DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Insulation (
-   -   Question about installing XPS insulation in basement (

Apollo276 06-13-2011 08:05 PM

Question about installing XPS insulation in basement
Hi all,

I've been working on finishing my basement. I started by working on framing walls around the exterior thinking I would just install fiberglass batts between the studs. Unfortunately after doing some research I discovered that the recommended approach seems to be to install a 2" layer of XPS insulation directly against the concrete walls and then put the framed walls up. This is a bit of a dilemma for me since I have made so much progress on the walls already.

Rather than taking the walls out and moving them around, would it be acceptable for me to install 0.5" XPS board between the studs and then unfaced fiberglass on top of that (and then drywall of course)? I found in one of the Building Science PDFs that it is fine to install XPS board and then unfaced fiberglass. But I did have some questions I hoped to get clarity on from all of you.

  • Would 0.5" of XPS be enough to help with potential moisture problems, or do I need more?
  • I would not always be able to get foam boards in contact with each other since the studs are in places too close to the concrete walls. I'm assuming I could fill the gap with some of the canned spray foam. Will this work?
  • Will there be major issues with stuffing 3.5" batts into the stud cavity along with the 0.5" XPS board? One of the reasons I'd like to use the 0.5" board is to reduce the space required. Since the basement walls are somewhat uneven, some places along the wall there would be plenty of room, but other places I think I would have to stuff the insulation in a bit. I know this will reduce the insulation effectiveness, but will it do so significantly?

Thanks for all your help.

HomeInsulation 06-14-2011 04:45 PM

Hey Apollo276,

I'd like to know where you live so I know what your climate is like.

What are you planning on using the basement for? Man cave?

Apollo276 06-14-2011 05:39 PM

I live in central Illinois. Yes, plan on mostly having it as a man cave - moving my office from upstairs down there and also buying a pool table and TV for it.

HomeInsulation 06-16-2011 04:04 PM


I wanted to know where you live to see what weather conditions you'll be experiencing. Living in central Illinois, you have cold winters and hot humid summers. So you'll have moisture concerns during most of the year.

I actually re-read this article from building science's Joseph Lstiburek to see if I could find a suitable solution for you.

If you're going to be spending a lot of time in the space and want to do it right. Moving the walls is your best option. I know it seems like a lot of work, but jamming rigid foam behind the existing walls might take more time in the long run.

However, if you want the quick and easy solution...Flash and batt is the way to go.

Using the flash and batt method, you would buy a DIY spray foam kit and spray the insulation directly onto the foundation wall. As the foam expands, it will air seal and insulate the space behind the studs in a few seconds. It will make your job of adding a layer of foam insulation a lot easier.

Then you can insulate the wall cavities with a fiberglass or rockwool insulation and move on with your project.

Do yourself a favor and divert all of the water from your gutters and downspouts as far away from your foundation as possible. Examine the grading near your foundation after a heavy rain to make sure no water puddles near your foundation. If so, fix the grading to keep the soil near your foundation as dry as possible.

This prevents excessive moisture from getting into your basement in the first place.

Enjoy your mancave,

Apollo276 06-18-2011 04:50 PM

Thanks for the advice. One more quick question. If I decide to move my walls, what should I do about the holes leftover in my basement slab? I used tapcons to anchor my bottom wall plate to the slab, and I am assuming I would need to fill the holes to prevent any moisture/seepage problems from happening. What's the best method for doing that?

ccarlisle 06-23-2011 08:18 AM

Just read your post; the whole point about using up to 2" thick XPS boards glued directly to the foundation wall is to prevent that concrete wall from becoming a transfer bridge from cold outside to warm inside, what they call a 'thermal break'; spray foam does the same thing.

So not only does the thermal break have to be thick enough, but it has to be directly on the concrete. If I read your post correcty, you are using 0.5" - which is too thin - and not agianst the concrete. Both are miscalculations, if I read you right.

Perry525 06-24-2011 12:55 PM

Basement insulation
Are your walls dry?
There are normally two things to overcome.

One is damp coming through the wall.
The other is damp/condensation landing on the walls from the air in the basement.

In either case the solution is to use sheets of polystyrene
that a more than 3 inches thick to overcome the "Dew Point" problem.

In situation number one the water/damp will come through the walls and run down to the floor.If the walls remain uncovered then the warm air in the basement, warms the walls and may keep them looking dry, if you cover them, they get cold and condensation forms and joins with the water coming through the walls and runs.

In the other the damp/condensation lands on the wall and may run down.

If you place wood or any other hygroscopic material against the wall it will get damp, mold will grow, the wood will rot and you may become ill, if the spores fruit.

The damp wall problem works round the wall temperature, is the wall below the "Dew Point" temperature?

The dew point is a mixture of air temperature and water
vapor content and the temperature of the wall.

Warm air holds more water vapor than cold air.

When warm air gets close to a cold surface (like a window in winter) or your basement walls condensation forms.

You cannot go below 3 inches.

ccarlisle 06-24-2011 01:22 PM

Cannot go below three inches?? :eek:

What are you referring to? XPS, EPS or spray foam?

Perry525 06-24-2011 02:05 PM


Originally Posted by ccarlisle (Post 673358)
Cannot go below three inches?? :eek:

What are you referring to? XPS, EPS or spray foam?

This is one of those areas where there is no exact science, merely a rule of thumb.

One can never predict how cold it will get in a given location and spray foam is very difficult to control.

If you want to go thin, then Aerogel will work for you.

One centimetre thick Aerogel equals 4 inches of EPS.

You could spend this coming winter running some experiments?

ccarlisle 06-24-2011 02:39 PM

Hmmmm; we may be speaking at cross-purposes...

In the first place, we are discussing a basement in a home in Illinois. I don't know their heating requirments but I have a suspicion it's moderate. It certainly isn't a 'cooling zone' (although some air conditioning is needed) and winters are relatively "dry". So the OP should be insulating his basement with a view of having the basement walls dry to the inside, where it is mostly warmer than the outside.

Secondly, I think we have a fairly good handle on insulation in basements; we know about XPS and EPS, we know about fibreglass and we have a good idea on spray foam, open and closed...we understand moisture control, mould and relative humidity. I've not heard about Aerogels in this context although I have worked with silica aerogels called 'Syloids'...

Up here in a cold zone, we recommend 2"of XPS fastened to the concrete basement walls - that thickness allows the walls to dry relative slowly to the inside while not allowing and moisture build-up and mould. Plus it gives about R12.

Perry525, if you are in Florida, there's a different set of requirements just as there is for people in the 'in-between' zones of Kenntucky and Tennessee. There it's a whole different kettle of fish! But I reckon Florida has some pretty humid days and nights and lots of AC's say nothing of 'basements' in general....

Gary in WA 06-24-2011 11:23 PM

Apollo 276, as ccarlisle said, you climate is Zone 4 or 5, which requires basement insulation (rigid or continuous) of R-10, actually Zones 4-8 all only require R-10:
Here is a study of colder climates than your 6200 heating degree days, with Minn. at 8500 HDD: http://www.buildingfoundation.umn.ed...timum-main.htm
Follow the back links for an interesting study.


Perry525 06-29-2011 09:37 AM


Originally Posted by ccarlisle (Post 673407)
Hmmmm; we may be speaking at cross-purposes...

s say nothing of 'basements' in general....

It does appear we are.
My point is how to avoid condensation on a potentially cold cellar wall.
Insulation is a different matter.
It can be said that in some southern areas one inch thick polystyrene will do the trick as the difference in temperature on both sides will be small and Dew Point will not occur, however, we then have people who turn their air con down to silly levels.
As you can never tell who may live in a home in the future its best to be safe and over prescribe.

concretemasonry 06-29-2011 09:50 AM

The temperature of the soil and concrete is possibly down to 55F in the winter and up to possibly 60F in the summer due to the thermal storage of Mother Nature. Not at all like being above grade where the are extremes.

It does not take that much insulation to eliminate condensation. Since it will be conditioned, just make sure you have good air circulation and cold air returns near the floor to get the benefits of the soil to help out the AC.


Perry525 06-30-2011 07:37 AM


Originally Posted by concretemasonry (Post 676469)
The temperature of the soil and concrete is possibly down to 55F in the winter and up to possibly 60F in the summer due to the thermal storage of Mother Nature. Not at all like being above grade where the are extremes.

It does not take that much insulation to eliminate condensation. Since it will be conditioned, just make sure you have good air circulation and cold air returns near the floor to get the benefits of the soil to help out the AC.


I agree that the temperature a few feet below the surface is around the 50f mark for most of the year.(In a lot of moderate places)

And therefore, the internal wall temperature will be about the same, (if the room is not heated.)

Adding heat to the room does raise the walls surface temperature and will often keep the surface dry. (of condensation)

Covering the walls will automatically lower their temperature to that outside.(and outside can be below freezing)

Heating a room to 70f and living in a room adds water vapor from breathing and sweating and possibly from cooking, washing, food, drinks, plants, fish tanks.

The normal humidity in a room will range about the 45-70% mark.

This brings into play the problem of condensation as air at 70f with a 70% humidity content will drop its water vapor onto a surface that is 60f or below.

The same air temperature 70f with a 60% humidity will reach "Dew point" at 55f.

As the temperature of air drops so does its capacity to hold water.

The molecules of water vapor are very, very tiny, much smaller than air molecules as such they require careful preparation to avoid any holes of cracks that will let them though to the cold surface. This is why it is best to fit a water vapour proof membrane (plastic sheet) over the polystyrene, the two combine to create a warm water vapor proof surface that is unlikely to attract water vapor from the air.

ccarlisle 07-01-2011 07:02 AM

That's good stuff; I'd just like to add that I have yet to see an "air molecule" - and I've 'been around' LOL - so people may get the wrong idea that everyone know what these are...

Rather, it should be understood that 'air' is a mixture of two main gases: oxygen and nitrogen. One molecule of water vapour 'weighs' 18 units, whereas one molecule of nitrogen weighs 28 'units' and one molecule of oxygen weighs 32 'units'.

'Units' are irrelevant for the sake of this discussion...

Water vapour is lighter than air (otherwise clouds would be near our feet versus in the sky). Water vapour management is thus just as important as air management in 'basement insulation' andthe two are quite distinct from "water management"....

All times are GMT -5. The time now is 08:13 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1