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Is Batt or Rigid Foam a Better Choice for Garage Insulation?

Batt feels like the obvious, blanketing choice when insulating the garage. I have seen plenty of batt-filled wall studs because the homeowner never got around to putting drywall over the insulation. However, is batt really better than rigid foam board?

Batt insulation is superior to rigid foam board on several metrics. Although it appears to cost more than foam board insulation, it has a higher R-value. Dollar for dollar, you get more insulation from batt insulation than from foam board.

Foam board is impervious to water. Where batt will not work if compressed, fiberboard has no compression failure vulnerabilities.

Which Insulation Has Better R-Value: Batt or Foam Board?

Reflective bcking on the batt insulation adds heat back into the insulated space
Reflective bcking on the batt insulation adds heat back into the insulated space

Foam board is not as insulating as batt. In terms of insulation value, the order from most to least insulating is:

  • Mineral wool batt
  • Fiberglass batt
  • Polyisocyanurate (polyiso) foam board
  • Extruded polystyrene (XPS) foam board
  • Expanded polystyrene (EPS) foam board

Does Garage Insulation Need a Vapor Barrier?

Garage insulation vapor barrier
Garage insulation vapor barrier

Vapor barriers are not a cut and dry subject (see what I did there?). You will need to look at the adjacent space to determine what you will do here.

If there is a large temperature difference between the adjacent room and the garage, the vapor barrier will prevent mold inside the home.

Since you are insulating to correct uncomfortable temperatures in the garage, then the vapor barrier probably is required. There is probably a big difference between the outside air temperature and the garage’s temperature; and between the garage’s temperature and the house interior’s temperature.

So long as you have these differences in temperature, you have condensation. The vapor will make its way to the warm side of the equation. When the temperature difference is on an outer wall, the interior garage wall will mold. When the difference is on an inner wall, the home’s interior wall will mold.

Understand that the Kraft-backed (paper backed) contains the vapor barrier function, so you only need the paper backing on the insulation to have a vapor barrier.

Whether you get batt or foam board, get the type with the paper backing. Look for the words “Kraft,” “vapor barrier,” “vinyl” or “paper” backing (or “facing”).

If the paper is not available, you will need a separate vapor barrier along with the insulation.

Install paper backed insulation with the paper facing into the room.

What is Batt Garage Insulation?

Batt insulation comes in fiberglass, mineral wool (also called rock wool or slag wool), plastic fibers, and natural fibers (which are usually cotton or wool).

Fiberglass Batt Garage Insulation

Fiberglass -- always use gloves, mask and goggles when handling!
Fiberglass — always use gloves, mask and goggles when handling!

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Fiberglass is the most batt material.

Fiberglass batts are cheaper than mineral wool verities.

Fiberglass batt insulation uses fine glass shards that really hurt when you touch them. Always wear gloves, a mask and goggles when around fiberglass.

The higher density fiberglass blankets have higher insulation value.

  • Low-density batt yields R-11 insulation value
  • Medium-density batt yields R-13 insulation value
  • High-density offers R-15 insulation value

Mineral Wool Batt Garage Insulation

Mineral Wool
Mineral Wool

Mineral wool is a fabricated from recycled plastic. Rock wool is a mineral wool manufactured from natural diabase or basalt (volcanic rocks).

Slag wool is a fabricated mineral wool made from the molten metal waste from blast furnaces (called “slag”).

Mineral wool is a few percent points better than fiberglass at keeping out extreme temperature air. A piece of mineral wool batt has the same R-value as high-density fiberglass.

Mineral wool offers a couple of points of R-value over regular density fiberglass.

A 2×4 mineral wool batt has an R-15 value, while a 2×4 fiberglass (regular density) is an R-13.

A 2×6 mineral wool bat has an R-23 value, and the equivalent fiberglass is R-19.

How Fiberglass is Better than Rock Wool

You can find fiberglass and mineral wool batts in rolls and in pre-cut sizes. You will find more selection in fiberglass, and it is cheaper than mineral wool.

How Rock Wool is Better than Fiberglass

Fiberglass is a nasty material. Fiberglass batts are made of thousands of pieces of glass. When papered to create a vapor barrier, one side of the batt is covered, but the other side still exposes you to glass fibers. These are especially nasty to breathe.

Fiberglass batts do not insulate well when wet or compressed. So long as they are dry and loose, they have decent but not spectacular insulation ability.

Which is Better: Fiberglass or Mineral Wool Batt?

Rock wool has a slight insulating advantage over fiberglass. Since every insulation point counts, that advantage is actually significant and preferable.

Over the long term, the rock wool will server you better by making the garage warmer in the winter and cooler in the summer.

Fiberglass insulation costs less money than rock wool, and works almost as hard.

Where in the Garage to Use Batt Insulation

Kraft backing is the paper backing on batt wool
Kraft backing is the paper backing on batt wool

Use batt insulation in the door, wall and ceiling.

The garage door sits inside channels it uses to roll up and down. Insulation tucks into these channels with the paper side facing you. Garage door insulation kits include pins to hold the insulation in place. Typically, kits also include gloves and tape. The side that sits in the channel has an adhesive strip so that it sticks to the channel interior.

Place batt insulation between the wall’s studs and the ceiling’s joists.

Either spray adhesive on the side touching the wall/ceiling, or staple the insulation to the studs/joists.

Do not staple the batt in the middle. Staple only the edges to avoid compression.

This is because stapling to the wall creates compression, which reverses the insulation value of the batt.

An easier method is to use spray adhesive on the wall/ceiling side.

What is Foam Board Insulation?

XPS Expanded Polystyrenis the pink one
XPS Expanded Polystyrenis the pink one

Foam board insulation is Styrofoam. In fact, the extruded polystyrene type of foam board is the same material in a takeaway coffee cup. It does not biodegrade, making it just as environmentally unfriendly.

Manufacturers of foam board insulation might add water-blocking material to their products. In this case, the water absorption factor is dependent on that change and not the foam itself.

  • Expanded polystyrene (EPS) offers only R-4 per inch of thickness. It is effective enough if you use two layers, and has the lowest cost per R-value of the three board types. EPS is better at retarding moisture than XPS, too, which adds to its value. EPS has a long life without degradation; it does not degrade over time.
  • Extruded polystyrene (XPS) offers R-5 per thickness inch. XPS is more likely than EPS to absorb water. XPS has a slightly higher R-value than EPS, but it degrades over time. Within 20 years, the XPS R-value will degrade to a lower value than the EPS’ R-value.
  • Polyisocyanurate (polyiso) offers R-6 insulation per inch of thickness. It has a reflective material on the back to capture and reflect radiant heat back into the insulated space. Polyiso has the highest R-value of the three types of rigid foam board insulation. Polyiso degrades in a similar manner to XPS, which lowers its insulation value over time.

Which is Better: Expanded: Expanded Polystyrene, Extruded Polystyrene, or Polyisocyanurate Rigid Board Insulation?

epx extruded polystyrene foam boardExpanded polystyrene (EPS) has medium moisture resistance. Polyisocyanurate (polyiso) is almost as good as EPS, and XPS has the worst moisture resistance.

Polyiso comes with a foil back that acts as a vapor barrier. Add a vapor barrier when using EPS or XPS.

Polyiso has the best R-value at about R-6 per inch of thickness. XPS is next with R-5 and EPS is last with R-4. However, remember you can double and triple up these boards to accumulate insulation value.

Batt vs. Foam Board Insulation Value

Batt insulation has a better insulation value than foam board. You will need to layer foam boards to get a good R-value.

Batt vs. Foam Board Cost per R-value Point

EPS Extruded Polystyrene board
EPS Extruded Polystyrene board

Foam board costs less than batt insulation, on the face of it, but they are actually about the same price.

You have to do the math to see how it works out for you.

First, take the number of layers you will use for your foam board. Each has an R-value. Multiply the R-value times the number of boards. Let’s say you are going three boards thick for this example.

If the R-value is four but you are using three boards, then you are creating a 12-R value with three boards.

Now multiple the cost per board times three to get the real price of foam board insulation.

Foam board starts out cheaper, but comparing them for equal insulation value is another matter.

To get a higher R-value from foam board, use several layers. Three layers of foam have the insulating value of a medium density fiberglass batt.

Add the R-values together to get the actual resultant insulation value of multiple foam boards in one space.

Batt vs. Foam Board Durability

Compression drops the batt insulation R-value. You could not compress the foam board under a car tire, so it is not an issue for foam board. There is no reason for the batt to be compressed. Compressed batt does not insulate.

Batt insulation is ineffective when compressed. This is why you cut it exactly to the size between the studs. Attach it with staples to the studs, but not the back wall, again, to avoid compression.

So long as the batt is in good shape, it has about three times better insulation value per inch of thickness than foam board.

Batt vs. Foam Board Moisture Resistance

Manufacturers add a hydrophobic coating to most if not all foam board insulation, which makes it safe from moisture.

Wet batt, on the other hand, compresses the fibers and ruins its insulation value.

If the batt does get wet, there is a bigger problem going on. There is a leak causing the wet fabric.

Vapor buildup does not affect the batt interior. The vapor barrier prevents the condensation from forming on the warm wall where cold and warm air meet.

That interior wall will have the nasty combination of warmth and moisture, fostering mold growth.

Vapor forms at the point of temperature change, which is not the middle of the batt. Therefore, wet batt got its moisture from somewhere else.

If the foam board does get wet, it will not affect the R-value. However, again, if there is water on the foam board, there is a leak in the wall. Water wrecks studs and allows mold to form. Insurance rarely covers slow-moving train wrecks such as moisture damage, so fix leaks right away!

You should get a vapor barrier on the foam board, and it looks similar to the barrier on batt insulation. Unless there is no temperature difference between the house and the wall, or the wall and the outside, the vapor barrier prevents mold.

There are many opinions about this out there, and your mileage may vary. Just remember that mold inside the house is much more dangerous than some cold air. It is not worth your health to expose your family to mold buildup because there is no vapor barrier on the insulation between the garage and the home.

Which is Best, Foam Board or Batt Insulation for the Garage?

Mineral wool batt has the highest R-value per inch of thickness.

Three layers of polyiso foam board have more insulation R-value than mineral wool, and a better moisture barrier. It includes a reflective barrier that repels heat back into the room.

Mineral wool is easier to cut than polyiso. You will end up with more useable material when cutting mineral wool than when cutting foam board.

Mineral wool is not effective when wet, but wet mineral wool means there is a leak that requires attention. You will need to replace mineral wool in this case, but not polyiso.

Except in the case of a leak, mineral wool edges three layers of polyiso because it is less wasteful.

 

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