Filling hole in the wall / ventilation

First Don’t completely block the hole with expanding foam or bricks.

Best option for filling behind is Breathable Draught-Proofing Membrane or Insulation

Use natural, breathable insulation like:

Hemp fibre insulation

Sheep's wool

Wood

Thanks for the details — here’s what I’d suggest:

Since the hole behind your decorative plaster vent was originally for ventilation (and is linked to an external airbrick), it’s important to maintain airflow to help prevent damp, but you want to reduce drafts and heat loss.

Good breathable filling options include:

Hempcrete or hemp insulation: Natural, breathable, and offers some insulation while allowing moisture to pass through safely.
Wood fibre insulation: Breathable and can be cut to fit the cavity; it reduces drafts but doesn’t trap moisture.

Vermiculite or perlite loose-fill insulation: Lightweight, breathable, and easy to fill irregular gaps with.

Mineral wool insulation: Breathable and insulating, but needs to be fitted carefully so it doesn’t block airflow entirely.
Breathable lime mortar or lime-based render: Can be used to fill gaps around the edges while allowing moisture vapor to pass.
Avoid using non-breathable materials like foam sealants or cement-based mortar directly behind the vent as they can trap moisture and worsen damp.

Once you’ve filled the hole with a breathable insulating material, you can fit the decorative plaster vent on top. This will help maintain the look while reducing cold drafts and supporting proper ventilation.

1. Understand the Purpose of the Ventilation Hole

The hole, paired with the external air brick, is likely part of a passive ventilation system to allow airflow and prevent condensation. Blocking it entirely could lead to moisture buildup and future damp issues.

2. Prepare the Area

Remove the half-brick and clean out any debris. Ensure the surrounding wall area is dry and free of mold or damp. If necessary, treat with a fungicidal wash.

3. Choose a Breathable Material

You need a solution that allows airflow but reduces drafts. Here are a few options:

Breathable Membranes: Install a breathable membrane (like those used in roofing) across the back of the hole. This will allow moisture to pass through while stopping direct drafts.

Insulated Vent Panels: Use a panel designed for ventilation but with built-in insulation properties (e.g., a vent with a fine mesh or filter). These are commonly available at hardware stores.

Perforated Metal or Plastic Mesh: Place a mesh backing over the hole. It won’t insulate as much as a membrane but will block debris and insects while maintaining airflow.

4. Seal Around the Edges

Once you’ve installed the breathable barrier, seal around the edges with a lime-based mortar (compatible with older properties) or a breathable sealant to ensure no gaps for drafts.

5. Install the Decorative Vent Cover

Affix the decorative plaster vent cover over the hole. Ensure it’s securely attached but removable for future cleaning or maintenance.

6. Monitor the Area

After installation, monitor the wall for any signs of damp or excessive drafts. If issues persist, consider consulting a professional specializing in older properties.

By using breathable materials and maintaining ventilation, you’ll strike the right balance between preventing damp and reducing cold drafts in your 1930s home.

1. Use a breathable material: Fill the gap with a product like lime mortar or a breathable insulation material (e.g., mineral wool). This will reduce drafts while allowing moisture to escape.
2. Fit a vent liner: Install a plastic or metal vent sleeve behind your decorative vent to control airflow and minimise drafts.
3. Check for damp causes: Ensure the exterior air brick is clear and inspect for other damp issues like blocked cavities or poor drainage.

This approach keeps the ventilation functional but reduces cold air intrusion.

When renovating your 1930s semi and dealing with signs of damp, it’s important to address the ventilation and moisture issues properly, especially when dealing with a hole in the wall that appears to be related to airflow.

Here’s some advice on how to approach the issue:

1. Ventilation Consideration: The hole in the wall that lines up with the air brick is likely there to maintain airflow and prevent moisture build-up inside the wall. It’s important to retain some level of ventilation to avoid trapping moisture, which could lead to further damp problems.

2.Filling the Hole: You can fill the hole with a breathable material, but it’s crucial to ensure that you don't block the ventilation entirely. One solution would be to use a breathable but insulating material such as lime mortar or breathable masonry sealant. These materials allow moisture to escape while still offering some insulation, helping to reduce drafts and heat loss. Be sure to leave some level of airflow to maintain the integrity of your wall’s moisture control.

3. Decorative Plaster Vent: You can install your decorative plaster vent over the hole, but again, it’s important that it allows sufficient airflow through it. Some decorative vents are designed to let air pass through while maintaining the appearance of the space, so ensure that the vent you plan to use doesn’t completely block ventilation.

Other Options:

Draught-proofing: Consider using draught-proofing materials around the vent (such as flexible draught excluders) to minimize heat loss while still allowing the vent to function as a breathable part of the wall.
Check Dampness: Before sealing or covering the hole, ensure that the underlying damp problem has been fully addressed. You may need to check the cause of the damp (e.g., a broken damp course, poor exterior drainage, etc.) to prevent moisture from accumulating in the wall again.
To summarize: you can fill the hole with breathable materials to reduce drafts, but make sure it doesn’t block necessary ventilation. The decorative vent can be installed once you’ve addressed the moisture and airflow needs. If in doubt, it may be worth consulting with a specialist to ensure the long-term health of your walls.

Let me know if you need further clarification!

Breathable Membranes: These are designed to allow moisture to escape while preventing drafts and water ingress. They are commonly used in construction to manage moisture levels and prevent dampness.

Wood Fibre Insulation Boards: Products like Gutex Thermoroom are made from natural wood fibres and are fully breathable. They allow moisture to pass through while providing thermal insulation.

Breathable External Wall Insulation: Systems like Baumit OpenSystem use vapour-permeable insulation that allows walls to breathe while maintaining thermal efficiency.