Insulating basement - Thinking aloud

[woodgeek]

Taped XPS IS a vapor barrier.

AND, the concrete is going to be (assumed) wet/damp anyway. So, nothing that molders in contact. THis is why I don't like the fiberglass/stud approach in a basement.

I tapconned 1x1's on 16" centers with the screws going through a continuous sheet of 1" taped XPS up against the concrete. Then put another 1" of XPS between the 1x1's, so I have 2" of XPS total and no thermal or moisture bridges. The wood can dry to the outside. I then screwed drywall to the 1x1s. They sell 'systems' of plastic studs that work similarly.

 

[semipro]

Taped XPS IS a vapor barrier.

I believe XPS is actually a vapor "retarder" with a perm rating of about 1.1 (ng/Pa·s·m2) per inch thickness. As sold, Foamular with the integrated plastic facing still comes in around 1.1 - 1.5 perms.
A "barrier" has a perm value of 0.1 or less.
This strengthens your case for avoidance of using mold-susceptible materials like wood studs in basement walls.
http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation
http://www.finehomebuilding.com/item/5090/whats-the-difference-vapor-barriers-and-vapor-retarders
http://ww3.owenscorning.com/content/docs/ASTM Document.pdf

 

[RichW_SC]

I am planning something similar. I have about 1000 sq/ft of dirt in my crawl space which is vented. It's pretty dry down there but no insulation in the joists. I'm going to put a 6mil vapor barrier on the dirt with 2" foam board on the brick walls. For the joists I'm gonna use 1" of spray foam to air seal and insulate. These old houses are so much fun...

 

[Dieselhead]

Anyone use heavy plastic against the wall then 1" airspace with traditional framing and fiberglass?

 

[semipro]

I am planning something similar. I have about 1000 sq/ft of dirt in my crawl space which is vented. It's pretty dry down there but no insulation in the joists. I'm going to put a 6mil vapor barrier on the dirt with 2" foam board on the brick walls. For the joists I'm gonna use 1" of spray foam to air seal and insulate. These old houses are so much fun...

I'd suggest you read up at greenbuildingadvisor, buildingscience, or energyvanguard before you do this.
As I understand it, there are 2 basic ways to go:
- insulate and seal the crawlspace and then connect it by duct to the living space for air exchange; or
- ventilate the crawlspace and isolate it from the living space with air sealing and thermal insulation.

From your description it sounds as if you are planning some sort of hybrid that may create more problems than you have now.

 

[woodgeek]

Indeed. You want to be careful with crawls...

Sounds like you want a separate air space insulated from the outside.

The 1" sprayfoam under the floor would be a nice way to airseal an old floor, but might not be necessary as Andy says.

 

[RichW_SC]

From your description it sounds as if you are planning some sort of hybrid that may create more problems than you have now.

Yes those are great sites and I'm planning to close off the vents, encapsulate and then monitor the moisture levels. I have a dehumidifier I can put down there if needed. Luckily it's a very dry basement/crawlspace. The spray foam will be to air seal and a little insulation to make my hardwood floors a little more comfortable.

To the OP here is a great site that covers basement insulation: http://www.homeconstructionimprovement.com/basement-insulation-guide/

 

[mass_burner]

So I put a little fan on the floor in front of my desk pointed toward stove. It helped move the colder air and it got up to 66 at my desktop a lot sooner.

 

[Seasoned Oak]

One downside to too much basement insulation is if you lose heat altogether for an extended period of time your basement will not benefit as much from the 55 degree below ground temps. Iv seen a house go thru 2 months of the worst part of the winter without any heat due to a n oil furnace that failed to come on and never freeze due to the basement being totally underground and no insulation under the first floor between the 2 floors. The basement stayed about 45 and the ist floor about 35. This situation would apply a house thats empty for long periods of time in winter like a seasonal home or second home.

 

[mass_burner]

One downside to too much basement insulation is if you lose heat altogether for an extended period of time your basement will not benefit as much from the 55 degree below ground temps. Iv seen a house go thru 2 months of the worst part of the winter without any heat due to a n oil furnace that failed to come on and never freeze due to the basement being totally underground and no insulation under the first floor between the 2 floors. The basement stayed about 45 and the ist floor about 35. This situation would apply a house thats empty for long periods of time in winter like a seasonal home or second home.

What do you mean by never freeze? Pipes or people?

 

[Seasoned Oak]

I m

What do you mean by never freeze? Pipes or people?

I mean the inside temp never went below freezing for 2 months despite no heat at all in the house and prolonged outside temps at 10 degrees overnight and 25 during the day. There were no people in the house for months as its a seasonal use house ,and the oil heat failed in mid december and wasnt discovered until mid feburary. I basement is all concrete block with no form of insulation on it and no insulation between the floor joists. Had there been it would have blocked the ground heat from traveling up to the first floor. As i said certain situations are better off with no basement insulation ,this is one of them.

 

[mass_burner]

I m

I mean the inside temp never went below freezing for 2 months despite no heat at all in the house and prolonged outside temps at 10 degrees overnight and 25 during the day. There were no people in the house for months as its a seasonal use house ,and the oil heat failed in mid december and wasnt discovered until mid feburary. I basement is all concrete block with no form of insulation on it and no insulation between the floor joists. Had there been it would have blocked the ground heat from traveling up to the first floor. As i said certain situations are better off with no basement insulation ,this is one of them.

Yea, I get what you're saying, but pipes often run along the perimeter of the main floor, close to below freezing temps. Whatever heat that comes up from the basement isn't gonna keep the pipes from bursting.

 

[sportbikerider78]

I m

I mean the inside temp never went below freezing for 2 months despite no heat at all in the house and prolonged outside temps at 10 degrees overnight and 25 during the day. There were no people in the house for months as its a seasonal use house ,and the oil heat failed in mid december and wasnt discovered until mid feburary. I basement is all concrete block with no form of insulation on it and no insulation between the floor joists. Had there been it would have blocked the ground heat from traveling up to the first floor. As i said certain situations are better off with no basement insulation ,this is one of them.

I live on a slope with a walk out basement. When I insulate, I'm only doing the walls that are above ground. It is not a normal living space with bedrooms and such.

I also had zero heat down there this winter as I was heating with a woodstove and no pipes froze.

 

[Seasoned Oak]

Yea, I get what you're saying, but pipes often run along the perimeter of the main floor, close to below freezing temps. Whatever heat that comes up from the basement isn't gonna keep the pipes from bursting.

It did keep pipes from bursting in this case, water pipes everywhere on the first floor above ground. When i entered the house in mid febuary it was 33-34 outside it was low 20s after an overnight in th emid teens and the basement was noticeably warmer about 40-45 and the heat had been off for 2 months.

 

[Husky]

I live on a slope with a walk out basement. When I insulate, I'm only doing the walls that are above ground. It is not a normal living space with bedrooms and such.

I also had zero heat down there this winter as I was heating with a woodstove and no pipes froze.

I'm just curious, but when I hear people saying they are only going to insulate half their wall how are you doing it. Do you leave the insulation exposed or drywall over half of it. I am only asking because I wanted to insulate my walls but in no way wanted to drywall to finish. Everything I have read says you can't leave foam board exposed due to the fire hazard and building codes.

 

[vinny11950]

Anyone use heavy plastic against the wall then 1" airspace with traditional framing and fiberglass?

This was done to the basement in my house somewhere in the late 80s. They used regular lumber, fiberglass and sheetrock, and the bottom sections that touch the concrete floor are rotting out. I have taken out some sections, and the lumber falls apart pretty easily and I can see there was no moisture barrier used between the lumber and concrete floor. Not sure how well the plastic works against the walls because it wasn't attached very well and I can see big gaps that allow plenty of airflow and moisture to get in there, especially in the summer. Not a fan of the system, which is why I am tearing it out slowly.

 

[Seasoned Oak]

I would cover the wall with cement board with a small gap behind it first ,or a large piece of roof rubber. Got to keep the moisture away from the wall

 

[sportbikerider78]

I'm just curious, but when I hear people saying they are only going to insulate half their wall how are you doing it. Do you leave the insulation exposed or drywall over half of it. I am only asking because I wanted to insulate my walls but in no way wanted to drywall to finish. Everything I have read says you can't leave foam board exposed due to the fire hazard and building codes.

My basement is a bit different. I have many different sections and not just a wide open square or rectangle. I have a block pantry and another area that is used for constructions. When I do it, I have the ability to do entire walls and then skip other walls without it looking very strange.

Right now, I have no plans to finish with drywall.

 

[DickRussell]

.... They used regular lumber, fiberglass and sheetrock, and the bottom sections that touch the concrete floor are rotting out. I have taken out some sections, and the lumber falls apart pretty easily and I can see there was no moisture barrier used between the lumber and concrete floor. Not sure how well the plastic works against the walls because it wasn't attached very well and I can see big gaps that allow plenty of airflow and moisture to get in there, especially in the summer. Not a fan of the system, which is why I am tearing it out slowly.

This comes up regularly, and this link:http://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
often is posted as recommended reading.

 

[velvetfoot]

Do you have a similar link for insulating a basement floor?

 

[iron]

also this: http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/

 

[velvetfoot]

I remember reading of osb tapconned onto xps foam, and then a flooring on top of the osb.

 

[moey]

I'm just curious, but when I hear people saying they are only going to insulate half their wall how are you doing it. Do you leave the insulation exposed or drywall over half of it. I am only asking because I wanted to insulate my walls but in no way wanted to drywall to finish. Everything I have read says you can't leave foam board exposed due to the fire hazard and building codes.

You dont actually need to "finish" the drywall just hang it. You would have boards up there anyways to hold the drywall up. Besides being safer its nicer to look at then blue or pink walls. Ive done our basement with 2 inch XPS with 5/8 drywall hung up. I didnt bother to finish the seams as there is no living space in the basement. Its much much more comfortable now. There were times when my basement would drop below 50F now I dont think it has been below 60F and theres very little heat going down there.

 

[velvetfoot]

I I think is cool is that you can run right into the wall and just bounce off-it's solid.

 

[mass_burner]

I I think is cool is that you can run right into the wall and just bounce off-it's solid.

What exactly is dangerous about exposed xps? It's out in the open at the BBS all the time.