Does doubling foam board double the R-value?

Author: Miss Serenity Wisoky  |  Last update: Thursday, September 4, 2025

For example, our polyiso rigid foam boards have R-values between 6.5 and 6.8 per inch. If two layers are installed, it doubles the R-value and cuts heat/conditioned air loss in half. For reference, plywood has an R-value of 1.25.

Does doubling insulation increase R-value?

The thermal barrier of a home should consist of a continuous layer of insulation on all sides—including the lowest floor, the exterior walls, and the ceiling or roof. Doubling the thickness of insulation will double the insulation's R-value, cutting heat loss in half.

Can you double up foam board insulation?

When two or more layers of polyiso insulation boards are used together to provide the specified insulation thickness for a commercial roof, you dramatically minimize thermal bridging. In order to create the most energy efficient installation, the joints of the polyiso insulation boards should be offset.

Can you stack rigid foam insulation to increase R-value?

Let's compare the R value of rigid insulation to these other types of insulation in the table below. You'll notice that we call out the R-value per inch. This is because rigid insulation allows stacking of boards to achieve a desired R-value.

Does expanding foam have R-value?

Expanding foam became a valuable form of insulation due to its thermal properties and resistance to heat flow, otherwise known as R-value. The higher the R-value, the more effective the insulation.

Which Foam Board? Polyiso vs XPS Insulation

What is the difference between Foamular 250 and 150?

Foamular 150 is a lower-density insulation product, while Foamular 250 is a higher-density product. This means that Foamular 250 is more resistant to compression and therefore provides better insulation performance.

Which rigid foam board has the highest R-value?

Polyisocyanurate: Known as polyiso for short, polyisocyanurate foam has the highest R-value per inch (R-6.5 to R-6.8) of any rigid insulation. This type of rigid foam usually comes with a reflective foil facing on both sides, so it can also serve as a radiant barrier.

Can you have too high of an R-value insulation?

The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space.

Does foam board lose R-value over time?

Over the course of time, the R-value drops because the added blowing agents found in the final product eventually is replaced with air making the product less heat resistant. An initially rated R-value 5.0 XPS product will eventually drop to closer to an R-value of 4.3.

What happens if you double up insulation?

By doubling the insulation layers in your loft, you increase its R-value, which in turn reduces its U-value (lower U-value is better). This results in better insulation and improved energy efficiency for your living space.

Do you need a air gap with foam board insulation?

Insulation boards are no exception to this rule, and it's important to leave an air gap between them and the surface they're installed on.

Does foam board insulation shrink over time?

Regarding dimensional stability, Dow Styrofoam XPS, states that the maximum shrinkage that can be expected is 2%. Over a 96" long board that's 1.9 inches.

How do you increase the R-value of insulation?

When calculating the R-value of a multilayered installation, add the R-values of the individual layers. Installing more insulation in your home increases the R-value and the resistance to heat flow. In general, increased insulation thickness will proportionally increase the R-value.

Is there a big difference between R13 and R19?

R-value is used to judge thermal insulation performance and the higher the number, the better the product's insulating ability. So R19 has better insulating ability than R13. Now R-Value ranges widely across products and home applications. Indeed you will see R60 in some places and R13 in others.

What are the disadvantages of double insulation?

Because its structure is more complex than single-layer insulation, the cost is relatively high. Additionally, in some cases, double-insulated cables may fail due to spalling between the inner and outer insulation layers.

Does doubling up insulation increase the R-value?

There are times when more than one layer of insulation will be used. An example of this is in an attic. If the existing attic insulation is an R-19, you may choose to add another layer of R-19. Two layers of R-19 at 6¼ ” thick will achieve a total R-value of 38.

What is the thinnest insulation with the highest R-value?

Thermablok® Aerogel is a revolutionary advancement in thermal technology offering the thinnest insulation available to prevent thermal and cold bridging. Classed as a Super Insulation, Aerogel has the highest insulation value of any known material with the lowest thermal conductivity value of any solid (0.015W/mK).

Is it possible to over insulate an attic?

Yes, it is possible to put too much insulation in a loft or attic.

What are the cons of foam board insulation?

Insulation Foam Boards Don't Work On Uneven Walls

This allows moisture to seep in. Excess moisture can cause mold, mildew, odors, decay, fungus growth and wood rot.

Can you walk on rigid foam insulation?

This means you can walk on rigid insulation without damaging it. Installation Method: Rigid insulation can be installed by fastening, adhering, or loose-laying modular boards to structural sheathing, studs, walls, or decking.

Does plywood have an R-value?

And we have three-quarter inch plywood and this plywood's R-value is also given as . 94, this is for 3/4". Whereas the fiberglass, each inch has an R-value of 3.7.

Do you need vapor barrier with Foamular?

Install a continuous and sealed polyethylene air/vapour barrier over FOAMULAR® Insulations (need one over both C-200 and InsulPink®). Install gypsum board over wood strapping. FOAMULAR® C-200 or InsulPink® can act as the air/vapour barrier with sealed joints.

What is the R-value per inch of Foamular 250?

3 FOAMULAR® 250 XPS insulation is classified as a Type IV product when tested in accordance with ASTM C 578 and provides a long- term thermal performance of R-5 per inch.

Can I use foamular 150 under slab?

Owens Corning FOAMULAR 150 extruded polystyrene insulation is ideal for wall furring, perimeter/foundation, cavity wall, crawlspace, pre-cast concrete, under slab, sheathing and other applications.

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