So, the ground is what 11-12C, and the basement floor is the ground. While this might be good in hot climates where cooling predominates, in cold climates where heating predominates, it is not so good.

There is a constant heat loss through the basement floor. So I have one area with some parquet flooring, but the rest is concrete, with some old carpet remnants covering a couple other areas. The only part of the basement that is finished is a gym at the bottom of the stairs.

We picked up some foam square puzzle piece flooring that we intend to use to replace some of the carpet and to cover some of the concrete.

We have about 300-400 square feet of the stuff, which will cover maybe a quarter of the basement. So I am asking myself, what is the R value of this flooring?

It is not very thick for insulation, but is very thick for insulation-like flooring.

You would have to seal the concrete against moisture of course, but I am wondering will covering say 70% of the basement floor with this stuff make a significant difference in energy loss?

Would carpet and underlay be better insulation? Perhaps some combination of both depending on the use of the space?

Will it influence heat loss significantly?

If you don't put REAL insulation under carpet you can still end up with mold conditions on the underlayment from room air condensation in summer when the outdoor dew points are above 11C. The carpet & underlayment insulate the slab from the room air, but don't block the air.

I don't know if the rubbery puzzle piece stuff will support mold growth (I suspect it does), but if it does, you have the same situation there.

Dry dirt has an R value of about R3 per meter of depth, but that is drastically reduced with higher moisture levels. Distance to the water-table counts, since it behaves more like a refractory heat sink than insulation.

25mm XPS sheathing has an R value of about R5, and is vapor-retardent enough that it won't transmit ground moisture upward very quickly. That's about the minimum R to put beneath ~11mm OSB or plywood as a sub-floor to guarantee you don't end up with mold on the rugs or sub-floor with slab temps that low. It's OK to use foam board construction adhesive to apply both the XPS and subfloor if you like. If you have issues around stairwells with minimum riser height, building a landing or ramp at first-step level is just one of several options.

And yes, it will make a measurable difference in heat loss. Whether it's "significant" or not depends on the heat loss from the rest of the house. With old-skool R11 batts and leaking single-pane window 1950s construction it's kinda "in the noise", but with R20 insulation and tight double-pane glazing it could be a double-digit percentage of the total seasonal heat load, depending on climate. The heat loss out the slab is something of a constant, not varying much with daily or even monthly outdoor temperature swings- it's a much bigger fraction during the shoulder seasons than in mid-winter. With deep subsoil temps of 5C or colder there's a good argument for going R10 or higher. R10 code-min for new construction even in some parts of the US, and I can imagine in the Canadian midwest R15-R20 would have a rational basis in a 25 year present-value financial analysis on fuel savings.

in summer when the outdoor dew points are above 11C.

The ground is only at 11C on the day you dig into it. Then it warms up to close to room temperature and condensation potential and heat loss drops dramatically. You will not find your basement floor at 11C in your no foam insulation example.

With a covering of even R2 it'll be damned close to 11C where the foamy-bits meet the concrete.

I my own home the deep subsoil is ~10C, the uninsulated but well-sealed slab runs ~ 13-14C without any floor covering. (Room temp averages ~18C +/- 2C depending on time of year). Even the insulation value of a single sheet of corrugated cardboard (even when placed on a sheet of poly to rule out ground moisture) is sufficient to create a localized drop in temp that results in mold during summer humid periods, despite the room humidity being controlled to 60% RH. On my "maybe someday" project list is to jackhammer out the slab and dig down and drop R15 of EPS under a new slab. (I could use about 8cm more headroom down there too.)

Moving crap out of my parents basement (helping them clean) this week.  Boxes stored on the slab are all moldy but dry now. Boxes stored within 6” of the slab are moldy but dry, boxes stored on higher shelves are fine. Poly, but no insulation under the slab.

The floor is very cold all summer, and  slab is  colder then room all year long.
 
My house, 2” xps under the slab + poly, no mold on things left on basement floor. The slab  is warm to walk on  in bare feet room temp =~ slab temp.  Basement stays warm all summer.

You can peddle the no insulation under the floor stuff all you want, but I like it. I fact I regret not using 5-6” of foam, which I would do the next time.

Cheers,
Eric

If I have even 1cm of air between a box & the slab I don't have mold issues, but that probably would change if I allowed the RH to get above 60%.

Slab & foundation insulation issues are something of the last frontier of what's getting it into building codes. Very few places in the US require it, but most could use it.

My basement slab is 59F with a room temperature of 61F - and that's with carpeting but no under slab insulation. Deep ground temperature is around 50F. It would not make sense for me to spend much on foam to reduce this heat loss - ie, trying to achieve a slab temperature closer to 61F with a room temp of 61F.

The big item is the amount of moisture in the soils. I have a 4000 sq. ft. slab on grade church that has radiant floor heat with no insulation under the slab but cost very little to heat and never has moisture problems because the soil conditions are quite dry. My on place 20 km away would always have problems if I don't use insulation under slab because it has very high moisture content. Both soil temps are around 42º F (Calgary Alberta area.

Another way to look at it is what is the equivalent heat loss downward vs through an exterior wall, accounting for the R value of soil and the fact that the deep earth temperature is much warmer than the outside air. Picking several temperature points, I get ~R16 - ie, no insulation under my basement slab loses roughly the same heat as an exposed wall with R16 insulation. It would only take R4 of foam to cut this downward loss by 50%.

...and if you did a 25 year net present value calc on R10 under the slab with reasonable assumptions on inflation & discount rates you'd find that it's cost-effective at that level using Type-I EPS (if you didn't have to jackhammer out the slab and pour a new one, that is.) And if you raised the basement room temp to 68F, R15 would probably still cut the financial mustard.

FBBP: With larger slab-on grade structures it's more likely to end up with low or reasonable losses to dry soil and a deep water table. Slab center on that church is probably ~10m from any edge, but ~5m or less for most single family residences. The smaller the lateral distances to the slab edges, the less you can count on the earth-buffering/insulating effects. Slab on grade only 2m above the water table it's still pretty lossy even with dry soil and an 100m wide slab with well insulated edges.

I agree - slab and water table depths matter. As does the slab temperature - a 90F radiant floor justifies much more insulation than a 60F one.

Let's hope your radiant slab never hits 90F- that's really cookin'! Most stay under 80F on design-day, even on the first floor, and certainly would in most basement slabs.

With carpeting and a cold day, 90F sounds within the normal range to me - but high for an average temperature. Depends on how well you insulated the rest of the house.

90F would be a typical WATER temps for design-day with slabs, not the surface temp in a concrete slab radiant app. But I s'pose with a rug the concrete might break over 80F on design day.

No, some people run over 110F for the water. Or as the Radiant Panel Association says - "generally under 85F" (for the slab temp). Or see http://www.welserver.com/WEL0241/ who was running today at over 100F for the water in his geo system- and it's not even winter yet.

In a basement slab?

No idea, but then that's not a qualifier that I've been using.

Posted By jonr on 07 Dec 2010 03:41 PM
No, some people run over 110F for the water. Or as the Radiant Panel Association says - "generally under 85F" (for the slab temp). Or see http://www.welserver.com/WEL0241/ who was running today at over 100F for the water in his geo system- and it's not even winter yet.

Hey that's pretty neat Layout! I wish some of the net zero guys would do something like that so we could see it in real time. Unfortunately I can't tell from that what temp. he is actually running in his floor. I do know that my body tells me that anything over 100 is a fever;-)