In Dana's link above, Building Sciences used BaseCalc, a Canadiian govt energy modeling program, to compare perimeter to fulll slab insulation in a Minneapolis basement. (An exercise that will seem familiar to long-time readers here.) The difference was about 5MMbtus over the heating season. That sounds like a lot until you realize that 1MMbtu costs a tad more than $3 at wholesale prices for natural gas. You might pay twice as much per MMbtu for gas delivered to the home, in which your savings from full slab insulation soars to $30/yr. As BS summarizes in Dana's link, "Because of the relatively stable and mild temperatures below a slab, especially after several years of heat provided to the soil through the slab, the annual and peak loss is modest." It goes on to say that in highly insulated, high performance houses the slab matters more, which is to say that at some point of reducing above-grade heat losses, $30 might actually seem like something. None of this strikes me as middle of the road unless we are talking about the Santa Monica Freeway. What's more, if Dana knows something about 2030 and can provide stock tips, I'm all ears.

Comfort is a different matter. If at some point you do want living space in the basement, the way to avoid that musty, moldy basement thing is to put insulation over the concrete, including the slab.

"the way to avoid that musty, moldy basement thing is"
to build a well insulated and moisture resistant foundation and floor. It's not rocket science and it's not expensive in real terms, let alone for the benefit derived from having a dry comfortable space as opposed to a "musty moldy" one.

It's not dogma either. As Dana says foam on top claims a slight energy penalty but how is bad that the ac runs more when the problems are stale air and humidity?

To clarify, Basecalc assumes that the basement is living space and conditioned year-round. And St Louis is not Minneapolis. Full slab insulation in a plain ole basement in the former might save -- what? -- $10/yr? $15? $20? The Canadian govt did an extensive study of basement heat loss about 10 years back, the result of which was to call slab insulation optional except when used for radiant heat. And, again, St Louis isn't Sasketoon.

From what I was being quoted for the cellulose batts it was like $3500 to $4000 for the insulation.  SOunds like this wouldn't be a great investment from what I am reading.

On another note, looking at windows should I just go for Low E and the correct SGHC or is there a better brand over another?  i have read plenty but at this point. I have gotten bids on Marvin Integrity windows $10k  vs  Pella and Anderson are only $7k.  Just wanting to see for energy effeciency if this is a good investment or should I focus elsewhere in the house.  Seems like Marvins get a glowing review on the forum here.

"the way to avoid that musty, moldy basement thing is"
to build a well insulated and moisture resistant foundation and floor.

I agree, but add "run a dehumidifier".

there has been no need to run a dehumidifier in the last few houses I've built. I have installed HRVs. Built right, the basement stays dry.

Posted By toddm on 28 Aug 2012 08:35 AM
To clarify, Basecalc assumes that the basement is living space and conditioned year-round. And St Louis is not Minneapolis. Full slab insulation in a plain ole basement in the former might save -- what? -- $10/yr? $15? $20? The Canadian govt did an extensive study of basement heat loss about 10 years back, the result of which was to call slab insulation optional except when used for radiant heat. And, again, St Louis isn't Sasketoon.

About 10 years back the cost of propane and heating oil were about 1/4 of what they are now too, eh? ;-)

And they weren't exactly factoring in the humidity and mold issues that go away with better-insulated basements too.

Posted By cards66 on 28 Aug 2012 11:00 AM
From what I was being quoted for the cellulose batts it was like $3500 to $4000 for the insulation.  SOunds like this wouldn't be a great investment from what I am reading.

On another note, looking at windows should I just go for Low E and the correct SGHC or is there a better brand over another?  i have read plenty but at this point. I have gotten bids on Marvin Integrity windows $10k  vs  Pella and Anderson are only $7k.  Just wanting to see for energy effeciency if this is a good investment or should I focus elsewhere in the house.  Seems like Marvins get a glowing review on the forum here.

If looking at efficiency upgrades simply as a financial investment there are two primary factors:

1. The net-present-value of the cost of the envelope-upgrades relative to future utility savings over a fixed period of time (such as "how long do I intend to live here?" )

2. Will the efficiency improvements have any residual value that can be recouped a the time of sale.

The answer to #1 is, in most cases at the typical 6-7 year trade in period you can't even make a financial argument for code-minimum if you're heating with natural gas. (But if heating with propane code-min is easy, even if 1.5x  or 2x code min might not be.) If you're looking at 20+ years it depends a lot on what you think future energy costs are going to be- the cost of the upgrades can be considered as an insurance policy, or a hedge against energy price inflation & price volatility. But it's not always super-simple you also need to measure the cost of financing the upgrades against the annual energy savings, keeping in mind that the increased mortgage interest is tax deductible, whereas utility payments are not. If you're in a higher tax bracket that can sometimes tip the balance.

The answer to #2 is, there's no credible evidence that minor upgrades over code will have a resale value above a code-min house, but there is some evidence that high-R houses retain at least some market premium if they can be sold as such, with demonstrably miniscule utility costs.

The less talked about aspect though is comfort and indoor air quality, etc..  Code-min might be 2x4 R13+5 or 2x6 R20-cavity-only but a house with R20+ whole-walls and better windows is more comfortable to live in, particularly at the temperature extremes. And a tight house with an insulated basement, with an active ventilation system & humidity control always feels good, with no lingering smells, no damp musty corners. And it's these factors that make people want to hang onto the houses once they've lived in them, more so than the fact that the utility costs are low.  From a comfort point of view going with WAY-better-than code windows (say U0.20-0.22 triple-panes) makes a bigger difference than better-than-code R-values on walls, even though it's a much lousier investment when measured in utility costs. (I've heard but not verified that Harvey makes some less-expensive triple-panes, if that's where you think you might put the money.  Some of the Net Zero builders in my area who previously used Paradigm have moved over to Harvey citing price/performance.)   When it's +15F outside you can FEEL the difference between a U0.30 window and a U0.20 window standing a few feet away.

From purely an investment point of view air-sealing ROCKS!  Defining a primary air-barrier and implementing during construction is the cheapest, most cost effective upgrade you can make, an it does pay off in short years on utility bills.  But finding a contractor that really knows how to make that happen requires some digging- even SIPs and ICFs are no guarantee that it won't have leakage well over 5-6 ACH/50 (that's air exchanges per hour at 50 pascals pressure, a common calibrated blower-door standard), yet it's not tough to build to under 2 ACH/50, and the savings on heating & cooling energy use is in double-digit percentages.

On new construction in US zone 5 building anything less than ~R20 whole wall would be something of a waste if you intend to live in it.  The 2x4 w/cellulose + 2" of exterior iso solution is dead-easy to build, fits doors & windows designed for 2x6 framing without a lot of fuss, and with better windows and an R50+ energy-heel truss attic could reduce your heating & cooling loads to within the output of ductless heat pumps that would run at nearly the efficiency of ground source heat pumps at a fraction of the cost, at comparable installed costs of separate heating & cooling systems.  If you orient the roof for optimal solar gains, when the price (or subsidy) of grid-tied photovoltaics reaches the an optimal point, with net-metering the utility costs become net-tiny if not net-zero.  (In many places where it's subsidized that optimal point is right now.)

The R and U values are only part of it, but a necessary part to hit those very low operating cost numbers, and often the cost of financing the upgrades on a 30 year mortgage is more than made up by the effectively-zero utility cost.  But where those crossovers happen will vary by quite a bit and are strongly affected by subsidies.  It takes a sharp pencil and some energy-use simulation in the planning phases to fully optimize any particular situation.

I don't turn cards66's wall query into a basement thread so, Dana, link your research that shows full slab insulation demonstrably reducing humidity across multiple climates vs perimeter slab insulation and I will go away. Seems to me BS offers a choice between R7.5 full slab or R10 perimeter in your earlier link so they probably need some enlightening too.

My direct experience in US climate zone 5 sez under-slab insulation raises the slab temp, with a direct impact on whether a cardboard box stored on the floor gets a moldy bottom or not. It's not a change in the absolute humidity in the basement, it's about the RELATIVE humidity- keeping the slab temp above the dew point of summertime basement air when deep subsoil temps are in the 50 (in my case, low-50s) whether there's a box (with it's own intrinsic R) resting the slab or not.

The BSC recommendations aren't a choice between R10-edge or R7.5 center slab. The boolean operator in effect is "AND", not "OR", it's R10 edge AND R7.5 sub-slab.

But go ahead and enlighten them, eh? Lstiburek has been known to change recommendations based on reliable data, but being a native-born Canadian with a keen professional interest in the topic I'm sure he knows more about the data & methodology behind Canadian BaseCalc basement heat-load modeling (and other basement models) than either of us ever will.

I guess you missed the part where BS acknowledges "the relatively stable and mild temperatures below a slab, especially after several years of heat provided to the soil through the slab." (Translation: whatever the deep subsoil temperatures in your part of the world, that's not what you'll find if you put a thermometer on your basement floor. Feel free to do just that.)

In fact, the BS report offers a window on the mid-slab temperature difference. According to its chart you'd pay a 5 million btu/yr penalty for insulating perimeter only in Minneapolis vs whole slab. Over 7 months of heating that works out to about 100 btu/hr divided by some unstated area of concrete slab. In my basecalc example I used 1000 square feet of basement floor, also in Minneapolis, of which about 600 feet would be uninsulated in a perimeter approach. In peak loss conditions in Jan, the midslab was losing about 400btu/hr, or .67 btu/square foot/hr. That would be a temperature difference between soil and concrete measured in hundredths of a degree. Again, basecalc assumes that the basement is heated; a well insulated unheated basement set in well drained soil will be cooler, but not the 50 degrees that Dana wants you to believe.

Make that well insulated where it counts. Conductive heat transfer is a straight-ahead process: flowing in one direction under the slab, in two directions at footer edges and in three directions at footer corners. Therefore it is imperative to break the thermal bridges between the slab and the rest of the foundation. By "slab edge" BS means the short, vertical break between floor and wall. (My bad for not reading it more closely; in my defense I was misled by Dana's earlier and equally erroneous boolean operator "if".) Perimeter foam breaks the bridge between footer and slab, and you are good to go in normal, well drained soil.

Understand that condensation is still possible in climates like mine (DC area) where dew points hits the 70s. The questions are how much full slab insulation would help given limited responsiveness in mid slab, whether the basement would qualify as comfortable even if it did, and how much mold you'd get regardless. I'm thinking dehumidifier. AC would give you air movement and filtering but the problem in basements is how to keep it running, short of putting insulation on top of the concrete. Still waiting, Dana. Show us multiclimate research comparing perimeter to full slab insulation.

I never said an uninsulated slab hits 50F if the deep subsoil is 50F.  I do say that uninsulated slabs in the eastern half of zone 5 will be at or below the summertime dew points.  Sections of the slabe where stored goods may be stacked will be even cooler, due to the R-value of the stored material leading to mold conditions.  In the eastern half of zone 5 summertime dew points average in the mid to high 60s F, and even with some mechanical dehumidication an uninsulated slab will have this issue.

Prime example:  I live in zone 5 (central MA).  Even with a basement air temp of 68-70F in the late summer, the uninsulated slab in the older section of my house measures about 60F in most places, and a bit below 60F on the north edge of the house. (Within the precision error of an infrared thermometer.) This well below the average outdoor dew points. But the insulated slab in an addition is above 65F.  The air is the same- there's a 40" full height archway connecting the two.  A dehumdifier in the older section keeps the RH in check, and uses about 300kwh/summer (down from 550kwh before I took more air sealing measures in the basement. Power is being logged with a Kill-a-Watt.) 

But even with the dehumidifer holding the line at ~55% RH @ 70F (53F dew point) a cardboard box on the floor in the north section will slowly develop mildew on the bottom over a summer even resting on poly sheet to rule out ground moisture wicking. But nothing resting on the insulated slab has that issue.  It doesn't have to hit the actual dew point to become musty/moldy- only a temp low enough that the localized RH is above 70% or so- the RH of 70F, 55% RH air crosses that threshold at 62F, which is barely above the average slab temp in that section of the house.

If a basement is unheated in winter the heat loss out the slab is lower, but the average temperature of the slab is lower, increasing summertime mold issue.

A timely discussion of those issues came up on this blog this morning (written by a guy with extensive experience building in zones 5 & 6, currently living on the warm edge of zone 5):

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/basement-insulation-part-1

On another note, looking at windows should I just go for Low E and the correct SGHC or is there a better brand over another?

Hmmm...let me see, thread is about best construction for the money.....is there a spouse involved?

As for Pella vs Marvin....both excellent quality. Don't pay a premium for one over the other.

That's a lovely orange you're describing, Dana, and I assume that the older part of your basement is not insulated to BS guidelines, if at all. Now back to apples, My question again is whether BS has any justification for specing full underslab coverage vs perimeter foam on a comfort basis. Applying common sense to Basecalc results suggests "no." If the fiscal difference is negligible because of a negligible temperature difference between soil and mid-slab, how can the physical difference be outsized?

I respect your knowledge and your willingness to help, and I generally agree with you (and Bob and BS) that it is wiser and greener to substitute insulation for HVAC capacity. But I draw the line at the basement stairs. Increasing insulation does nothing about relative humidity. RH in the basement is exacerbated by convection (open the basement door and cold slides down the stairs) and thermal mass (all that concrete is cold to begin with.) This is climate specific, of course, but if I wanted living space in a basement in my part of the world, I'd be looking for ways to keep keep air stirring below grade. Hvac that blends upstairs and down is one approach. So is covering the slab with XPS and floating a floor on top of it.

To avoid kumquat or pawpaw replies let me reiterate. I agree with BS's guidelines in every respect except to say that full underslab coverage isn't worth the money.

The older part of the basement has been retrofitted to R18 on the walls & band joists. The BSC table spells out R15, for zone 5 but I had access to reclaimed 3" fiber-faced roofing iso and didn't feel the need to wait for thinner stuff,eh? ;-) ) The addition was built on an R16 ICF foundation but the sub-slab foam is only R5, not R7.5 as in the table, which is not a big deal in the grander scheme- it's proving to be enough for moisture control. Insulating the foundation walls & band joist on the older foundation reduced the heat load and fuel use by more than 15%, which was within expectations.

On the older section jackhammering out the slab and digging down, relocating drains that run 30' just under the slab, and digging up the sewer in the yard to lower IT to match has no economic rationale over the anticipated life of the building. I'm pretty much stuck with the slab where it was poured in 1923, and focus my efforts elsewhere.

Putting insulation under the slab does EVERYTHING for the relative humidity of the air films adjacent to the cool slab, where the box comes to rest. (And wall insulation does the same, well below-grade.) It has a more modest effect on the whole-room's RH by raising the average summertime temp in the room a few degree. This is pretty straightforward stuff if you have a psychrometric chart (or calculator) handy.

The savings are more than just the space-heating savings, when the increased dehumidification load is factored in. I'm paying ~$50/year on the dehumidification now, and that would be cut by more than half with an insulated slab, since the overall temp would rise and I could hold the line at 60%RH or maybe a bit more without a detectible whiff of mold.

Is the cost of the foam worth the $25/year in power savings + whatever it is in heat load to add R7.5? It certainly is over the lifecycle of the building, if not the short term.

But is it worth never having to sniff a musty basement and keep what I store down there from stinking? HELL YES!! (Even in the 1-year time frame!)

It's like the comfort vs. NPV on reduced energy use savings arguments- making the basement a more comfortable, dry, and more usable space (even if it 's just for storage) has value well beyond any simplified financial model.

Hmmm...let me see, thread is about best construction for the money.....is there a spouse involved?

As for Pella vs Marvin....both excellent quality. Don't pay a premium for one over the other.

I like the fiberglass windows. however, after researching the Pella ones they seem to have gotten pretty bad reviews where Marvin seem to get pretty much good reviews.

As for a spouse being involved how did you ever guess?

As for a spouse being involved how did you ever guess?

Once you choose the general type and construction of window, let the spouse choose the rest.

I looked at them both, and I didn't buy fiberglass, but it would surprise me if Marvin made a good one and Pella made a bad one. They are both quality outfits. When you are talking about billions of dollars worth of windows, much of that online review stuff is jiggered.

I looked at them both, and I didn't buy fiberglass, but it would surprise me if Marvin made a good one and Pella made a bad one. They are both quality outfits. When you are talking about billions of dollars worth of windows, much of that online review stuff is jiggered.

What window did you end up going with if you don't mind me asking? Are you happy with their performance?

So, in summary, your comparison of R5 full underslab insulation coverage in your addition vs zero sub slab in the original house tells us nothing about the relative comfort merits of full slab vs perimeter (and slab edge which icf gives you in the addition). The perimeter counts; the middle doesn't. In normal soil anyway. IIRC, you have water table issues as well.

You don't dispute that full slab coverage saves very little money. You don't dispute Basecalc's estimate that a Minneapolis basement in January loses heat from the middle of the slab at the miniscule rate of 0.67btu/sq ft/hr. Yet, somehow, sloooow changes in midslab make a major difference in comfort. How? And where is the proof?