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How To Insulate A Bonus Room: Three Options Compared

How To Insulate A Bonus Room: Three Options Compared

When we built our new home my wife designed the blueprints to allow for a bonus room upstairs. The extra space would be nice but I knew I would be dealing with a heating and cooling issue. Bonus rooms built in attics face constant temperature challenges due to walls that are adjacent to unconditioned space.

I researched this until I couldn’t see straight and this is what I learned.

You have three basic options when insulating a bonus room:

  1. spray foaming the exterior walls
  2. use traditional blown in insulation and wrap the exterior of the walls with foil-backed foam board
  3. install traditional fiberglass batts

Spray foam offers the highest level of energy efficiency but can be costly and does come with certain health concerns. Let’s look at each of these options so that you can determine the best approach to insulate your bonus room.


Best Option – Insulate Your Bonus Room With Spray Foam

Yes, it’s pricey. And yes, there are health concern issues associated with it. But if you want the absolute best insulation value, there is no approach to insulating a bonus room that tops spray foam insulation. 

Spray foam provides an air-tight thermal boundary between your bonus room and the unconditioned air space in your attic.

This insulation should only be installed by trained professionals. Inappropriate mixture or application can result in suboptimal insulating results and hazardous fumes. If applied correctly, however, you will have a superior boundary to the thermal envelope of your bonus room. 

To learn more about the benefits and concerns surrounding spray foam, read Foam Insulation: Pros, Cons, And Is It Worth It For You?

Here is a short but interesting time lapse video of spray foam insulation being applied to an exterior wall with impressive results shown at the end.


Good Option – Blown-In Insulation And Foil-Backed Foam Board

This approach involves wrapping the wall cavities with 1) sheetrock on the interior and 2) foil-backed foam insulation board on the exterior. Between the cavities, fiberglass or cellulose insulation is blown in creating a tight thermal envelope for the bonus room. 

This is the option I chose.

While this approach does not provide the same level of air-tight insulating that spray foam offers, it is a cost-conscious, energy-efficient solution to insulating a bonus room. There are several advantages to this insulation strategy:

  1. Cost – The price for blown-in fiberglass and foam board is significantly less expensive than spray foam.
  2. Air-Sealing – The foam boards butt together and provide clean seams that can be air-sealed with Great Stuff expanding foam for improved energy efficiency.
  3. Radiant Heat Blocking – The foil-backing of the foam insulation boards provide a radiant barrier to reduce the amount of radiant heat that is transferred into the conditioned space during the summer. To learn more about radiant barriers, read Is Radiant Barrier Effective? The Research-Based Facts
  4. DIY Installation – This is a project that can be taken on by the homeowner.

These benefits put this approach to insulating a bonus room in the sweet spot. Not quite as effective as spray foam but cheaper. Much more effective than traditional fiberglass batts but still able to be Do-It-Yourself project. I did most of the work myself but had my insulation blown-in by a commercial company while they were insulating my attic.

Here’s a picture of the bonus room’s exterior walls from inside my attic. Notice the expanding foam around wires and other entry points. 

Bonus room insulation - foil-backed foam board

I nailed the foam board to the studs with roofing felt nails. You can see that I also applied silver ducting tape to the seams. Under that tape is Great Stuff foam that I applied to all of the seams throughout the thermal envelope.


Budget Option – Fiberglass Batts Offers The Cheapest But Least Effective Approach To Insulating A Bonus Room

This is the defacto insulation approach for the vast majority of bonus rooms. Although it does provide some level of heating and cooling protection of the thermal envelope, traditional fiberglass batt insulation is the least effective method for insulating a bonus room.

The reasons for this are plentiful.

First, it is very difficult to fully enclose the wall cavity with batts. Although exterior wall studs are usually on 16-inch centers, there is often a little variance. Batts do not allow for this.

Additionally, ensuring proper insulation around pipes, wires, corners, and other odd areas can be next to impossible. 

Finally, batt insulation provides no air-sealing for the multitude of cracks and crevices that exist in exterior walls. 

If you choose to insulate your bonus room with this option it is imperative that you take the time to properly air-seal every gap, crack, and airspace that you can find. 

i would also recommend you watch this video on YouTube related to properly installing fiberglass insulation batts.

How To Decide Which Bonus Room Insulation Option Is Right For You

With the advantages and disadvantages of each approach clearly laid out, it’s time to decide which approach to insulating a bonus room is right for you. There are several factors that need to be considered.

  1. Budget – The amount of money that you have budgeted for insulating your bonus room will play a major factor in your decision.
  2. Turn-Key vs. DIY – How much of the work are you planning on doing yourself? 
  3. Length of Stay – Depending on how long you plan to be in the home, you may not want to invest in a more expensive option since it will take longer for the energy savings to justify the upfront costs.
  4. Health concerns – If you are uncomfortable with the potential health concerns of spray foam, you may wish to consider the other options. 
  5. Energy Savings Priority – If saving money on your energy bills every month and ensuring the most comfortable living area in your bonus room is your priority, investing in a higher efficiency option may make sense for you. 

I considered each of these factors when deciding on the insulation choice for my bonus room. In the end, I wanted the highest energy efficiency possible but my budget was somewhat limited and I was not sure how long we would be staying in our home.

Had we planned to live in the home for the rest of our lives, I likely would have gone with foam insulation. Since I couldn’t be sure, however, and the length of time that it would take for the energy savings each month to justify the upfront cost, I chose to have insulation blown-in with foil-backed foam board wrapping the exterior walls. 

I would strongly encourage you to consider each of these factors as you decide on how to best insulate your bonus room.

Unless you are on a very tight budget, I do not recommend the traditional fiberglass batts. Yes, it is the least expensive option but you will likely pay for that decision with higher electrical bills and an uncomfortable living environment in your bonus room.

Importance Of Air-Sealing A Bonus Room

If there is one area that I obsess over when it comes to energy efficiency, it is air-sealing. I went to great strides to air-seal my home. It is one of the primary ways that energy is wasted in the home.

A key advantage that spray foam has over the other two options is that the application process both air-seals and insulates at the same time. If you are installing blown-in or batt insulation, however, you will need to take the time to do this manually.

It is not very time consuming but does require you to inspect areas for cracks and gaps. A few cans of Great Stuff expanding foam spray and a weekend is all that is required to make a big impact on your bonus room’s energy efficiency. 

For more information, read Common Questions On Air-Sealing A Home.

Use Deep Exterior Walls For Improved Bonus Room Insulation (R-Value)

When I built my house, I insisted on 2 x 8-inch exterior walls for the upstairs. My framer thought I was crazy and kept trying to tell me that 2 x 4 studs would adequately support the structure. What he didn’t understand was that it wasn’t the additional structural support I was after, it was a deeper insulation cavity. 

According to a report by the Washing State University Extension Energy Program, blown-in fiberglass insulation provides an R-value of between 2.2 and 2.7 per inch

A 2 x 4 wall cavity with blown-in fiberglass insulation will provide, on average, an R-13 to R-15 insulation value. A 2 x 8 wall, by contrast, provides between R-21 and R-24 insulation value (source). While I spent more on framing costs for these exterior walls, it was still significantly less than the cost of spray foam would have been.

Add to that the foil-backed foam boards with air-sealing and radiant heat transfer reduction, and you have a pretty hefty thermal envelope for a bonus room. 

One option if the walls are already in place is to fir them out with additional 2 x 4s. I did this in a previous home. I screwed 2 x 4 studs on top of the existing 2 x 4 exterior walls to create deeper wall cavities. 

However, you choose to approach it, increasing the R-value of the exterior walls of your bonus room is a worthwhile project. You can significantly improve the insulation value which will assist your HVAC unit to heat and cool the area more efficiently.

Which brings us to our next point…

Bonus Room Air Conditioning And Heating

Another very important consideration with a bonus room is how you will provide heating and cooling. If the area isn’t currently heated and cooled, you have a couple of choices.

The first is to run ducting from your current system. This may or may not work as HVAC systems are sized based on the square footage of conditioned space. By increasing that square footage, you stand to put a strain on the HVAC system and may find that it does not effectively maintain desired comfort levels. 

The second option and the one I chose is to have a second, smaller HVAC unit to manage the bonus room. The advantage of this is that you can more accurately control the temperatures in the bonus room. Since upstairs areas are generally warmer it is sometimes hard to find a thermostat setting that is comfortable both upstairs and down. This solves that issue.

Another advantage of a separate system is that it can be shut off when the bonus room is not in use. In our case, for example, the bonus room is only used when the family gathers for games or a movie. Otherwise, it sits vacant. No sense spending money to heat and cool it while we aren’t using it.

I do leave the thermostat on, but I keep the comfort settings at 50 degrees for winter and 80 degrees for summer when we aren’t using it. As a result, the system hardly runs when we aren’t using the room. Best of all, I’m able to control the settings via my phone or our Amazon Echo because I have set up a lot of smart home devices in my house including smart thermostats

The route you chose to go to heat and cool your bonus room will depend on your situation but take the time to consider your options before committing.

Bonus Room Knee Wall Insulation Considerations

Knee walls are common in homes that have an upstairs bonus room. This is a small, unconditioned area that usually runs along one or two sides of the bonus room. The space is generally shaped like a triangle making movement through the space difficult. 

As a result, this is often the most overlooked opportunity for air-sealing. 

The below video from a commercial company clearly outlines the challenges of not adding an air barrier in the knee wall space.

I approached this area with the same strategy as the rest of the bonus room, covering the insulated wall with 1 inch foil-backed foam insulation board. 

The foam board provides a radiant barrier to reduce the transfer of radiant heat as well as air-sealing (because I took the time to apply Great Stuff spray foam to all cracks and seams.


Making the right decision on how to insulate a bonus room is about making the choice that is right for your situation. There are certain disadvantages to each approach but finding the right solution for you will depend on the factors outlined in this article. 

I can say that I am very pleased with my choice. We’ve lived in the house since 2016 and our bonus room is a comfortable, cozy place for my family to gather for games and movies. Yes, its more work than just stapling up some fiberglass batt insulation and it isn’t as effective as spray foam likely would have been but for my situation, it was the right choice. 

My takeaway for you is this: Don’t let someone who is selling you the insulation make your decision for you. Make that decision based on your situation, budget, and needs.

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