Finished Basements – Insulation Method
Finished Basement Insulation
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.
Recommended Basement Insulation Method
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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[...] between the studs. If you want to learn more about this method, check out Todd’s website, Home Construction Improvement for the intricate details. I bugged Todd a couple times with some questions and he has helped out [...]
[...] interested in learning more about insulating basement walls check out my previous posts, Finished Basement Insulation – Step 1, Basement Insulation and Framing Update, and Basement Storage Room. Tags: home improvement, home [...]
[...] out my previous post, Finished Basement Step 1 – Insulation where I explain the methods of insulating in detail. Also check out How To Insulate Basement Walls. [...]
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.
[...] out my previous post, Finished Basement Step 1 – Insulation where I explain the methods of insulating in detail. Also check out How To Insulate Basement Walls. [...]
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.
[...] interested in learning more about insulating basement walls check out my previous posts, Finished Basement Insulation – Step 1, Basement Insulation and Framing Update, and Basement Storage Room. Tags: home improvement, home [...]
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.
Just wondering…. If I got some 2 inch foam board, cut them to fit snuggly into the stud space and then seal the edges next to the studs, would I effectively be taking care of my potential problem? I would then not need to use batt insulation and could finish as I was planning with drywall. No vaper barrier then would be needed, right…??? Thanks again.
@ Joe – That is a MUCH better solution.
Todd, I am planning on finishing my basement with 1″ R5 foamboard glued to the conrete, then 6″ R22 roxall bats inside of 2×6 framing with a vapour barrier on the inside of the framing. My question is I would rather not leave an air space between the foam board and the bats of insulation, however I do not want to spend the extra for 1 and a half inch foam board like you would’ probably reccomend. Since I am skimping on the insulation would a 1″ apace probably be the best way to prevent water from getting on the insullaion bats, and do you think the vapour barrier is a good idea? Thanks for your help so far, this forum is great!
@ Stu – The air space certainly is an improvement. However, if the water vapor condensates on the surface of the foam board then it’s likely to take longer to dry and may over time be absorbed by the fiberglass. In your case I think it’s very important to use a good vapor barrier.
Todd, just to clarify, the vapour barrier would be on the inside of the framing(the warm side), the side next to the drywall right, not imbetween the foamboard and the bats, sorry if this is a dumb question, my week old child is crying.
@ Stu – You are correct.
Todd, I am redoing my basement and have gutted the walls. When I did I found a white haze on the bottom of the walls. This was probably due to poor grading on the outside walls which I have corrected. I have cleaned off this white haze and plan on sealing walls with Dry Loc. Then using 1 1/2 of the Dow foam insulation and then R13 insulation. I am a little confused as to whether or not to leave a gap between the foam and insulation from the prior posts. Also where the stud meets the floor do you use any different size foam insulation? And at the top of the wall where the sill plate and upstairs floor meet, should I spray “Great Stuff” or do you recommend another product to fill that gap? Thanks for your advice.
@ Mike – That white haze is efflorescence from the concrete. It’s typically a sign of water but it sounds like you have addressed that problem.
The gap is optional, just be sure to seal the foam joints well.
I like to use foam board between the joists as well and foam them into place.
Good luck.
Todd,
Our finished basement has no insulation. None in wall or ceiling. We would like to add some to make our house more energy effecient. The house is 8 years old. What is our best options?
@ Tonya – What are the details of the existing wall systems?
the walls are framed drywall.
@ Tonya – You have a difficult situation. Sounds to me that you will probably need to remove the drywall and then use some type of foam product.
Hi Todd, I am going to finish my basement and am curious if your foam board/faced insulation – faced r-13 against the drywall mehthod still apply. My home is 1 year old with a 10” poured foundation – no water found. I have the “tuff and dry” system on the outside(fiber board plus sealer). I have seen foam in a part of my unfinished walls where the cement wasn’t smooth (extra insulation or barrier?) Based on what I have, is the 1.5 foamboard still necessary? And is the moisture resistant green board drywall a good idea or wasted money?
Also, I have a vapor barrier beneath my cement floor – can I just put some vapor barrier plasic then lay low pile carpet with pad? Or should i still use your foamboard and plywood method?
Thanks!!!
Chris – Some of your comment is hard to understand (cement wasn’t smooth…you see foam?)…..I would definitely recommend at least 1-1/2″ of foam in order to create a good vapor barrier along the existing concrete wall. Moisture resistant drywall certainly can’t hurt, however, if you deal with the moisture problems it’s probably a waste of money. As far as the floor goes, you can get pad and carpet that work well on slabs, it’s really a warmth issue if you already have a good vapor barrier under the slab.
Thanks for the quick response.
The “foam” is in the wall – meaning there are some rough spots on the finished wall and I am seeing foam sticking out. I will talk with the builder to see what this is. I wasn’t sure if this was an added moisture barrier or not with poured foundations. Or someone sprayed foam on the outside for whatever reason.
I was hoping with the poured foundation and the tuff n dry system that I would not need to use the 1.5” foam board. But it is what it is and I don’t want mold.
I was originally going to use drylock and the greenboard. But you mentioned that greenboard is overkill. If i use regular drywall and buy the foam – the costs may almost equal out.
The vapor barrier under the floor is 6mil plastic. I don’t know if that is “good” or not or if there are tears underneath. So the setup of an 8lb top moisture barrier(for spills) carpet pad with commerical low pile and be ok? Or would you still maybe lay down some plastic vapor barrer directly on the slab?
Forgot to add that I do have crushed stone against the foundation and french drains at the footer but this may not matter. The basement walls are all below grade.
Chris – That foam might be a foam block that was used for a block-out for utilities….some times foundation contractors will place a solid piece of foam in the form to create a void.
The 6 mil poly should be sufficient.
Thanks again Todd for your help.
Todd, great info. I live in South West PA – I am trying to make sense out of this basement finishing and moisture control.
1. What adhesive(brand) is proven to hold the 1.5” foamboard against a poured foundation and not fail after years of moisture?
2. The DOW site is saying to push the studs against the foamboard and says to use adhesive as a temporary way to hold the foam there. In another example it mentioned furring strips attached to the cement wall. I was originally hoping to leave a half inch of space between the studs and the foam like you suggest. I am just concerned that after a couple of years the glue will fail and my board will fall leaving an exposted wall. Would it be better to almost paint the adhesive on and put beads every 12”?
3. Also based on your info it seems ok to use faced FG rolled insulation — with the face – facing the warm room?
4. I am not sure I can get unfaced insulation and I am not sure how to secure it to the studs (no paper to staple)
Thanks!!
5. Forgot to ask one more – it is or isn’t ok for the rolled FG Insulation to come in contact with the foamboard?
thanks again!
Chris – Recently we’ve discovered http://building.dow.com/na/en/products/sealants/gspwallfloor.htm DOW Great Stuff foam adhesive. This stuff is amazing and it works VERY well. If you use that stuff plus tape the seams well with Tyvek tape you won’t have any problems. If you go with unfaced FG it will hold itself in place by friction if you’ve framed the walls correctly. The fiberglass can come in contact if need be, I just like having a gap for an added level of separation.
Thank you Todd for your quick answers- it seems that you recommend the unfaced vs. faced in my application?
I will look out for this product and try to get that as my #1 choice- have you had any experience with the probably easier to get Liquid Nails – http://www.liquidnails.com/products/product.jsp?productId=34
thanks again!
Chris – Liquid nails will work as well although not as well.
I have almost completed the job with 2” Pink F150 Owens – including the tedius rim joist area as you suggested. I have the R30 faced insulation pieces that fit up in the rim joist and figured why not reuse them and stuff them behind the foam board. Is this ok and does it matter which way the kraft paper is facing? it is all above grade. Originally(before removal), the unfaced portion was facing the room.
Chris – It shouldn’t hurt anything. Keep the kraft faced towards the finished side.
Sorry – so I am clear – the kraft facing the finished – warm room (so i see it) OR against the foam board (which is against the wood joist).
when they built the house the unfaced side was facing the warm room. (which could have been wrong)
Thanks again for your help in this project. Most appreciated. It was more work and more expensive than FG rolled but it will be worth it.
one more thing- the kraft side isn’t perfectly cut and won’t be an effective vapor barrior. it is a little ragged from pulling out
Chris – Kraft side should be on the warm side, which is AWAY from the foam. No worries if it’s torn, just patch it up with some tape.
Todd,
I am re-doing half my basement, a workshop-storage side. I live in the northeast and my house is a raised ranch that was built in 1965. Recently, I gutted the drywall that was covering the “knee-wall” foundation to find that there was no insulation or vapor barrier. Only some straping that ran horizontally for the drywall to attach to. Based on pervious posts I’ve read on this site, I anticipate doing the following:
1. Install 1 1/2″ Foam Board, adheard with proper adhesive. All joints to be taped with Tyvek tape or equivelant.
2. Build 2×4 framed wall that butts up against foam board. (Do you recommend leaving a space between stud wall and foam board?)
3. Install R-13 Faced insulation in framed wall and regular wall that sits on top of knee wall.
4. Rim joists to be insulated with 1 1/2″ Foam board as well. (Is it ok for this to be exposed?, No covering?)
There doesn’t appear to be any moisture problems, past or present. I am also planning on painting the floor with a 2 part epoxy paint for looks.
Just wanted to see if everything appears correct. Should I also insulate between the floor joists in the above ceiling? It is wide open now with a drop ceiling planned in the distant future.
Appreciate any help and insight!
Thanks,
Dave
Dave – Everything sounds fine. As far as the rim joist it really depends on the local code. Many codes do not allow for foam to be left exposed. Some codes will allow a foil faced foam (PolyIso). Check with your local building official.
My question. My basement had no insulation when purhased. Panal was on the walls. I removed the panals to find straping nailed to the wall. When i insulated i used 1.5 inch foam cap nailed to the straping leaving me a gap between the wall and insulation. Will this be ok? Some say it is better having a gap so your home can breath.
Billy – That is ok, over time the strapping will most likely decay.
Todd,
Thanks for the great info.
I live in Southwestern PA and recently discovered that my finished basement bathroom had started showing signs of moisture (the bottom of the baseboard was wet, as well as signs of mold). My semi-finished garage (no studs on the back wall) connects to the finished bathroom. The block in the garage were extremely wet due to gutters overflowing about 3 or 4 feet high (I corrected that problem). I discovered the bathroom wall on the garage side was wet as well near the floor, and up the side of the block. I cut the drywal to see how bad the damage was. I discovered the previous owner (original builder) put R-13 batt insulation against the cement block with no foam board or anything. This, of course, meant that the water just saturated the bottom of the insulation and then wicked its way onto the studs and drywall. I’m in the process of cutting more drywall out to see how bad it is. From what I can tell it doesn’t appear the water got beyond halfway over into the bathroom.
At this point I’m thinking of tearing down the entire back wall to expose the R-13 insulation. My plan would be to take out the drywall and insulation leaving the studs and cement block visible. The framing is about 1-2″ away from the cement block. My hopes would be that I could somehow get the foam board in-between the studs and cement block and seal it up, then put the insulation back in the studs.
What would your recommendations be for this job?
Thanks!
— Jason
Jason – Don’t feel bad, what was done to your home is done far too many times in this Country because people just don’t understand the situation. First off you need to remove all the old drywall and insulation and most likely dispose of all of it. Secondly I think you have two choices. You could have someone come in and spray foam the wall cavities or you could install some foam board. In your situation I think I would stick to just foam board and no fiberglass. I say this because it’s unlikely you can get sufficient insulation behind the studs and truly seal every thing up. I think you’re better off installing a thinner sheet behind the studs, sealing, then installing another layer between studs, sealing again. Make sense? In PA you probably want at least 2 inches of closed cell foam.
Todd,
Thanks muchly for the information. That makes sense, except for the “thinner sheet behind the studs, sealing, then installing another layer between studs, sealing again…”. What do you mean by that (thin sheet, then sealing, another layer, sealing)?. I was thinking about putting a coat (or 2) of DryLok on the cement block, then the foam board behind the studs, then as you recommended and using foam board instead of the batting insulation.
My next question is, if the foam board is against the cement block and water does happen to make it through the DryLok, wont the water get trapped between the cement and foam board? Or eventually make its way to the floor?
Thanks again!
– Jason
Jason – First off I think Drylok probably an expensive “belt and suspender” approach that may do little to no extra good. First off I would install a layer of foam as thick as you can behind the studs. Be sure you can get it all behind the studs and seal the seams with tape as indicated in the article. If you can’t get at least 2 inches behind the studs I recommend a second layer installed between the stud bays, pushed back to touch the first layer of foam. Again, this layer should be sealed, I would seal it to the studs with canned spray foam like Great Stuff.
Water between the concrete and foam will not be a problem. It may drop down to the floor and you should plan for that in your wall coverings to prevent wicking of water into finished materials like drywall.
Hey Todd,
Thanks! So that I have a better understanding, the foam board serves as what? An insulator and it protects the wood/drywall from direct leaks, or just an insulator, or just a leak protector? If the water can still get to the floor, what good is the foam board?
A few more question:
o) In my situation, you said you’d recommend using the foam board instead of fiberglass insulation, why’s that?
o) You say that the water may drop down between the block and foam board and go to the floor, and I should plan for that. Are you meaning I should use a water-resistant drywall (e.g., green board) instead of regular drywall, and pressure-treated wood for the studs (I believe they already are)?
Thanks!
– Jason
Jason – Some of your questions can be answered in the articles on the site. The quick answers are that closed cell foam acts as an insulator and vapor barrier if you have a minimum of 1-1/2 inches in thickness. If you install a min 1-1/2″ thickness, tape the seams and seal around all penetrations you can stop water vapor from the concrete and/or block from entering the wall cavity and getting trapped in fiberglass thus creating mold/mildew.
In your case, if you can’t get a good layer (1-1/2″ min) behind the wall, sealed properly then there’s a chance that water vapor could get to the stud cavity. So, I recommend you use ONLY foam as this gives you assurance that there will be no fiberglass growing mold.
No matter how well you seal the foam, if you get water behind it, it’s likely to drop down to the floor. What I’m suggesting is keeping things like drywall 1/2″ or so off the floor to prevent wicking. If your basement is prone to more water than that I would recommend not finishing the space.
Todd,
Ahhh…I didn’t even check out the rest of the site yet ;-) Sorry. I’ll check it out after posting this. What you said makes perfect sense now. Thanks so much for the info!
– Jason
Jason – Good luck. If you have more questions don’t hesitate to ask, it’s complicated stuff and unfortunately every situation is a bit different.
Todd, I read all the posts and still have a question. I will be insulating my basement in northern Wisconsin. 2″ foam board and tape 2 x 4 walls and fiberglass bats. Should I paint the walls first with DryLok? Will DyLok cause a problem with the foam adhesive? Should the fiberglass bats be faced(paper or foil)?
Thanks you have a great site!
Fred – Thanks for visiting! With 2″ of foam you may or may not need additional insulation, i.e. fiberglass. That really depends on the R value you need/want. If you do go with fiberglass I would use unfaced seeing that you have 2″ of foam.
I’m not sure DryLok will buy you much. The foam is going to take care of moisture. Just my 2 cents.
Hello Todd,
This is a very useful website. I have problem I am hoping you can help me solve I live in Northwestern Ontario. I have one wall in my basement that has condensation behind the vapor barrier, which made the insulation wet. I took the wall down and discovered that there is no mold so probably happens only in the summer months. The previous owner put:
-stud wall 1″ away from the foundation wall
-insulated in between the studs with fiberglass insulation R12
-sealed the stud wall with a 6 mill vapor barrier with acoustic sealant,
-drywall.
I took everything apart except for the stud wall (1″ away from the concrete), now how do I go about fixing the condensation problem.
I was thinking about having some one come in and spray foam that one wall, then insulate (R14 Roxol), do I need a vapor barrier? then drywall. What do you think?
Alex
Alex – Thanks for the compliment. So far everything sounds good, I’d stick with just spray foam, be sure it’s closed cell foam. No need for Roxol if you spray foam it.
Good luck!
Hello Todd,
Thanks for the reply, If i do not use the spray foam what other options do I have to get rid of the condensation problem?
Alex – That’s tough with only 1″ of space. I’d recommend getting a layer of foam behind the framing, taping/sealing all joints, then another layer of foam between studs.