Insulating my basement

[sportbikerider78]

I thought I would bounce this one off you guys. I have read a great deal online and pretend to be just one of the uneducated masses that moonlight as a DIY'er on the weekends. Nothing more...so here is the situation....

I have a large (1900 sq ft) uninsulated basement with nice flat, block walls. I have very little moisture (just enough to keep a dehumidifier going) and never see any water run down the walls or on the floor,,,anywhere. With any uninsulated block structure, I do get damp looking areas in the corners of the basement where it is above ground. The below ground sections (90% surface area) show no discoloration any time of the year.

There is insulation on the outside of the block foundation below grade, not above. I do not know if the poured floor has insulation below it. My guess would be yes.

I have no plans to finish the basement in the next 3-5 years, if ever. Although, I want to start this project with the intention of being able to finish it at some later date.

I am not going to use any combination of foam boards and fiberglass insulation. Too many horror stories of fiberglass insulation getting wet and causing mold. Not an option for me.

This takes me down the decision pathway on deciding what type of foam board to use. EPS or XPS. The XPS claim to fame is that it actually forms a great vapor barrier and absorbs less moisture than EPS. The price is about double for XPS. For a large basement, that difference is quite large.

My question boils down to this. Does the EPS really absorb so much moisture that I would have issues if I was to drywall over it? Is it possible to put up a cheap plastic vapor barrier and then the EPS?
Anyone with experience?

 

[kennyp2339]

There are companies that make interlocking foam board insulation that act as a vapor shield at the same time.

 

[semipro]

I'm not sure the answer to your questions but I can tell you that I found XPS much easier to work with than EPS.

I wasn't aware there were water issues with EPS thought I know there are for polyiso boards.
You may want to check out greenbuildingadvisor and buildingsciencecorp websites if you haven't already. Its likely any EPS water issue has been covered there.

Edit: good related discussion here: http://www.greenbuildingadvisor.com...urability/34462/xps-versus-eps-underslab-foam

 

[sportbikerider78]

Great link..I read it top to bottom. Good food for thought even though it was centered around insulation below grade and under a foundation.

I'm thinking that with how many people have zero issues using good 'ole fiberglass insulation with a dehumidifier, I'd be fine with the EPS. That's my 'hunch'.

 

[semipro]

Another related article, though a short one.
http://www.finehomebuilding.com/tool-guide/articles/insulation-rigid-foam.aspx
I think my primary problems with EPS in the past stems from the grade of EPS. The stuff I encountered was brittle and hard to manage. It seems that better grades of EPS are available and may be best suited for your basement application.

 

[sportbikerider78]

Do you recall what type it was?
I can get 4'x8' by 2" thick sheets of type 1 EPS for about $21/sheet. It has a foil on both sides to keep it all together and reflect heat.

Data sheet link
http://www.cellofoam.com/BuildingProducts/epsinsulationtds.html

I still don't understand which type is better for my application. Density is not really a concern...nor compressive strength. I think I only care about R value and moisture absorption. ?

 

[ironpony]

I thought I would bounce this one off you guys. I have read a great deal online and pretend to be just one of the uneducated masses that moonlight as a DIY'er on the weekends. Nothing more...so here is the situation....

I have a large (1900 sq ft) uninsulated basement with nice flat, block walls. I have very little moisture (just enough to keep a dehumidifier going) and never see any water run down the walls or on the floor,,,anywhere. With any uninsulated block structure, I do get damp looking areas in the corners of the basement where it is above ground. The below ground sections (90% surface area) show no discoloration any time of the year.

There is insulation on the outside of the block foundation below grade, not above. I do not know if the poured floor has insulation below it. My guess would be yes.

I have no plans to finish the basement in the next 3-5 years, if ever. Although, I want to start this project with the intention of being able to finish it at some later date.

I am not going to use any combination of foam boards and fiberglass insulation. Too many horror stories of fiberglass insulation getting wet and causing mold. Not an option for me.

This takes me down the decision pathway on deciding what type of foam board to use. EPS or XPS. The XPS claim to fame is that it actually forms a great vapor barrier and absorbs less moisture than EPS. The price is about double for XPS. For a large basement, that difference is quite large.

My question boils down to this. Does the EPS really absorb so much moisture that I would have issues if I was to drywall over it? Is it possible to put up a cheap plastic vapor barrier and then the EPS?
Anyone with experience?

This is so water drains down the outside of the wall not insulation. part of the water proofing system.
Highly unlikely that there is insulation under the slab, unless you have radiant floor heat.
as far as type of insulation I have no recommendations. however to test for moisture, tape a piece of plastic, sealing I all the way around, to the floor and wall for a few days an see if any moisture collects behind it.

 

[moey]

You have moisture its concrete unless your in a desert. At some time during the year the concrete will have moisture if not now when it rains alot. I have XPS glued on my basement walls I was hesitant to use EPS with that foil layer. Ive read its OK to use it though.

 

[sportbikerider78]

Thanks for the pointers guys. I have done quite a bit of reading in the last couple of days and likely read 100's of comments from lots of different construction sites. Lots of learning going on over here.

For basement moisture, it appears you are ok to secure foam board with adhesive as long as you aren't getting any type of "wet-to-touch" moisture. Then you have a grading issue that needs to be resolved. My basement is way drier than wet-to-touch and will only get better, as I am addressing a soggy yard issue with additional french drains this summer.

Fire protection is my next question. I really have no plans to finish the basement. If I was to proceed forward with any foam board, install it all around the basement walls, and leave it exposed, is this a fire risk and against code?

I think this might be a city/town/county question that has to be answered locally.

 

[moey]

Its against code its not a local question. You need to put up a fire barrier a.k.a 1/2 drywall on top of it. Thermax foam board can be left exposed but its expensive and not many places sell it. You usually put boards 1 by 3s on top of the foam to hold it to the wall while the adhesive drys. You can attach the drywall to that more work and another $8-10 a sheet.

 

[ironpony]

around here it would be against code, has to be covered with drywall, fire resistant covering. if it did catch fire it would go fast and hot with a lot of toxic fumes.

 

[sportbikerider78]

Its against code its not a local question. You need to put up a fire barrier a.k.a 1/2 drywall on top of it. Thermax foam board can be left exposed but its expensive and not many places sell it. You usually put boards 1 by 3s on top of the foam to hold it to the wall while the adhesive drys. You can attach the drywall to that more work and another $8-10 a sheet.

Ok..thanks for the heads up. This Old House has a quick video on just what you are describing and the method I will be using.

http://www.thisoldhouse.com/toh/video/0,,20503345,00.html

 

[semipro]

I did something similar.
I installed the foam the same way and then installed a full 2x4 stud wall and then insulated between the studs with Roxul.
I did the full stud wall to:
-end up with a plumb wall (some concrete walls were not plumb)
-accommodate extra thermal insulation
-accommodate wiring and standard electrical boxes without cutting into the foam.

 

[moey]

I used some shallow electrical boxes and cut into the foam in some spots where I had to put in a thicker box. You still have 1 inch foam against the wall in those spots which is all you need for a vapor barrier. Its a pain but doable. I thought about putting up a 2 *4 wall but my basement is unfinished and not that big I just wanted insulation like you. A 2*4 wall would have taken away essentially another 4.5 inches on each wall.

 

[ozzy73]

I did the same as semipro, worked great. Total R value or R24 in basement, R10 ( 2" XPS ) and R14 ( Roxul ). No VB needed over the 2 inch foam. I wish I would have done the floor with 1" XPs also.

 

[semipro]

. I wish I would have done the floor with 1" XPs also.

Funny you should say that. I'm planning to do that next.

 

[ozzy73]

The only thing that kept me from not doing the floor is the 2000 tapcons ( + broken drill bits ) I would have needed to secure the foam + plywood to the floor. DryCore panels seem like less of a hassle and be quicker.

 

[semipro]

The only thing that kept me from not doing the floor is the 2000 tapcons ( + broken drill bits ) I would have needed to secure the foam + plywood to the floor. DryCore panels seem like less of a hassle and be quicker.

I plan to lay the foam down and install a floating floor above it (without a subfloor).
I've done this in other rooms in the house and its held up very well going on 10 years.
Our slab has a poly water barrier under it so moisture should not be an issue.

 

[ironpony]

I did 2" foam under the floor and a 2" foam thermal break around the interior of the foundation, between the foundation and the slab.