly comparable because producers must keep fracking, and there's less pressure to produce before Bubba down the road does. Fact is Marcellus production in 1H 2015 was up up 6 percent.

Second, solar economics depend not only on govt subsidies but also on aid from nonsolar utility customers that is under fire. http://www.nytimes.com/2015/04/19/business/energy-environment/solar-power-battle-puts-hawaii-at-forefront-of-worldwide-changes.html (At scale, net metering involves costs all utility customers pay; An AZ utility just slapped a $50/mo surcharge on solar customers. What's more, NJ's once lucrative market in solar credits, a direct transfer from nonsolar to solar customers, also went on hiatus and came back decidedly less lucrative.)

This isn't to say that solar is a bad bet or that low fossil fuel prices will stay low. But risking $3k on a new condensing gas furnace is 6 times safer than $20k on a 10kw solar array plus heat pump, particularly if the latter will be demonstrably cheaper when the furnace gives up the ghost.

Shale wells typically crap out in 3-5 years, not 20. They have to keep producing or even ramp up production on existing wells at the low price to service the financing of the already sunk costs, but securing financing for new wells at a low gas price is expensive when possible, and not always possible. Producing faster to make their payments shortens the time to depletion, and hastens the time to when these operations go bust. The fact that production was up year on year in the Marcellus isn't exactly an indication that this is a healthy and expanding market. (I get frequent blow-by-blows on this one from a relative in the oil & gas biz, who recently had to bail on his projects in Alberta. He owns some fairly easy proven coal seam gas in Oregon that can't be put into production unless the price roughly triples to be able to cover the infrastructure cost.)

The NYT article linked to is behind a paywall, but the date is old enough to be irrelevant, since only in the past couple of months Hawaii rescinded net metering. HECO has been horribly inept at handling the influx of PV, and as a company is getting their just desserts. The regulators have been increasingly critical of the company, but at the high penetration of PV have relented somewhat on the net-metering for now, but in a way that isn't breaking the back of the solar companies there.(

Local utilities will of course be defending their turf, but it's a losing battle. The repeal of net metering in Hawaii grandfathered retail net metering for already approved systems, but even when compensated at the wholesale price of electricity installing PV still a financially rational thing to do in the land of 34 cent retail electricity, even if the Federal tax incentives go away. In fact, in Hawaii under the newly adopted regulations it financially rational to install batteries on your side of the meter to minimize the amount of power exported, though that takes a sharp accounting pencil at this week's battery prices. Solar City has been offering out & out grid-defections as well as grid-attached solutions in the Hawaiian market for months now, at a price that undercuts grid-retail.

The biggest broadside to PV in recent weeks has been in Nevada, where they didn't grandfather-in net metering for existing installed systems for more than a handful of years. The way it was handled there invites outright grid defection when it becomes financially viable, unlike how it was handled in Hawaii.

At current retail electricity pricing and heat pump efficiency the marginal cost of heat pumps at NJ temperatures isn't dramatically more than condensing gas, if waiting on the solar is deemed more prudent. I'm not sure how fat the SREC subsidies are/were in NJ, but the PV that went onto the grid deferred grid infrastructure upgrade capital expenses for the non-solar customers. (And limited the size & cost of the post-Sandy rebuild stuff.) Whether the SREC cost more than the total benefits of lower peak power energy costs and lower capex cost to other customers, and the health benefits of lower emissions from peaker plants etc. needs to be carefully estimated to know whether that was a direct cash transfer from non-solar customers into the pockets of solar customers. But that's an SREC design problem, not a net-metering problem. Massachusetts has excessive SREC compensation too, but SRECs go away in MA once 1600MW is subsribed state wide. In MA net metering alone is financially rational on a lifecycle basis, but the value to other ratepayers at the paltry penetration rate here is much higher than grid-retail- it's a good deal for the non-solar customers.

Viewing the cost declines strictly from the prism of the US market is myopic. The cost trends for the PV and battery hardware are a worldwide phenomenon. The US is nowhere near the largest PV market. India alone is on track to exceed the total current amount of PV in the US to date by 2020. Behind the meter batteries are financially viable and markets rapidly growing in both Austria & Germany, not just Hawaii. In the developing world the capex of village scale PV + battery micro-grids is lower than bringing the fragile grid to remote locations. This stuff will continue to get cheaper even if more states go the path of Nevada in attempting to strangle the baby in the crib.

Not yet in evidence: Did NJ need additional capacity through the recent unpleasantness in the economy? If so, would it have been cheaper to meet it with NG peaking capacity even if the glut gets worked off in a short timeframe, complicated by the fact that solar needs peaking backup anyway? (Surely it would fairer to make utilities help poor customers with weatherproofing rather than rich ones with pv.)

Neither Dana nor I are smart enough to divine the future, and said skill is certainly beyond the NJ legislature. I'm not a classic believer in efficient markets -- the notion that everything known is correctly priced from minute to minute. More like if you average out all the ways that millions of us can be wrong you'll be pretty close to dead on.

My path to humility was paved by mistakes. so much so that my linchpin is to ask if this action would constitute a big mistake or a little one. If you are considering a great deal for PV in an expensive market for electricity and a favorable climate for solar production, you may be looking at a big one.

With third party ownership models in play in NJ it's unfair to characterize the average homeowner with PV as "rich". It doesn't take perfect credit to sign a PPA with a solar company to put PV on the roof and get a discount from the standard retail rate.

Solar doesn't need peaking backup any more than a nuke does. Everything backs everything else up on the grid. The biggest annual peak loads in NJ are air conditioning peaks, and even though the hourly output curves of PV is not perfectly aligned with the grid load peaks, there is a large overlap. The presence of PV in NJ reduces the amount of other peaking power required, rather than expanding the peaking requirements.

If the utilities really cared about cross-subsidies of the poor ratepayers to rich ratepayers, all houses with central air conditioning (a reasonable proxy for "rich") would be assessed demand charges on the highest use 15 or 30 minute intervals during the billing period to pay for the grid, just like commercial & industrial customers. The grid infrastructure necessary to support high-draw loads that have a high correlation with peak grid loads is what adds size & cost to the grid. These are some of the costs that PV reduces for the other ratepayers, since site-located generation frees up capacity at both local distribution grid as well as the long haul grid, lowering grid congestion at substations, lowering the peak temperature of the transformers & wires.

In Australia the grid costs are enormous, since the utilities were encouraged to over-build capacity for a least a decade after it was clear that growth in demand was flattening. Without the increased kwh sales to support the financing cost of those investments, regulators have allowed rate increases to keep those companies afloat. Between efficiency measures and launching the solar development with (arguably excessive, at first) very rich feed-in-tarriffs, solar mushroomed got cheap, and has pretty much guaranteed that demand for centralized power sources will never again see growth rates like the latter half of the 20th century. At current PV pricing in Australia it's cost-rational to put up a few kw of panel to offset the 30 cents+ power, even though any exported excess is effectively donated to the utility in many areas, or only remunerated at a low energy cost only rate in others. The gross kwh sales are poised for decline, not growth.

Australia is a better harbinger of the future for the US than Hawaii, since the penetration rate in Oz is about 2x that of Hawaii. There are now many locations in Australia with more than 25% of all rooftops having at least some PV, most with room for expansion as battery costs fall, and utility rates rise. As some utilities & their regulators have assessed large fixed fees for solar customers, they've tipped the scale toward adding even more PV, and some battery, not as backup, but as a means of preventing export to the grid, which is not exactly in anybody's interest. That is proving to be a very BAD response to take by the utility, for both the ratepayers and the investors, but it hasn't fully played out yet. The battery game is only now getting started in earnest, but it's the financial math numbers driving it in no-net-metering environment, not subsidies. So far none of the utilities with redundant but not-really-operating antique thermal coal plants have been willing to even take the write-down on even those assets (delaying the pain of de-commissioning costs), let alone write down any of the gold-plated grid infrastructure to nowhere that will clearly never be needed. Hopefully the US regulators will do a better job of managing the transition. I have real hopes for that in NY or MN, not so much in GA, NV or AZ.

In the midatlantic in Aug, you can SEE the humidity in the air, unlike more favorable climates where ACs and PV cells go into concurrent hyperdrive and solar can be considered peaking power. In fact, the NJ legislature has a relatively modest target of 4.1 percent solar by 2028. With homeowners eager to produce it, SRECs crashed from $700/Mw to $40, since rebounding to about $280. (The people who make out in gold rushes are the folks selling shovels.)

That tax shelter investors are stripping off the credits while offering homeowners a price break on electricity in no way makes this a program for the unwashed masses.

Is that 4.1% solar for NJ 4.1 of all power going onto the grid, 4.1% of the annual peak, or some other 4.1%?

Net metering in MA is currently capped at 4% of nameplate output relative to the annual peak draw within any utility's service area. That ridiculously low number was chosen nearly a decade ago as something that is guaranteed to be safe for grid stability. Since back in the day when they settled on that number it's become pretty clear that even 4-5x that much power is not going to create problems. The cap has been reached in many parts of the state, and the MA legislature is still struggling to come up with a re-write, which would hopefully be a real long term staged forward looking plan for how it steps away from net metering as installed capacity increases rather than a short term lift to kick the can down the road.

The "...unwashed masses..." are a bit of straw man argument. Something like 90% of the residential PV in Nevada was third-party owned installations, with most of the benefit accruing to the homeowner. If the definition of "wealthy" is anybody who qualifies for a home mortgage, then the home mortgage tax deduction is an even more egregious wealth transfer of the less well off to the "wealthy" than having all ratepayers subsidize PV. In the latter case the non-PV ratepayers still get the benefit of lower grid infrastructure costs, and lower peak power pricing, overall lower power pricing and the health benefits of low-carb power. Only at fairly high market penetration of PV (levels not yet seen in NJ or NV) are the unwashed masses really subsidizing those with enough credit to qualify for third-party owned PV on their (mortgaged to the hilt) roofs.

Without distributed PV more and bigger ongoing grid upgrades are required. The whole notion that net metered PV owners aren't paying "their fair share" of grid costs hasn't been borne out in any real in-depth analysis. But solar critics (usually utility industry insiders with oxen & other Holy Cows to be gored) treat it as a zero sum game for easy headlines type analysis:

Grid cost = X

total kwh sales= Y

Grid cost per kwh = X/Y

As Y shrinks from loss of sales to net-metered PV, the grid cost per kwh goes up, soaking the not PV customer.

Sounds simple, but that isn't the way it works.

Just as Y isn't a constant, subject to erosion via efficiency upgraded and PV, X isn't a constant either. The cost of grid maintenance goes down with more distributed PV, and the need for grid capacity expansion drops with more distributed PV too, as PV reduces the absolute peaks. Most PV owners are nowhere near net-zero electricity too- for them the net effect is more like a massive efficiency upgrade. Are we going to attach charges to efficiency upgrades that unfairly impact the grid costs for the less well off too? The net effect of the presence of PV on the local grid to other ratepayers is better than a mere efficiency upgrade though- since it lowers power drawn from remote generators, power that requires and uses more grid capacity than locally sourced PV power.

Increased prevalence of PV affects the revenues & margins of investor owned utilities far more than it affects the other ratepayers, and that's a regulation & business model problem, not a cross-subsidy problem. But it's not in the short term interests of the investor owned utility to treat it as such, as long as they can have their way with the regulators. There has been a century of guaranteed profits on capital improvements that have driven the growth of utilities, but when those capital improvements are no longer needed, or not growing fast enough, it hurts the bottom line, investor dividends, and the share price. But that's not to say that the ratepayers (or PV owners) OWE the utilities a monotonically growing revenue stream. It's a tough thing to sort out on the regulators' end, but utilty business models will simply have to change- the 20th century model doesn't really work any more.

My N Ytimes lin k above has a different view. It works for me, but here is a snippet:

xxx In solar-rich areas of California and Arizona, as well as in Hawaii, all that solar-generated electricity flowing out of houses and into a power grid designed to carry it in the other direction has caused unanticipated voltage fluctuations that can overload circuits, burn lines and lead to brownouts or blackouts.

“Hawaii’s case is not isolated,” said Massoud Amin, a professor of electrical and computer engineering at the University of Minnesota and chairman of the smart grid program at the Institute of Electrical and Electronics Engineers, a technical association. “When we push year-on-year 30 to 40 percent growth in this market, with the number of installations doubling, quickly — every two years or so — there’s going to be problems.” xxx

Could be, as Dana claims. this is the empire striking back. Most certainly the empire will exploit problems as they arise. Dana notwithstanding, The specter of Darth Edison will make most people more wary rather than less. I repeat, if you're looking at a great deal in a place with expensive electricity and 365 sun, it may be too good to last. And, no, as NJ"s SREC market illustrates, PV is not an investment. It is an expense meant to cut ongoing costs.