Key Takeaways
| Factor | Requirement |
|---|---|
| Moisture test | Mandatory—calcium chloride test or plastic sheet method |
| Vapor barrier | 6-mil polyethylene minimum for below-grade slabs |
| Underlay choice | Rubber or synthetic—never rebond foam on concrete |
| Curing time | New concrete needs 60–90 days minimum |
| Temperature | Slab must be 18°C+ at install |
| Adhesive | Moisture-resistant adhesive for direct glue-down |
Why Concrete Slabs Need Different Treatment
Concrete is porous. It pulls moisture from the ground beneath it. This moisture travels up through the slab and hits whatever you’ve laid on top. Carpet and underlay absorb that moisture. Mold follows. The carpet buckles or develops a smell within months.
Ottawa basements sit below the frost line. The ground stays damp year-round. Even newer homes with external waterproofing still deal with humidity transmission through the slab itself. You can’t skip the moisture test. Every concrete floor installation starts there.
Wood subfloors breathe. Concrete does not. The prep work is longer. The materials cost more. But the alternative is pulling up a ruined carpet in two years and starting over.
Testing for Moisture Before You Install
You need a calcium chloride test or a plastic sheet test. The calcium chloride method is more precise. You tape a small dish of calcium chloride salts to the slab, seal it, and wait 60–72 hours. The salts absorb moisture. You weigh the dish before and after. If the moisture vapor emission rate exceeds 3 pounds per 1,000 square feet per 24 hours, the slab is too wet for carpet installation.
The plastic sheet method is faster but less accurate. Tape a 2-foot-by-2-foot plastic sheet to the concrete. Wait 24 hours. If condensation forms under the plastic, moisture is present. If the concrete under the plastic darkens, the slab is releasing vapor.
Both tests work. The calcium chloride test gives you a number. The plastic sheet test gives you a yes-or-no. Either way, you need to know before you proceed.
New concrete takes 60 to 90 days to cure. Fresh basement slabs in Ottawa new builds need the full cure time. Installing carpet early traps residual moisture. The carpet adhesive won’t bond properly. The underlay compresses unevenly. Wait the full period.
Vapor Barriers: When and How to Use Them
Below-grade slabs—anything underground—require a vapor barrier. Use 6-mil polyethylene sheeting. Overlap seams by 6 inches and tape them. Run the barrier up the wall 2 inches. This prevents lateral moisture from creeping in at the edges.
Some installers skip the vapor barrier if the moisture test comes back low. This is a mistake in Ottawa. Ground conditions change with the season. Spring thaw brings water. A dry test in January doesn’t mean the slab will stay dry in April.
The vapor barrier goes directly on the concrete. The underlay goes on top of the barrier. Do not puncture the barrier with staples or nails. Use tape to secure edges.
If you’re installing over a slab with radiant heat, confirm the barrier is compatible with heat. Standard polyethylene works for most systems. Check the radiant system specs before you lay anything down.
Choosing the Right Underlay for Concrete
Rebond foam underlay—the recycled foam type common in residential installs—does not belong on concrete. It absorbs moisture. It compresses under weight and doesn’t recover. Use rubber underlay or dense synthetic foam rated for concrete.
Rubber underlay costs more. It lasts longer. It blocks moisture better than foam. It provides a firmer feel underfoot, which some homeowners dislike, but it won’t degrade when exposed to occasional dampness.
Synthetic underlay (often polyurethane-based) offers a middle ground. It resists moisture better than rebond foam and costs less than rubber. Look for products with a built-in vapor barrier or moisture-blocking layer.
Thickness matters less than density on concrete. A thinner, denser underlay outperforms a thick, soft one. Aim for 6–8 pounds per cubic foot density minimum. Avoid anything marketed as “plush” or “luxury” unless it’s specifically rated for concrete slabs.
For basement carpet installs, rubber underlay is the standard recommendation. Basements flood. Humidity spikes in summer. The underlay needs to handle it.
Direct Glue-Down vs. Stretch-In Installation
Stretch-in installation uses tack strips around the perimeter. The carpet stretches over the underlay and hooks onto the strips. This method works on concrete if you use masonry nails or concrete adhesive to secure the tack strips. The underlay and vapor barrier go down first.
Direct glue-down skips the underlay. The carpet backing glues directly to the concrete. This method is common in commercial settings and high-traffic areas. It’s thinner. It doesn’t shift. But it requires a perfectly level slab and moisture-resistant adhesive.
For residential basement carpet in Ottawa, stretch-in installation is more common. It’s easier to remove if the basement floods. The underlay adds warmth and sound dampening. Glue-down is harder to reverse and offers less cushioning.
If you’re doing glue-down, use a urethane-based adhesive rated for high moisture. Water-based adhesives break down on damp concrete. The carpet will lift at the edges within a year.
Level the slab before either method. Grind down high spots. Fill low spots with self-leveling compound. Carpet will telegraph every dip and bump in the concrete beneath it.
Temperature and Humidity Requirements at Install
The slab needs to be at least 18°C (65°F) at the time of installation. Carpet adhesive and underlay both require stable temperatures to set properly. Cold concrete slows adhesive curing. It can also cause the carpet backing to contract, leading to seam separation.
Ottawa winters drop basement slab temperatures. Heat the space for 48 hours before installation if the slab feels cold to the touch. Use portable heaters or run the furnace. Don’t install in an unheated basement in January.
Humidity should be between 30% and 60% during installation. Too dry and the carpet backing can crack. Too humid and moisture gets trapped under the carpet. Use a hygrometer to measure. Run a dehumidifier if levels are high.
Carpet needs 24 hours to acclimate to the room temperature before installation. Unroll it in the space where it will be installed. Let it relax. This prevents expansion or contraction after the install.
Common Mistakes on Concrete Slab Installs
Skipping the moisture test is the top mistake. Installers assume the slab is dry because it looks dry. Moisture vapor is invisible. The test is not optional.
Using the wrong underlay is second. Foam underlay marketed for wood subfloors will fail on concrete. It absorbs moisture. It compresses. It doesn’t recover. Use rubber or synthetic rated for concrete.
Failing to level the slab creates permanent bumps in the carpet. Concrete self-leveling compound costs less than replacing a carpet that looks wavy.
Not sealing the vapor barrier seams lets moisture through. Tape every overlap. Run the barrier up the wall edges. Treat it like a waterproof membrane because that’s what it is.
Installing too soon after concrete is poured traps residual moisture. New slabs need 60–90 days. There is no shortcut.
For proper carpet installation over concrete, the prep work takes longer than the carpet laying itself. Rushing the prep guarantees problems.
When to Call a Professional Installer
Moisture testing requires equipment most homeowners don’t own. A calcium chloride test kit costs $30–$50, but you need to know how to interpret the results. Professional installers carry moisture meters and know the local building code requirements for Ottawa.
Leveling a concrete slab requires a grinder and self-leveling compound. Mistakes create permanent ridges. Grinding too much weakens the slab. A professional has the tools and the experience to level the slab without overdoing it.
Vapor barrier installation looks simple but mistakes are common. Poorly sealed seams let moisture through. Barriers that don’t run up the wall allow lateral moisture infiltration. An experienced installer seals the barrier correctly the first time.
If you’re installing in a basement prone to flooding or high humidity, a professional can recommend moisture-blocking underlay and adhesive that matches the conditions. They’ve seen what fails. They know what lasts.
For concrete slab carpet installation in Ottawa, especially in basements, the cost of a professional install is insurance against moisture damage. Get quotes from installers who specify moisture testing and vapor barriers in their process. If they don’t mention it, find someone else.
FAQ
How long does concrete need to cure before carpet installation? 60 to 90 days minimum. Fresh concrete releases moisture as it cures. Installing carpet before the cure completes traps moisture and can damage both the carpet and adhesive.
Can I install carpet over a painted concrete floor? Yes, but the paint must be fully cured and bonded to the concrete. Loose or peeling paint will prevent adhesive or tack strips from holding. Scrape off any loose paint and clean the surface thoroughly.
Do I need a vapor barrier on a concrete slab above grade? Test the slab first. Above-grade slabs are less prone to moisture but can still release vapor, especially if the ground beneath was recently graded or if there’s poor drainage around the foundation.
What happens if I skip the moisture test? Mold, mildew, carpet buckling, and odor. Moisture trapped under carpet creates an ideal environment for mold growth. The carpet may need to be removed and replaced within 1–2 years.
Can I reuse old underlay on a concrete slab? No. Old underlay has compressed and may have absorbed moisture or odors. Use new underlay rated for concrete. Rubber or synthetic only.
How do I know if my basement slab has a moisture problem? Run a plastic sheet test or calcium chloride test. Visual signs include efflorescence (white mineral deposits), dark spots, or a musty smell. Any of these indicate moisture issues.
Is carpet a good choice for a basement that floods occasionally? No. Carpet absorbs water and is difficult to dry. Consider carpet tiles that can be removed and replaced, or choose a different flooring type. If you proceed with carpet, use a waterproof vapor barrier and moisture-resistant underlay.
Can I install carpet over a cracked concrete slab? Small hairline cracks are fine. Large cracks (wider than 1/8 inch) should be filled with concrete crack filler before installation. Movement in the slab can cause the carpet to separate at seams.
What type of tack strips work on concrete? Use tack strips with masonry nails or apply them with concrete adhesive. Standard tack strips with wood nails will not hold. Pre-drill holes if using masonry nails to avoid cracking the concrete.
How much does professional carpet installation on concrete cost in Ottawa? Expect to pay $3–$6 per square foot for materials and labor, depending on the carpet grade and underlay choice. Add $200–$400 for moisture testing and slab prep. For a detailed quote, visit our contact page.