Sometimes. And sometimes it makes it considerably worse. Here is the honest answer, which depends entirely on what kind of damp you have.
Render can fix penetrating damp. It will not fix rising damp, and on a solid old wall it can cause damp that was not there before.
The question is never "will render stop damp". It is "what kind of damp have I got, and where is the water coming from". Get that wrong and you can spend a lot of money making the problem worse.
They look similar on the inside. They are completely different problems.
Water coming in through the wall from outside, driving rain on an exposed elevation, failed pointing, cracked render. Render can genuinely fix this.
Moisture drawn up from the ground through the base of the wall. Render will not stop it. Rendering over it just moves the problem up the wall.
Warm, moist air inside meeting cold walls. Extremely common, frequently misdiagnosed, and rendering the outside does absolutely nothing for it.
If you have a west or south-west facing wall in Devon taking the full force of the driving rain, and the water is getting through the brickwork or through failed old render, then a good breathable render is exactly the right answer.
A silicone render is well suited to this: it is water-repellent, so rain beads off the surface rather than soaking in, but it is still breathable, so any moisture already in the wall can get back out. That combination is what you want. It sheds the weather without sealing the wall.
Put a cement render, which is not breathable, on a solid cob, stone or old brick wall, and you have sealed it. Moisture still gets in, from the ground, from a leaking gutter, from a bit of cracking. But now it cannot evaporate back out through the wall face.
So it goes somewhere else. It sits in the wall, it rots the timbers, it pushes the internal plaster off, and it often comes out higher up the wall than the original problem. We see this constantly on Devon cob properties, and it is an expensive thing to undo, the render has to come off and be replaced with lime.
Rendering an old solid wall with cement is one of the most reliable ways to create a damp problem in a building that did not have one.
Look at the boring things first, because they are usually the answer and they are far cheaper to fix. Blocked or leaking guttering. A cracked downpipe. Ground levels built up above the damp course. A failed sill. Pointing that has gone.
Rendering a wall to solve a damp problem caused by a leaking gutter is an expensive way to avoid a twenty-minute job. We would rather tell you that than take the work.
The ones we get asked most.
It depends on the type of damp. Render can genuinely fix penetrating damp, where water is driven through the wall from outside. It will not fix rising damp, which comes up from the ground, and it does nothing for condensation. On a solid cob or stone wall, a non-breathable cement render can actually create a damp problem that was not there before, by trapping moisture inside the wall.
No. Rising damp is moisture drawn up from the ground through the base of the wall, and external render does not address the source. Rendering over it typically pushes the visible damp higher up the wall rather than curing it.
Yes. A cement render is relatively impermeable. Applied to a solid cob, stone or old brick wall that needs to breathe, it traps moisture inside the wall. The moisture cannot evaporate through the wall face, so it rots timbers and pushes internal plaster off instead. This is a common and expensive problem on older Devon properties.
A breathable, water-repellent render such as a silicone render works well for penetrating damp. Rain beads off the surface rather than soaking in, but any moisture already in the wall can still evaporate out. That combination sheds weather without sealing the wall.
Check the simple things first, because they are usually the cause and are far cheaper to fix: blocked or leaking guttering, cracked downpipes, ground levels built up above the damp proof course, failed window sills and perished pointing. Rendering a wall to solve damp caused by a leaking gutter is an expensive way to avoid a small repair.
Send a photo on WhatsApp and we'll tell you straight, including if the answer is "leave it alone".