How to Insulate Plaster Walls in a Home

If you notice your home is drafty, or your energy bills have been creeping up, you may consider insulating your plaster walls. Doing so can come with benefits—like better energy efficiency—but there are certain situations where it’s not advisable. 

Insulating your home’s plaster walls can be a challenging DIY project, but it is doable for handy homeowners. Learn how to insulate plaster walls without the help of a professional and whether it’s a good choice for your specific situation.

Why Insulate Plaster Walls

If your home was built before 1960 or has poor insulation, you likely want to improve its energy efficiency. While adding insulation to a modern house is always advisable and required by building code, installing insulation in an old house with plaster walls isn't always the best option. 

However, in many cases, you can install insulation behind plaster and enjoy the benefits of potentially reduced energy bills and increased energy efficiency. The task, though, requires precision and a thorough knowledge of insulating principles. 

How Much Does It Cost to Insulate Plaster Walls  

The cost to insulate plaster walls depends on how you wish to tackle the project. While the insulation part of the job costs between $1 and $4 per square foot of wall space, the other associated tasks when dealing with plaster walls will increase the cost by about double.

How to Prep for Insulating Plaster Walls

Preparing to insulate plaster walls starts with determining whether the job is worth it. The design of older homes with plaster walls allows the walls to breathe. Older houses don't have a vapor barrier between the interior and exterior. Installing insulation where a vapor barrier doesn't exist can trap moisture, creating an environment for peeling exterior paint, mold growth, insect homes, and rodent activity.

To combat the lack of a vapor barrier, the best approach is to remove the plaster and start from scratch. However, that's a big job. Only a qualified insulation professional who installs insulation for a living can determine if adding insulation without removing the plaster is possible and whether it will help or hurt the home.  

How to Insulate Plaster Walls 

There are three ways to insulate plaster walls. The first is to attempt to add insulation without removing the plaster. While this approach sounds like the easiest, it's also the most concerning for the health of your home. 

From the Interior

Insulating existing walls without removing drywall is a common project. However, it's important to note that modern walls with drywall covering most often have a vapor barrier in place behind the drywall. This isn't the case in most plaster walls, meaning adding insulation can cause more harm than good. However, you can complete the task by blowing in insulation from the home's interior.

1. Drill holes in the plaster in each stud bay. Depending on whether you use spray foam, loose fill, dense fill, or polystyrene bead insulation, you'll need at least one hole for each space between studs in the wall and sometimes two or more holes. 

2. Fill the space with your selected insulation material by blowing the insulation into each hole until the stud bay is completely full. Repeat the step for each stud bay in the wall.

3. Fill and patch the holes in the walls with plaster or drywall patches. 

4. Finish and repaint the walls. 

From the Exterior

The same task of insulating plaster walls without removing the wall covering from the interior is also possible from the outside. This project was common in the late 1970s and 1980s. It involves drilling holes in the home's exterior cladding and blowing in insulation from the outside.

1. If your home has removable siding, such as vinyl or metal, remove the pieces where you'll drill holes in the exterior portion of the wall. For wood or stucco siding, drill through the siding. 

2. Drill one or more holes in each stud bay, depending on the type of insulation you'll install. 

3. Blow in the insulation.

4. Fill the holes with wood plugs or stucco, depending on your siding type.

5. For removable siding types, reinstall the removed pieces. 

Remove the Plaster

The final and best way to install wall insulation is also the most challenging. While removing the plaster from the walls to install insulation may seem like overkill, doing so ensures the best insulation installation is possible. Removing the wall covering allows you to remove old insulation if necessary and start from scratch with proper preparation. The additional cost of doing it this way will pay for itself over time in the form of reduced monthly energy bills. 

1. Put on your demolition outfit and personal protection gear, including a respirator, and remove the plaster and lathe from the walls.

2. Remove debris and dust from the wall cavity.

3. Install your choice of new insulation in the spaces.

4. Install the vapor barrier.

5. Re-cover the walls with drywall and finish. 

Seal Window and Doors 

In many cases, you and your contractor will decide that installing insulation in your plaster walls is either too costly and time-consuming to be worth it or isn't good for the well-being of the house's structure.

It's not uncommon to conclude that spending many thousands of dollars just to save a couple of hundred dollars a year on energy bills isn't worth the investment. However, there are still steps you can take to increase your home's energy efficiency without adding insulation. 

1. Use a high-quality, paintable caulk to seal windows, doors, and their trimwork. 

2. Hire a professional to complete an energy audit of your home and follow the recommendations in the results. 

3. Insulate your attic space to meet building code. 

4. Encapsulate your crawlspace if applicable. 

5. Seal around any protrusions, including pipes and vents, in your home's exterior walls.

Taking Care of Insulated Plaster Walls

If you choose to remove the plaster and add insulation and a vapor barrier to your home's exterior walls, there's nothing more to do to make sure your insulation lasts as long as possible. However, if your project involves insulating without removing the plaster, follow these tips.

• Monitor for cracks in the plaster and repair any that appear as soon as possible to help keep as much moisture out as possible.

• Use vapor barrier paint to help protect your new insulation. While these products have limited success, they can be worth the effort, and you'll paint the walls anyway after insulating.

• Avoid creating holes in your plaster walls. Consider adding a picture rail from which to hang decorations. 

DIY vs. Hiring a Pro

Regardless of the method you choose to insulate your plaster walls, it's a big job. If you're an avid DIYer with insulation experience and a firm understanding of insulation principles, it's possible you can do the work on your own. Doing so can save up to 40% or 50% of the job's price by eliminating labor costs. 

However, DIY insulation installation comes with a variety of risks, especially when a vapor barrier isn't present. Installing insulation improperly can create significant damage to the house and your wallet. Most homeowners choose to hire a nearby insulation expert to handle the project. 

A local expert will have the experience and knowledge to determine if the project is a good fit for your home and the skills to pull off the job correctly. Additionally, hiring a pro will come with a work guarantee that a DIY job simply can't replicate.