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How much am I going to save by insulating slab?

I have a daylight basement three story with an uninsulated slab in Southern Maine. It presently has 3/4 sleepers with eps between them. Is it under renovation, is it worth the work to pull sleepers to better insulate, also is radiant a better fit than baseboard hydronic. Worth the extra expense? Thanks

Asked by Robert Mason
Posted Feb 4, 2013 3:34 PM ET


7 Answers

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It's hard to know where the thermal weaknesses are in your house without a site visit.

If your "daylight basement" includes a door, then the area near the door is usually a thermal weak point. You certainly need vertical rigid foam insulation directly under the door threshold, extending down at least 3 feet into the soil.

If your basement has uninsulated concrete walls, they certainly need to be insulated.

Whether it makes sense to improve the floor slab insulation is a judgment call. You don't have to pull up the sleepers if you don't want to. You could install a continuous layer of 2-inch-thick rigid foam right on top of the sleepers and EPS, followed by a layer of plywood. (Of course, you'd need to adjust your door threshold if you do that, assuming you have a door.)

Answered by Martin Holladay
Posted Feb 4, 2013 4:05 PM ET


In new construction in a southern ME location heated with electric baseboard, oil, or propane it's worth hitting R10 under the slab if unheated, (R15 if radiant) would make sense. For retrofits with foam above the slab it's more labor intensive and has other considerations about adjusting stairs & thresholds to accommodate the changing floor level, which adds significant expense.

At the slab edges & above-grade perimeter it's all about heat loss, but with mid-slab insulation it's as much about mold & moisture control and comfort: Summertime outdoor dew points are much higher than the subsoil temps in a ME climate, and a box left on the floor of an uninsulated slab will get moldy, even if there's a capillary break and vapor barrier under it. (It wouldn't surprise me if the existing 1x sleepers were a bit punky or moldy on the bottom side where they contact the cool slab, not from ground moisture, but from summertime moisture from the conditioned space air.) While R3 (3/4" EPS) is enough to mitigate the worst mold issues, another inch would buy you some margin. If you leave them in and install more foam over them it's probably worth putting 6-mil poly between the sleepers and new foam.

From purely an operating cost point of view there's rarely a good economic case to be made for the expense of radiant floors, but from a comfort point of view it's huge as compared to fin-tube baseboard. In-between is low-temp hydronic panel radiators, which are far more comfortable than baseboard, yet far cheaper than radiant floor. If it's a finished basement you plan on spending some time in winter a radiator approach or even radiant floor may be "worth it", but if it's workshop space or intermittent use space, maybe not.

What is your heating fuel & space heating equipment?

Answered by Dana Dorsett
Posted Feb 4, 2013 6:34 PM ET


The space is used as year round living room,kitchen areas.The room is 23 feet across contributing to the cool feel I imagine. The perimeter has 2 inches of eps 4 feet down on the outside of frost wall. I heat mostly with a PT105 pellet boiler with oil backup. The sleepers are PT. What a great tool this site is and the people who contribute. Many thanks.

Answered by Robert Mason
Posted Feb 5, 2013 5:13 PM ET


I assume you meant the Harman PB105 (not PT...) boiler? Seems like a lot of boiler, but it can probably still run at reasonable efficiency even if it's (probably) 2-3x oversized for the actual load. (And pellet pricing in ME sure beats oil with huge margin!)

If it's a year-round living-room there's a comfort argument for radiant. Without more technical specs on the pellet boiler it's hard to say if running low-temp radiant involves any greater design/implementation difficulty than it would be running an oil boiler, but it's probably not going to be as happy & thrilled as a gas-fired modulating/condensing boiler at low-low temps. Any good heating system design starts with a careful heat load calculation, from which all the rest flows.

Answered by Dana Dorsett
Posted Feb 6, 2013 6:31 PM ET


Thanks Dana, We have 6000sf but I think it will handle it, with Martin's previous advice we found a lot of air escape in the roof system, which was wasted BTU,s we can use in the radiant. A dealer online says a DIYS system for 800 st would be $3000. Would you do it with the present degree of insulation,headroom is an issue that my family says would kill the looks of both rooms. I'm planning on mixing 180 boiler supply.

Answered by Robert Mason
Posted Feb 6, 2013 8:05 PM ET


If you don't have the headroom to accommodate a higher-R floor the floor losses of a heated floor will be significant. (I'm not 100% sure, but IIRC in ME there are minimum R values spelled out in building codes for heated slabs that would be in effect here, even as retrofit.) Radiant between-the joists ceilings (with aluminum heat spreaders behind the gypsum) can work if your heat loads are low enough (there's a not-to exceed temperature limit on the gypsum when going with radiant walls and ceilings), but low temp flat panel radiators can be as (or more) comfortable than radiant ceiling approaches.

Answered by Dana Dorsett
Posted Feb 11, 2013 5:51 PM ET


Thanks for your help

Answered by Robert Mason
Posted Feb 11, 2013 10:19 PM ET

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