You want the room darker. Which word are you looking for?
Blackout and thermal get used as though they mean the same thing, and shops do not always help. They do not mean the same thing. They describe two different intentions, and understanding which one you actually have will save you buying the wrong curtain.
Blackout is about light. Thermal is about the layer of insulation between the room and the glass. A curtain can be built with both in mind, but they are separate goals and they are solved in slightly different ways.
What "blackout" is trying to do
A blackout construction is aimed at reducing the light that passes through the curtain itself. That is normally done with a dense lining, or a fabric woven or coated so that light does not travel through it, hung behind or bonded to the face fabric.
The important word there is "through". A blackout lining works on the fabric. It does nothing about the light that arrives some other way, which is the part almost nobody is told before they buy.
The blackout lining offered here is a three-pass coated lining: a woven base carrying three layers of coating, which is what stops light travelling through the cloth itself. It is heavier and denser than a standard cotton sateen lining, so it changes the curtain as well as the light, giving a flatter, more solid drape and a firmer feel in the hand.
The honest bit: no curtain makes a room fully dark on its own
This is the single most useful thing to know before you spend money on blackout curtains.
Light comes round a curtain, not just through it. It comes over the top, between the heading and the wall or ceiling. It comes down the sides, in the gap between the leading edge of the curtain and the wall. It comes through the centre, where the two curtains meet. And it comes under the hem, especially where a curtain stops at the sill and daylight rises off a pale windowsill into the room.
You can fit the densest lining made and still have a bright line down each side of the window at six in the morning. If a room is not dark enough, the lining is often not the thing at fault.
Fitting matters as much as lining
If darkness is your goal, plan the hardware and the curtain size around it. Every one of these does more for a dark room than upgrading a lining will.
Fit the pole or track higher and wider. Mounting well above the frame and running the hardware past the reveal on both sides means the curtain covers wall rather than stopping at the edge of the glass. That closes the two biggest gaps at once.
Get a return to the wall. A curtain that finishes flat against the face of the wall, rather than hanging in mid-air at the end of the pole, blocks the side gap. Returns are easier with a track, or with a pole using a bracket that lets the curtain wrap back, and they make an obvious difference.
Overlap in the middle. Two curtains that simply touch will show a line of light where they meet. Curtains that cross over each other in the centre will not. A track with an overlap arm at the centre does this by design.
Drop below the sill. A curtain that finishes at the sill leaves the sill lit. Taking the curtains to the floor, or well past the sill, cuts the light coming up from below.
Consider mounting to the ceiling. Where there is no wall above the window, a ceiling-fixed track removes the gap over the top of the heading. Ceiling-mounted curtain tracks covers when this is worth doing.
Fitting is not a consolation prize for people who cannot afford the right lining. It is the larger half of the job.
What "thermal" is trying to do
Thermal describes the intention to put an insulating layer between the room and the window. In practice that means a lining, or an interlining between the face fabric and the lining, that adds bulk and traps a layer of still air.
The general principle is not complicated: a window is the thinnest part of an external wall, and a curtain is another layer over it. Interlined curtains are heavier and denser than unlined ones, and they hang with more body as a result.
There are two ways to build that layer. A thermal lining replaces a standard lining and adds an insulating layer at the window: the room feels warmer near the glass and there is less of the draught that comes off a cold window in the evening. Interlining goes further, a soft layer sandwiched between the face fabric and the lining, which adds real bulk and gives the heavier, more furnished drape that comes with it. Interlining is a made-to-measure option, because it is built into the curtain rather than added to a finished one.
What either one changes in your room depends on the window, the fabric and how well the curtain closes off the space behind it.
The same fitting points apply, and for the same reason. A curtain that sits tight to the wall at the sides, overlaps in the centre and reaches past the sill encloses the window far better than one that hangs loose in front of it.
Can one curtain do both?
Often, yes, because the two goals overlap. A dense lining tends to be a substantial one, and an interlined curtain is thick enough to cut a good deal of light. Many people buying for a bedroom want both at once, and that is a normal thing to ask for.
But do not assume it. Blackout and thermal are separate linings here: the blackout lining is the three-pass coated one, chosen for what it does to light, and the thermal lining is chosen for the layer it puts between the room and the glass. Interlining is the option that does a good deal of both at once, because the extra layer is dense enough to cut light as well as to insulate. Check the lining specified on the curtain you are ordering rather than reading across from the name of the collection you found it in.
You can start from either end: the blackout curtains or the thermal curtains. What matters is the lining specification on the curtain you end up with, not the collection you arrived through.
Choosing by room
Bedrooms are usually where light control wins, particularly for a child's room, a shift worker or a bedroom facing east. Living rooms are more often about comfort in the evening and the look of the curtain when it is closed. A north-facing room used in winter has different priorities from a south-facing room that overheats in summer. Curtains by room is a useful way in if you are working through the house.
Privacy is worth separating out too. Privacy at night is about not being seen, which a mid-weight lining handles perfectly well, and it does not require a blackout construction.
Working out what you need
Decide which problem you are solving first. If it is light, plan the fitting as carefully as the lining. If it is comfort, think about the whole layer rather than one product feature. If it is both, say so, and check the specification rather than the label.
Choosing curtain linings goes through the options in more detail. If you want to talk a specific room through, including how high and wide the hardware should go, the free design consultation is the place to do it.