Basement Insulation Detail
Basement Wall Insulation
Basement Wall Insulation Detail
Lately I’ve been swamped with questions about insulating basement walls properly. So I’ve written this article to share with you a Basement Insulation detail that I hope will clarify some issues.
Moisture Problems
Before you choose an insulation method for your basement it’s important to understand the moisture problems associated with basements. Whether your foundation walls are concrete or masonry the moisture problems are really the same.
Concrete and masonry walls have a significant amount of moisture (water) contained within the material. The moisture will be present in the foundation materials for years and will never fully dry out. The foundation wall will go through drying cycles over time. The upper portion of the wall will dry to the outside and inside while the lower portion will dry to the inside of the house.
The drying cycle releases moisture which can get trapped in adjacent materials causing moisture problems such as mold, mildew and rotting. Knowing that this cycle will continue year after year it’s important to choose an insulation detail that prevents that drying moisture from entering vulnerable materials such as wood framing, fiberglass insulation, drywall and other moisture sensitive materials.
We also need to prevent water vapor from the finished space from coming in contact with the cold foundation walls and forming condensation. Unlike traditional walls where we have vapor barriers on the outside and inside foundation walls present some challenges. In the proposed basement wall insulation detail we’ll provide a moisture/insulation barrier along the face of the foundation wall and then a vapor barrier on the warm side of the framing of the basement walls.
Insulation Detail – Insulating Basement Walls
The insulation detail shown in the figure is a detail that I’ve used several times with great success. This detail is my preferred method for insulating basement walls short of having the walls sprayed with foam. There are several variations of this detail that can be used to achieve the same result.
- Install rigid expanded polystyrene insulation from the slab up to the top of concrete (masonry) wall. Install the same insulation along the top of the exposed foundation wall and then up along the rim joist to the bottom of the sub-floor.
- The vertical expanded polystyrene insulation can be attached to the wall with a multi-purpose adhesive designed for foam. The top portions along the top of wall and rim joist can be installed using spray foam such as “Great Stuff”.
- Be sure to seal ALL the joints in the foam with tape. The best tape I’ve found for adhering to foam board is Tyvek tape.
- Frame a wall in front of the expanded polystyrene insulation. The wall can be as simple as strapping shot through the foam into the concrete or a traditional framed wall. If you frame a wall I recommend you install a piece of 1x composite decking below the bottom plate. This will prevent moisture from wicking up into the framing and finished wall.
- Optional – Install insulation in the stud cavities. Depending on the thickness of the foam you can save some money and supplement insulation in the stud cavities, it’s fine to use fiberglass or cellulose in this step. See Insulating A Basement with Cellulose and Basement Storage Room for more details.
- Install a vapor barrier. Either use faced insulation in the wall cavity or install a vapor barrier (if you use Spray-In-Place Cellulose Insulation you may omit the vapor barrier).
- Be sure to leave a minimum 1/2″ gap between the slab and finished wall board.
Once you’ve effectively sealed off the drying cycle from entering the house side the drying cycle will have to happen to the portion of the wall above grade on the outside. As i said there are several alternative approaches to this method that will work. The key is to cut off drying moisture from the foundation wall and prevent it from entering the finished wall cavity.
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Todd,
I live in Boston and am much more concerned about mold than cost. I appreciate your help in answering the following questions:
1. Do you need plastic vapor barrier between studs and drywall when using 2″ foil-faced Polyisocyanurate foam board against the concrete?
2. Would it be sufficient to use 2″ Polyisocyanurate foam without a fiberglass insulation between the studs?
2b. If you still need fiberglass, wouldn’t plastic-wrapped rolls of fiberglass create double vapor barriers, which seems to be agreed upon as a bad idea?
3. Would metal studs be better than wood studs in preventing mold and mildew on studs and drywalls or will metal transfer more cold into itself and the cavity and create higher odds for mold and mildew?
4. If you use metal studs, do you need any type of insulation between the slab and the footer?
Thanks very much,
Sam
@ Sam – Great questions, thanks for stopping by the site. As I’m sure you’ve figured out there is no perfect answer for insulating basements, however, you’re on the right track.
1. I would suggest you do not need a vapor barrier. In fact, if you properly seal the joints the foil will act as your vapor barrier.
2. Whether 2″ of Polyiso is sufficient depends on the specifics of your home and the current Energy code in MA. I haven’t lived in MA for several years so I’m not familiar with the current code situation down there. My gut feeling is 2 inches of Polyiso is probably darn close to being sufficient.
2b. If you’re going to use the Polyiso I’d try to skip the fiberglass and get the thickness of foam you need.
3. This is a question I get pretty often. You hit on the pro’s and con’s quite well. Again, no perfect answer. If you’re worried about mold, and you definitely use Polyiso only, then metal studs offer a better protection against mold. However, metal studs will transfer cold quicker to the drywall surface than wood studs.
4. I would use some “sill seal” (a commonly used foam pad used between foundations and wall framing) to create a nice tight seal. I’d also foam the back side of the track with some great stuff which will seal it even further.
Good luck!
Todd, This is very helpful. Much appreciated for your quick response.
What if you sealed the rigid foam to the back of the framing and then left a 1 inch air gap between the foam and the concrete or block wall for air movement to dry out any excess moisture?
@ MikeR – That would work. Might be hard to actually do though. Be sure you seal it all very well.
could also install 1 inch rigid insulation,pre fab walls & wrap back with tyvek then nail up walls allowing 1 inch air gap,fill with fiberglass insulation and install 6 mill platic on inside.This would prevent any any air or water getting into the insulation and let any water vapor to escape thru the tyvek.I am going to use delta dl on the floors but not sure whether to run delta under the outer walls to maintain a continuous air gap around living area and allow any water to move to the basement drains or to run it up to the bottom wall plate and keep water out.
I was leaning toward running dl under walls to let any water drain and to allow air movement & use of dehumidifier to help dry out.This would prevent insulation from getting wet and eliminate mold growth.
Todd,
Mike again. Queston:
When I glued the two 3/4″ panels to the concrete walls (toal of 1 1/2 ” thick) I did not remove the film faced plastic (for damge control) from the boards. Please tell me this will not make any difference. Reading your earlier article you didn’t mention that you removed yours either.
Thanks in advance.
@ Mike – I wouldn’t worry about it. I’ve actually never seen a protective plastic film on any of the foam I’ve bought before. Must be something new! Don’t sweat it.
Good luck.
Thanks for the quick reply, Todd. I won’t sweat it. Appreciate your time!
Todd,
I have a basement wall project where condensation was an issue and mold formed on and behind finished drywall that was attached to firring strips directly to the concrete block wall. The finished wall had no insulation nor moisture barrier. The old drywall and firring strips were removed, the Kilz or Dryblock paint was scraped off to bare concrete block, (yes about 400 sq ft. completely scraped off, been since spring off and on doing this). The paint was remove because it had bubbled in some spots and I worried about the adhesion on the rest of the walls. The walls were cleaned with a strong Clorox solution rinsed and dried for weeks using a dehumidifer.
I built a 2 x 4 stud wall with treated lumber for the bottom plate and left it 2 1/2″ away from the concrete block. I have spray foamed the 2 1/2″ cavity behind the stud wall and the rim joist area with closed cell Handi-foam insulation.
Finally, my question? Because of the cost I do not wish to fill the 3 1/2″ stud wall with the spray foam. What would be my best option for the stud wall space? Do I leave the stud wall empty? Do I use unfaced fiberglass insulation? Do I use faced fiberglass insulation? Do I need a moisture barrier behind the drywall since I have a tight sealed envelope against the concrete block walls? I have never been a fan of plastic film for moisture barriers?
@ Dan – I would say you should be done at this point with the 2-1/2 inches of spray foam. The closed cell foam should have sealed out any moisture problems. Sounds to me as though you are read to put up drywall or whatever wall finish you’ve chosen. Good luck.
Thanks for the quick response.
I just worried that the 3 1/2″ gap created by the studding might cause a temperature difference between the foam insulation and the drywall and allow condensation to form.
I like your idea of the the 1X composite decking board beneath the bottom plate. Wish I had that tip before I built my stud walls. I did use treated lumber for my bottom plate, though. I am going to use mold resistant drywall for the finished wall. Would you suggest using a 1/2″ strip of composite decking material cut with a table saw to place under the bottom of the drywall. Between the drywall and the concrete floor? I had planned on using a piece of treated lumber to fill the 1/2″ gap to keep the drywall off the concrete floor.
@ Dan – Should be no problem with the air space, there shouldn’t be a temperature differential in that space.
I would just leave a 1/2″ gap at the bottom of the drywall. Just cover the gap with base trim. I think the gap is better than leaving wood/composite there so that it’s not in contact with anything that might get wet.
Thank you so much for the information!
If insulation is omitted in the wall cavity could/should plastic be used as a vapor barrier,on the outer area of the stud wall before the drywall is put up? I plan to put up 2″ T&G foam taped with a stud wall.
@ Mike – If you install 2″ and tape and seal all the joints really well there is no need for plastic.
Hi,
I’m planning a basement redo and after reading a lot of info on sites like this I plan on using rigid foam (1.5 in) and then stud in front of that.
My question is that in my basement I have a step back in the wall. it goes up about 4′ish feet and then steps back 1.5 to 2 inches. How do I handle that step back and keep everything nicely sealed.
thanks for any advice.
-Mike
Mike C – Just cut the foam and step it as well. You can use a combination of spray foam from a can and tape to seal everything nicely.
makes sense, thanks for the insight Todd.
My basement will become a woodworking shop with some very heavy equipment (800 pound planer).
For the insulated subfloor I will exceed the rigid foam load rating if I place the rigid foam under the sleepers.
How would you recommend I install and insulate the subfloor?
Pete – With equipment that heavy I think you’d be better off not insulating the floor.
Thanks Todd.
I have tested this subfoor design. 2 x 4 sleepers under 3/4″ tounge & groove plywood. It’s strong enough to support the load. The sleepers are 16″ on center and laid on the 4″ side further reducing the span.
Question I have not is how best to insulate between the sleepers and what to use for a vapor barrier?
Pete
Pete – I would insulate the space with 1-1/2″ rigid foam. You could use a layer of poly for a vapor barrier if you choose. This would be similar to the details posted here: http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/
Todd,
I am finishing a basement with a crawl space on either side, an interior block wall for support, and one exterior wall. Therefore only one wall is exposed to the elements with the other two walls being under the house with significant amount of roof protecting the ground that comes into contact with these walls. My question is should the walls that are next to the crawl space have the foam board insulation installed on them or not.
Thank you
Russ
Russ – I would for sure. Even though those walls are protected, they are still cold and damp.
Good luck.