Insulating basement - Thinking aloud

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kennyp2339

kennyp2339

Minister of Fire
Feb 16, 2014
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I'm will be starting my insulated the basement project this weekend. A couple questions or thoughts out load before I get this rolling.
My existing basement is concrete block wall. The wall is approx. 8ft high with 3ft exposed to weather the other five feet underground, I have (2) rectangular windows maybe 12ft x 20ft that swing open vertically. The basement connects to my garage which is walk out to the driveway (house is a raised ranch style)
Last year I used a basement water sealing primer to give all the blocks a uniformed white color, I have never had water in there, but in the summer I do run a dehumidifier and empty the 2 gal tank every 3 to 4 days, In my opinion it seems like I can call it a dry basement.
My idea for insulating is to install (glue) ridged foam board in the R-10 range (looking at owens corning foamular 250) taping all the seams. I will install a 2x4 along the rafters to make a clean line where the foam board meets the ceiling. I will tape all seems with a heavy duty tape. I have bats of roxual insulation left over from a different project that I intend on stuffing in between the rafters where they meet the 2x4 to the outer sill plate. I want to frame out the inside wall with 2x4 after the foam and install electrical outlets and drywall.
Right now my wood stove is approx. 10" away from the block wall. I plan on buying a couple 45deg dvl elbows and create an offset to move the stove away from the wall.
My main question is plastic vapor barrier - If I install the rigid foam board glued and tape all seems the foam wall should be air tight keeping the cold block from creating moisture right? Any input on this or the project in general will be greatly appreciated. Thanks KennyP.
 
I did this in my basement almost exactly.
There a good article here on basement insulation http://buildingscience.com/documents/information-sheets/basement-insulation Another good reference here: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
The folks at Building Science have since changed their view that the walls need to dry inward saying that any water that might accumulate between the masonry wall and foam board wasn't going to cause any problems. There's an article out there somewhere on that. The XPS is a pretty good liquid water barrier but does allow some water vapor through. A separate vapor barrier is not needed. I took the precaution of using only latex paint on the mold resistant drywall I hanged on the wooden studs. I also used Roxul, which is well suited for this application.
I'd also add that any wood that touches masonry should be treated so consider that with your sill plates. Some recommend using metal studs also.
My walls were a bit taller than the foam board so I had to install some fire-stop spray foam at the top of the stud bays.
You may also want to consider insulating your floors similarly so that may affect your wall design.
 
Me too, but a little different. I made a couple threads here. One is: https://www.hearth.com/talk/threads/basement-wall-foam-board-idea.30400/ .

The only wood I used was in the furring strips, and they were buried in two sheets of 2" foam. There are no fasteners going all the way from the outside through to the wall. I also put foam pieces in the sill plate area. Used router for wiring. Used Densarmor plus mold resistant drywall w/fiberglass facing.

You can run into the wall and bounce off, there's hardly any mold friendly stuff, but it took a while. A long while.
 
All good stuff, i did mine in similar fashion. Glued and screwed 2" foam directly to the concrete and then studded 2x4 walls with fiberglass insulation in them. If I remember the building science page correctly, foam and then fiberglass was OK since the foam is in essence your vapor barrier. I did all this before i put the stove in, i like cutting wood but don't like wasting it by heating the outside. Fyi, had a hard time finding what tape to use, all articles just said "tape the seams", i used 3M all weather flashing tape from Amazon, really good stuff.
 
What is the expected benefit here? I have a similar basement setup and with the stove running for 3-4 hours the temp is a comfortable 69 d within 20' of the stove. Temp decreases about 1d every 6-8' and is coldest, 66, at the far end and corners. I have no insulation, and open joist bays.
 
velvetfoot said:
I used Tyvek tape from Home Depot.
Click to expand...
Tyvek tape is good stuff and expensive.
Owens Corning also makes a pink tape that's meant for use with their foam insulation panels.
 
mass_burner said:
What is the expected benefit here?
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The less heat that is sucked up by cold walls and what's outside them means more heat for your house.
Also, if the basement is occupied it will feel much warmer. There's nothing like a cold masonry wall nearby to suck the heat right out of you.
 
semipro said:
Tyvek tape is good stuff and expensive.
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Hah, that the least of your expense concerns! I don't even want to add up what the 4" of foamular cost. It was a slow project, so I'd pick up some foam evenings on the way home. They are light and just fit into my car.
 
semipro said:
The less heat that is sucked up by cold walls and what's outside them means more heat for your house.
Also, if the basement is occupied it will feel much warmer. There's nothing like a cold masonry wall nearby to suck the heat right out of you.
Click to expand...
Less heat, yes, but how much less? And is the difference worth the significant outlay in time/expense? And if you want heat in your house upstairs, why have ceiling insulation.
 
mass_burner said:
why have ceiling insulation.
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For me, it was there and I didn't want to mess with taking it down. At the time I thought I'd want a finished basement, hence the walls. Now there are 6 tons of pellets down there, so I guess that ship has sailed.
 
mass_burner said:
And if you want heat in your house upstairs, why have ceiling insulation.
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There' s no reason other than for sound deadening.
If you heat source is in the basement the best insulation scenario is basement walls and floor insulated, ceiling not.
The numbers I've seen estimate 50% of heat produced in a basement goes to heating uninsulated walls and the world outside (no reference). That's not too hard to believe when you consider masonry has an R-value of about 1 per ft. thickness.
 
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so the opinion on the vapor barrier is??? none?

i'm looking to do a similar project this summer, but plan to use the spray on closed cell foam instead. seems cost comparable and should give good air sealing properties with a good installation rate (vs trying to cut sheets perfectly, tape, etc)
 
Yeah the general consensus is no vapor barrier, at least from what I've read online
 
semipro said:
There' s no reason other than for sound deadening.
If you heat source is in the basement the best insulation scenario is basement walls and floor insulated, ceiling not.
The numbers I've seen estimate 50% of heat produced in a basement goes to heating uninsulated walls and the world outside (no reference). That's not too hard to believe when you consider masonry has an R-value of about 1 per ft. thickness.
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But doesn't being underground, geothermal effect, give you a headstart on heating? When it's 10d outside, my basement is a steady 62d.
 
mass_burner said:
But doesn't being underground, geothermal effect, give you a headstart on heating? When it's 10d outside, my basement is a steady 62d.
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Sort of but that just raises the base temp. Thermal insulation slows the rate at which heat moves. It doesn't prevent temp equalization.
When you start heating the basement at 62d you warm the masonry and surrounding fill also to increase its temp. Masonry and soil have a high heat capacity so it will take a lot of BTUs to raise their temp. This heat would be better used to heat the interior of your house.
 
semipro said:
Sort of but that just raises the base temp. Thermal insulation slows the rate at which heat moves. It doesn't prevent temp equalization.
When you start heating the basement at 62d you warm the masonry and surrounding fill also to increase its temp. Masonry and soil have a high heat capacity so it will take a lot of BTUs to raise their temp. This heat would be better used to heat the interior of your house.
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I guess it all comes down to the many variables, layout, size, etc. It also depends on your use of the space. I work there 3 days/week and also do wood working, other fix it projects, and occasionally watch football games. During none of these activities am I ever cold/uncomfortable. In fact, if I didn't work, watch football, I wouldn't need the stove, moving around is enough to keep me warm.
 
mass_burner said:
I guess it all comes down to the many variables, layout, size, etc. It also depends on your use of the space
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Not really.
With a heat source in the basement your basement temps won't change much if you insulate the walls as long as that heat can move upstairs.
Simply put; if you insulate your basement walls you'll either use less fuel to maintain current temps throughout the house or temps in the house will increase.
I'm assuming here that heat produced in the basement has a way to move to upper levels. If its trapped in the basement then basement temps will most certainly go up.
 
Don't forget to install 1/2" sheet rock above the top plate of the framed wall prior to installing the wall. This is fire blocking. Easy to do prior to the all construction.
 
semipro said:
Not really.
With a heat source in the basement your basement temps won't change much if you insulate the walls as long as that heat can move upstairs.
Simply put; if you insulate your basement walls you'll either use less fuel to maintain current temps throughout the house or temps in the house will increase.
I'm assuming here that heat produced in the basement has a way to move to upper levels. If its trapped in the basement then basement temps will most certainly go up.
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My basement stairs are outside the inner basement envelope. There are 2 sliding aluminum clad 2" doors, one on each side of the two car garage. I think these doors have foam inside cause they are pretty light for their size.

No other way for heat to escape freely upstairs.
 
So, is it worth insulating if you're not trying to heat upstairs?
 
mass_burner said:
So, is it worth insulating if you're not trying to heat upstairs?
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Not as much so. You're still burning more wood than you'd have to if the walls and floor were insulated. However, since you're only heating the basement and it has restricted use it just may not make sense to invest time and money there.
That said, unless your basement ceiling is very well insulated and air sealed (not just FG stuffed between the joists), a lot, if not most, of the heat you produce in the basement is migrating upstairs.
 
semipro said:
Not as much so. You're still burning more wood than you'd have to if the walls and floor were insulated. However, since you're only heating the basement and it has restricted use it just may not make sense to invest time and money there.
That said, unless your basement ceiling is very well insulated and air sealed (not just FG stuffed between the joists), a lot, if not most, of the heat you produce in the basement is migrating upstairs.
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I have no insulation at all, open joist bays everywhere. Wood floors above benefit. I have an l-shaped sectional/coffee table area 3' in front of the stove.
 
mass_burner said:
I have no insulation at all, open joist bays everywhere. Wood floors above benefit. I have an l-shaped sectional/coffee table area 3' in front of the stove.
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Had to feel it for myself, but man am I using more wood in the basement. If I used the same amount upstairs yesterday, the house would be 85.

I can't tell if the heat is floating to the further end of the basement, if cold air is coming in from the garage, or if the walls are soaking up the heat.
 
You'd be amazed the amount of heat the walls and earth beyond suck up. I was.
 
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