Floor pad over slab: Recommendations

Heatmeister_2

This is a two part question:

1) My basement is roughly 1200square feet of heated slab. Before I lay the carpet I wanted to learn more of the impact of carpet padding in this application. Normal pading of 3/8" have an R Value roughly of 1.25-1.50. This is made up of synthetic sponge material ground up and glued together. However, they also make a pad made of true rubber that has a more spongey feel to it and is designed for heated floor applications with an R of .5 but costs TWICE as much. Over a slab with tremendous wall insulation, does it matter?

With such low R values with either pad, will it matter?

2) If so, Since I re-trofitted in floor between my floor joists upstairs, should I replace the padding in the rooms upstairs that have carpet?

ALH_4

Carpet and radiant

Unfortunately carpet cuts radiant floor output severely compared to tile or hardwood. The carpet over the slab might be a problem because, assuming a 15btu/sf design heat load, the required fluid supply temperature might go from 90°F to 140°F (very approximate numbers). Charging up a slab with 140°F water might cause over/underheating of that zone since it could take hours to warm up and cool. Also, a warmer slab loses more heat to the ground beneath itself.

Definitely go for the radiant friendly pad if you decide to stick with carpet over the slab. What radiant floor installation method did you use upstairs?

Unknown

we generally recommend shooting for a maximum R2 combined pad and carpet, so having a 1.5 in just the pad doesn't leave you much for the carpet.

It does depend on your heat load though. The lower your heat load, the less it matters.

It's more important in an underfit method, and your loads are probably higher up there. Again though, the heat load calculation is what tells you if this is a problem or not.

Brad White

Rubber, not urethane

As Rob and Andrew said, keeping the R value low, to a maximum of 1.5 or 2.0 total is the best advice, even if you have to change the padding. I am fighting such a battle now.

Even so, provided your heat loss is met even with a reduced output, having a little R value above the floor makes the floor surface temperature nice and even. A small bonus but it is there.

bruce_21

How much in absolute terms, total dollars, not percent does the R=.5 pad cost? Is that affordable to optimize your heat system? If you go with the cheap pad you'll need to have R=10 ( two inches of styrofoam) at least under the slab to keep the heat from going down too much, same with the other floor, R=15 min. below the tubing, given you've got a wood subfloor also insulating the tubing from the room. Going with the low R pad gives you more options and slack with the rest of the system.

Heatmeister_2

Slab and Floor

The slab has 2 1/2 inch of styrofoam under it. The floor has R 15 under the tubing.

The rubber backing is over $5/yard vs. $2.5 for the urethane which comes out to $240 more for the whole carpet job. Small price to pay when you figure the whole basement with is exponentially more.

Mike T., Swampeast MO

Here's an extremely low R pad (.097 R). It certainly won't be thick and cushy, but carpet installers tell me that thin, high-density pads are best for minimizing carpet wear.

Watch out as not all pads are suitable for below-grade use. Supplier claims all of their pads are, but I'd still verify.

Harold

There seems to be a bit of a disconnect between "Outstanding thermal and sound insulation." and .097 R valus.