Short answer: put your money in the underlay before you upgrade the carpet. A mid-range carpet on a good underlay feels and lasts better than an expensive carpet on a thin one. Here is what you are choosing between.

What underlay does

Three jobs. Comfort, because it is what makes a carpet feel deep. Wear, because it absorbs the footfall instead of the carpet backing taking it, which is why carpets on poor underlay go flat at the door first. And warmth and sound, because it is a layer of insulation between you and a cold or noisy floor. In a tenement or a semi it is the cheapest sound insulation you will ever buy.

The types

PU foam is the common one. Made from recycled foam, sold by thickness and density, and good value. The trap is that thickness is not quality: a thick, light foam squashes flat within a year. Look at the density figure, not the millimetres. We fit a heavy-density foam as our standard because it holds up.

Rubber (sponge or crumb) is the traditional choice. Heavier, firmer and very long lasting. Crumb rubber is the one for stairs and halls where the carpet is under real strain; it does not bottom out. Sponge rubber is softer and a lovely feel in a bedroom or living room.

Felt and wool underlays are firm and warm and suit wool carpets, particularly loop piles, where a soft underlay lets the carpet stretch and ripple.

Which goes where

Stairs and halls: a dense foam or crumb rubber. Comfort matters less than survival here. Living rooms and bedrooms: a thicker, softer foam or sponge rubber for the feel. Anywhere over a concrete floor: a heavier underlay for warmth. Under a wool loop carpet: something firm.

Why it changes the quote

Underlay is priced per square metre like the carpet, and the gap between a budget foam and a proper one over a whole house can be a couple of hundred pounds. That is usually where a suspiciously low quote has found its saving. Ask what underlay is included and what it weighs. If the answer is a thickness with no density, be careful.

Can I keep the old underlay?

Occasionally, if it is a good rubber that is not crumbling. Usually not. Old foam is flat, old rubber turns to dust, and either way you are putting a new carpet on the thing that wore the last one out. We will tell you honestly when we come to measure.

Every quote we give lists the underlay by name and grade so you know what you are getting. Book a free home measure or see the carpet ranges.

Paul James Home, Co-op Department Store, Clydebank. Monday to Saturday 9 to 5. 0141 266 0400.