I’m preparing to “finish” one of the rooms in the basement. The room is actually going to be a storage room so I use the term “finish” with a grain of salt. I plan on building some shelves to store some of our “junk” that has accumulated in the portion of the basement that will one day become a family room. The storage room will be approximately 12’x14′ and it’s in the front corner of the house that has full 8′ high concrete walls. Before I build any shelves I want to build two walls in front of the concrete walls and insulate them. Before finishing your basement I recommend you get this book, Black & Decker Complete Guide to Attics & Basements.
Insulating the concrete walls will drastically improve the overall energy efficiency of the house. In my opinion the best way to insulate a foundation wall is the following:
1. Apply an extruded polystyrene foam insulation board (I happen to use “blue board” that DOW makes) to the concrete wall. You can apply the foam insulation board to the wall using a couple of methods; using an adhesive or shooting 1×3 strapping into the concrete wall to hold the insulation in place. I plan on using an adhesive. There are two types of adhesive, one comes in a caulking tube and it generally says foam insulation adhesive or you can use Great Stuff spray foam.
2. Tape all the seams with a tape that is appropriate for adhering to foam board. I like to use Tyvek Tape. The tape helps keep moisture from getting to the wood framing.
3. Build a 2×4 stud wall just in front of the foam board insulation. Fasten the bottom pressure treated plate to the concrete with powder actuated fasteners and nail the top of the wall to the floor framing.
4. Insulate the wall with R11 fiberglass insulation.
For my scenario I plan on using 1″ of blue board (R=5) and R11 fiberglass for a total R = 16. I’ll write a post in the next day or so about my progress.
Check out the followup post, Complete Guide To Insulating Basements.
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what brand of foam adhesive did you used ?
@ Ron - We've used so many different ones. Check out your local hardware or big box store. Look in the caulking isle and find an adhesive rated for foam. You can also use "Great Stuff" foam to adhere it.
Todd,
I'm still in the researching phase of doing a finishing my basement. I like your foam insulation board then fiber glass as another buffer. Does the fiberglass lay or touch the foamboard? If so, do you have any ideas on how to hang the insulation so that I maintain an air gap b/w the board and the studs?
Reason I ask is that the building code for where I live requires me to offset my studs from the foundation wall by 2".
...ok so I found some more info in your other threads that probably answers my question. Since my code requires me to have an air gap, I'll just go with a thicker foam board and just frame with no added insulation. I live near St Louis, MO.
Hi Todd,
Great post(s) on the basements. I am just wondering about one things. I live in the Toronto area in Canada, and really don't know if that will make a difference to the above post, or my question. I don't notice any mention of a vapor barrier? With the combination of a foamboard (DOW) and then an insulated wall, does this eliminate the need for the barrier?
Thanks
Jeremy
@ Jeremy - Vapor barriers are a tough issue in this application. By using the foam, and sealing it properly, the theory is you prevent water vapor from moving through the section. Some folks feel that's not true, others do. I personally think the foam will eliminate the vapor problem. So far the walls I've done that way have performed very well.
I like this idea of 1" polystyrene onthe walls with stud and battsover that,but what about insulating the floor with polystyrene to,or what would you recomend
@ Darren - Lots of people insulate the floors with polystyrene and a plywood sub-floor. There are also lots of basement sub-floor systems on the market and some of them have insulation incorporated into them. Typically we deal with radiant heat so it's not an issue.
Should there be an air gap between the batt insulation and the foam? Please answer pros and cons. Thanks!
@ Bill - No need for an air space if you've installed a sufficient thickness of foam and properly sealed it. If the foam is thick enough the surface of the foam will not be cold enough to cause condensation of any water vapor the penetrates the wall surface and reaches the foam surface.
Todd - Thanks for answering my previous question regarding an air gap between the batt insulation and foam. Is 1" DOW insulation a sufficient thickness or should I go thicker?
@ Bill - I would suggest 1-1/2" min.
Hi-
I have read all your posts...I am in in southern N.Y. and have a historically dry basement - my contractor is suggesting to seal my basement cinder block walls with drylock, then stud with wood posts and fill with batt insulation ( not sure if it will have paper or not), then use greenboard instead of sheetrock to finish the wall. Any thoughts on this approach compared to your approach? I really do not want to make an expensive mistake. Thank you in advance for any advice you can offer.
@ Lisa - If it were my house or a house that my crews work on I'd chose not to have that approach done. Drylock is a decent product but it's not going to keep all the moisture out and you're just asking for a mold problem once water gets into the fiberglass. Either spray foam it or install a layer of foam board properly sealed before installing any fiberglass or cellulose insulation. I wish you luck.
Hi,
I live in north central Iowa. Have a dry basement made of poured concrete. House is 5 years old. I have started to finish my basement and began by putting a 1/2 inch pink foam board next to the wall and followed with standard stud wall. Thought that would be an easy way to get some extra insulation. I placed batts into the stud wall and left it that way for a while until other aspects of the job could be completed. Had to remove a section of batts and noticed moisture had accumulated on the foam board and had made the insulation a little wet! Things dried out within an hour or so... If I put the batts in and cover with vapor barrier will I be OK, or is the foam board causing the problem? If I do use a vaper barrier, will I be trapping any moisture between the foam board and vaper barrier causing future problems? This "moisture" problem was only an issue on the part of the cement wall that has an outside exposed surface. All other walls constructed using the same method are fine. Not sure if this is also an issue, but the foam has a plastic barrier on each side. Should I have removed this prior to putting up???? Hate to move forward just to have issues down the road. Thanks for your opinion.
@ Joe - There are two issues. First off it sounds like you insulated with fiberglass prior to a vapor barrier and vapor passed though it and hit the cold foam. The foam is cold because it's only 1/2" thick. Water vapor can also travel from the concrete through the 1/2" foam into the stud cavity and cause trouble there. You REALLY need more than a 1/2" of foam to stop both things from happening.
So... What should I do at this point? I guess I should not have skimped with the 1/2 inch foam board (should have read your info first!!!), but that is what I have. However, at this point all the framing, rooms, closets, etc.. are finished. The foam board does have a plastic film on both sides - will that effectively stop the moisture from coming in from the cement? Do I move the outside walls out from the wall and add thicker foam board? Hate to that if there are other options. Many basement are done in these parts without any foam board at all;Just stud wall, batt insulation, vapor barrier and then drywall. Should I cut out the 1/2 inch foam board and just have that as dead air space in that part of the basement that I noticed the moiture. Not sure what to do???? Thanks in advance for your advice.
@ Joe - It's hard to say the best approach at this point. It's possible that your 1/2" with poly is an effective vapor barrier.......if so you can dry out the fiberglass really well..cover it with a good vapor barrier....and hopefully that will take care of things.