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Foam Board Insulation R Values
Foam Board Insulation Types
I’ve written several posts about how to insulation basement walls in which I promote the use of foam board insulation as the first line of defense against moisture and mold. Because of this I often get questions about which type of foam board insulation to use and what R values these products provide.
There are three basic foam board insulation products on the market produced under several different manufacturer names. They include expanded polystyrene, extruded polystyrene, and polyisocyanurate unfaced or foil faced.
Expanded Polystyrene Foam
Expanded polystyrene foam (EPS) is the cheapest and least used foam board insulation product on the market. This product typically has an R value of 3.6 to 4.0 per inch of thickness. Expanded polystyrene insulation is similar to the foam used for packing “peanuts” and it’s typically used for insulated concrete forms also knows as ICF’s. It is also sometimes used on commercial buildings for roof and wall panel insulation which is typically sandwiched between light gauge metal.
Cost = Cheapest of the foam insulation boards.
Extruded Polystyrene Foam
Extruded polystyrene foam (XPS) also known as blue board or pink board comes in many different thicknesses and edge profiles. This insulation board is probably one of the most widely used foam board insulation products in the residential construction industry. XPS has an R value of 4.5 to 5.0 per inch of thickness.
This is the product that I typically use to insulate basement walls. It’s reasonably priced, light weight and easy to use. This product is also used to insulate the outside of foundation walls and even under slabs.
Cost = This product is the middle of the road for these types of foam board insulation products.
Polyisocyanurate
Polyisocyanurate also known as polyiso is seen in all kinds of commercial building applications and more recently with residential building projects. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved. The foil facing also makes it very easy to seal with good quality foil faced tapes.
Cost = Polyiso is the most expensive of the foam board insulation products however it’s the highest R value.
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[...] Foam Board Insulation R Values [...]
Also Polyiso is the least moisture resistant. Not only is it subject to breakdown from extended exposure, it also loses significant insulation value when wet.
Use expanded or extruded in any potentially damp locations.
All good info. Add to that the ability to purchase recycled/once-used rigid foam insulation boards of all types and sizes and you have a eco friendly solution.
Thanks,
Mike
[...] that stop moisture movement and prevent mold growth. Basements are the perfect location for foam type insulation products. Cellulose is also an option for basements but it’s not a product that I’ll cover here. [...]
[...] rigid foam board (read more about Foam Board Types and R Values) is an excellent alternative to spray foam. Foam board is cheaper to install and it can easily be [...]
I am going to build a new home on the Brad’Or Lake in Cape Breton Island Nova scotia. It will be a two story with a furnished walkout basement (built on a hill facing the water). The lower walls will be poured 8 inch reenforced concrete. On the inner side I intend to use one inch of rigid foamboard then 2″X 4″ 16 in OC framed wall. Fiberglass batton insulation and 1/2 inch gypsumboard. My question is, is this a good idea, what type of rigid board should I use? My understanding is that the polyiso foamboard does not hold up to any type of moisture but has a higher r factor per inch than the extruded but the extruded holds up better to moisture. If the polyiso is foil faced does that protect it against moisture and act as a vapor barrier. And in either case what is the best to use for a vapor barrier and where??
@ Robert Rein – Building a house that far north I would opt for a minimum of 2 inches of DOW extruded tongue and grove (other brands are fine as well), joints taped and sealed. Then go ahead and frame with 2×4 with R11 fiberglass insulation. No need for a vapor barrier if you install 2 inches of foam and seal all the joints really well. Best of luck!
With not much room to build a 2×4 wall frame and insulate along the side of our basement stairs, we have opted for polyiso sheets with foil and plastic face. Now on the inside of the wall, which face (the plastic or the foil), goes against the block wall and do I still need a vapour barrier between it and the drywall?
@ Debbie – I would place the plastic side against the concrete. No need for vapor barrier if you seal all the joints well. For foil faced polyiso i suggest you get a good quality foil tape (used for duct work…..but NOT duct tape).
As long as you’re building new and have the luxury of doing it right the FIRST time, why not put the insulation, and the waterproofing on the OUTSIDE of the concrete?
You get the advantage of the thermal mass inside, to moderate temperature changes, and it doesn’t cost you any more.
Also, instead of conventional insulation, do superinsulation and cut your heating bill by 80%- you can save most of the cost of the insulation by installing a TINY (and much less expensive) heating system, and pay $300 a year to heat instead of $3000 (most of your heating would come from waste heat inside the building). A savings of $2400/year would pay for about $48,000 greater expense in construction.
@ PeterL – Insulating the outside is certainly one option. However, it’s not as simple as that. The big issue with insulating outside is the detail where the foundation comes above grade. Making the transition from below grade to above grade and transitioning to the interior insulation without a break it difficult to do properly. Furthermore, many people in the industry agree that exterior insulation still doesn’t’ address the severe moisture problem that exists throughout the life of concrete products.
Your point is well taken on spending up front to save in the long run. We’ve built several homes in the last two years that use less than 50% of the energy that previous homes use. Thanks for your input.
I am planning to insulate a large basement, over 200 ft of outside block wall. It is damp in basement but no water problems.There are mildew problems, especially in summer. We run two dehumidifiers constantly, which uses alot of electricity. The floor is concrete. I am wondering how much good it will do to put two inches of extruded or iso sheet on exterior walls, if I don’t cover floor with anything.
@ Charlie – You’d be amazed at the difference 2 inches of foam on the walls can make.
I’m going to use the XPS on basement walls, but i’m not sure what to do about the space above the concrete walls between the floor joists. Currently batt insulation is placed there. Do I need to do something special in this space? I don’t want to spend the time and money on the XPS only to have the moisture barrier benefits negated because I didn’t address this space. Thanks!
@ Mike – I recommend you replace the fiberglass with XPS. You can cut pieces to fit between the joists. Use spray can foam (Great Stuff or similar) to seal each piece in place.
todd….We are in need of any of your advice. We have a walkout basement, three framed walls,the other wall concrete. the framed walls have a paperfaced R 19.the basement is unfinished and has no drywall.The slab was poured over 2″ foamboard and plastic on top as a vapor barrier. In the winter, the framed walls have moisture and black mold behind the insulation. the dehumidifiers keep the basement @ 30% humidity. There is also a pellet stove which keeps the 1200 sq. basement @ 70 degrees. The exterior consists of vynil siding and Tvec. My husband dug the cellar hole,he stated, the ground is pervious, with no ground water. Any help or advice you could lend would be greatly appreciated. Thank you, Dawn Dicarlo
@ Frank – From the sounds of it the kraft faced insulation was not properly sealed and moisture has been trapped between the fiberglass and sheathing. If the basement will not be finished I recommend a poly vapor barrier, properly taped and sealed. At this point it sounds like you need to remove the damaged fiberglass, clean the mold, and start over.
[...] including fiberglass batt rolls, blow-in insulation (such as NuWool), spray foam insulation, rigid foam board insulation, and [...]