What happens to carpet when the padding is not thoroughly fastened to the subfloor

I just recently finished my basement project. I spent a lot of time scouring the web and grilling friends and co-workers about their basements to gather information to try to make an informed decision on this very subject.

Here are the options I considered:

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Carpet tiles directly over concrete

PROS

• cheap

CONS

• cold feet
• hard, unforgiving floors
• Potential for condensation/mildew, even if floor is mostly dry

VERDICT

• I couldn't find a tile with a padding thick enough to make the floor even close to comfortable in a midwestern US winter

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Traditional carpet and pad directly over concrete

PROS

• Pretty cheap
• Many choices of carpet and padding thicknesses available

CONS

• Floor is still much harder than wood subfloor
• Even thick padding can still leave the floor a bit chilly
• Potential for condensation/mildew, even if floor is mostly dry

VERDICT

• Thick padding can help the floor feel less cold, and less hard, but it's no match for wood, especially if you have a kid's playroom or a workout room in the basement
• It seems like asking for moisture/mildew trouble to not allow air circulation between concrete and carpet

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Rigid foam insulation under 5/16" plywood subfloor

PROS

• Cheap
• Good R-value to provide insulation from cold
• Feels almost as warm and soft as a regular subfloor

CONS

• Opinions vary about how long the foam will last before breaking down
• Installation instructions vary wildly (some anchor directly to floor, others anchor through wood bracing, etc.)
• Opinions vary about how much air circulation the foam would allow

VERDICT

• Although it doesn't seem like a terrible idea, there just wasn't enough information about this approach to make me feel comfortable trying it

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DRIcore-style subfloor tiles

PROS

• Dimpled underside allows excellent air circulation over concrete
• Feels almost as warm and soft as a regular subfloor
• Most come in 2'x2' tiles so there are no 4'x8' sheets to lug around
• Easy install, much like a "floating" hardwood floor
• Nothing anchored into the concrete

CONS

• By far the most expensive option
• Each 4 sq foot panel is about $5.50 [as of 2016] USD

VERDICT

• I had a fairly large amount of floor to cover, and this was just too expensive. It would have approached 20% of the total budget for the project.

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Dimpled underlayment under 5/16" plywood subfloor

UPDATED March 2019

The Platon underlayment I used years ago seems to be hard to find these days. Home Depot currently offers what appears to be a similar underlayment product called DMX 1-STEP, and others may be available from other manufacturers.

PROS

• Dimpled underside allows excellent air circulation over concrete
• Feels almost as warm and soft as a regular subfloor
• Much cheaper than the DRIcore tiles, because the Platon is purchased separately in large rolls

CONS

• More difficult to install than probably any other option
• 4'x8' plywood subfloor is no fun to lug around
• Requires the subfloor to be anchored to the concrete

VERDICT

• Although it required more manual labor, the cost was less than 10% of what it would have cost to use the DRIcore tiles
• I still got it done in just a few days, with some help moving the plywood
• I used a hammer drill and Tapcon concrete screws to secure the floor
• My basement floor is as warm and soft as any other floor in my house, and it didn't cost an arm and a leg

You didn't mention how large your room was, but if it is fairly small you may get away without having to restretch the carpet. You will probably need new tack strip as it is difficult to remove the old without breaking it. Worse case would be that you may need to rent a knee kicker to stretch it back if the results without are disappointing. If the room is large, you may want to have a kicker ready to go. In either case, don't forget to have a 3 or 4 inch chisel or stiff putty knife to tuck the carper edge back down behind the tack strip. Good Luck.