Foam Board insulation over Kraft Batts?

[Nickmm]

Hello guys,

So I've got a 30x40 garage, and I've began the process of insulating it using kraft faced batts and rolls. My walls are 24" on center w/2x4s. So I'm maxed out at R15 that I can cram between the walls as far as batts. I've read of another project where a guy took foam board and installed it over his fiberglass/studs, basically inbetween the studs and sheetrock. If I go with a 1.5" foam board at 7.5r value, would this work out as far as my vapor barrier being the kraft batts already inbetween my 2x4s? It would then be sheetrock and foam board before getting to the studs on my walls. If I had built this garage I would have opted for 2x6's in our climate, honestly.

I have a smaller 5000w 21 amp heater, and I'm trying to tax it as little as possible. I am able to heat the place to about 40degrees in 20 degree ambient temps currently, but we easily see -20 in the jan/feb months, and I'd like to be able to use the garage if possible during this time.

 

[Chaz]

I did that in an old home I once owned. I don't know if it meets code, but the added benefit of having insulation OVER the studs seemed to make more of a difference than the added insulation in the bays. It worked beautifully!

 

[joe--h]

It will work. Before you do it, mark the stud location top and bottom. Gets hard to hit them after they're buried.

As to vapor barrier, it's a garage. Moisture from cooking & showers probably isn't something to worry about?

 

[walrus]

Hello guys,

So I've got a 30x40 garage, and I've began the process of insulating it using kraft faced batts and rolls. My walls are 24" on center w/2x4s. So I'm maxed out at R15 that I can cram between the walls as far as batts. I've read of another project where a guy took foam board and installed it over his fiberglass/studs, basically inbetween the studs and sheetrock. If I go with a 1.5" foam board at 7.5r value, would this work out as far as my vapor barrier being the kraft batts already inbetween my 2x4s? It would then be sheetrock and foam board before getting to the studs on my walls. If I had built this garage I would have opted for 2x6's in our climate, honestly.

I have a smaller 5000w 21 amp heater, and I'm trying to tax it as little as possible. I am able to heat the place to about 40degrees in 20 degree ambient temps currently, but we easily see -20 in the jan/feb months, and I'd like to be able to use the garage if possible during this time.

I did it on the outside of the walls and on the underneath of my trusses. Click the link in my signature to see it

 

[yeldogt]

The problem with any wall system is how is it going to dry -- every wall system is going to have moisture.

What you propose to do can work if the wall will dry to the outside -- but you can't have the kraft .. and you have to make sure that no penetrations exist that will allow air to bring moisture into the walls.

Go look at GBA -- green building advisors.

The reason that the foam is normally used on the outside of the sheathing is that it can be taped and will have few penetrations -- so few leaks. And walls will dry to the inside as they normally have conditioned space.

Moisture gets into walls from air movement.

 

[walrus]

I have very few penetrations in my interior walls. I brought down romex and came out into a surface mounted 4 square box. I then used conduit on surface of wall for outlets. Instead of having a bunch of 2.5 by 4 inch holes in walls I have on 1/2 inch hole in which I caulked tight. I do have some outlets recessed for specific purposes, I caulked or canned foam those tight also. Moisture is a concern in cold climates as I bring in vehicles that have ice or snow on them.

 

[fury9]

You might want to take a knife and slice the paper (vertically) on the batts so you don't have a double vapor barrier. Then tape up the foam seams.

 

[rburke65]

And, you might want to update your profile and list, at least, a state of residence. Good luck!

 

[Nickmm]

And, you might want to update your profile and list, at least, a state of residence. Good luck!

Too busy working in the garage!
Central Minnesota.

Thanks for all the insight, guys! Actually, a good point, in that I do bring in snow and ice in the winter, and soaked jet skis in the summer, there will be some moisture to worry about. Although, the hope is by summer time I can air it out through the doors.

So, the trick would have been to go unfaced, and then seal with foam board. Now, I have kraft, so I need to open those up and use faced foam board with tape between the seems. I can handle that. My hope is I can be at R22.5 or so with the R15+r7.5, but all together it will make for a solid garage.

Thanks everyone

 

[walrus]

Too busy working in the garage!
Central Minnesota.

Thanks for all the insight, guys! Actually, a good point, in that I do bring in snow and ice in the winter, and soaked jet skis in the summer, there will be some moisture to worry about. Although, the hope is by summer time I can air it out through the doors.

So, the trick would have been to go unfaced, and then seal with foam board. Now, I have kraft, so I need to open those up and use faced foam board with tape between the seems. I can handle that. My hope is I can be at R22.5 or so with the R15+r7.5, but all together it will make for a solid garage.

Thanks everyone

You should be over R10 on the foam board and don't tape the cracks between sheets, either foam them if big or caulk them if smaller. The R-10 figure comes from www.buildingscience.com

 

[CKS1955]

I think you will have moisture problems. If you installed the foam board first then unfaced insulation and then your interior wall finish you would be fine. Please see this website http://www.buildingscience.com/doctypes/digests .

Jay

 

[Nickmm]

I think you will have moisture problems. If you installed the foam board first then unfaced insulation and then your interior wall finish you would be fine. Please see this website http://www.buildingscience.com/doctypes/digests .

Jay

I thought its the other way around? Because I'm in a cold climate, your vapor barrier goes inside, not outside. This is in a detatched garage. Am I mistaken here? In the south, the vapor barrier goes outward because they are more concerned with summer time air conditioning and humid conditions, where as we are concerned with condensation in the winter months, or am I mistaken?

 

[Average_Joe]

The vapor barrier goes on the inside in the south too

 

[CKS1955]

Please see this link regarding condensation in wall cavities for cold climates.
http://www.buildingscience.com/docu...ondensation-using-insulation/at_download/file

In one house we had kraft faced insulation in the basement, but it did not slow the transfer of air and we would have frost on the interior of rim joist. The frost would eventually melt and cause mold. We removed the kraft faced fiberglass insulation. Then we installed 2" XPS (R10) caulked and taped all the joints. Then installed the unfaced fiberglass insulation (R15). The XPS slows the transfer of air so by time it comes in contact with exterior wall it is closer to that temperature preventing condensation.

Jay

 

[walrus]

I thought its the other way around? Because I'm in a cold climate, your vapor barrier goes inside, not outside. This is in a detatched garage. Am I mistaken here? In the south, the vapor barrier goes outward because they are more concerned with summer time air conditioning and humid conditions, where as we are concerned with condensation in the winter months, or am I mistaken?

You are worried about condensation in the winter. Warm moist air in the dwelling goes into the wall cavity with a dew point at lets say 45 degrees f. Lets say its 15 degrees f outside. The sheathing is at 30 degrees f, that warm moist interior air hits the sheathing, goes thru dew point and look its water condensating on the sheathing. If you put foam on the inside you have to make sure its tight, tight , tight. So any penetrations in the foam is a cause for worry. Electrical outlets would probably be #1 worry. All cracks between foam would have to caulked or using canned foam if too big for caulk. I wouldn't trust tape but it might work if you use something besides cheap duct tape.

 

[Nickmm]

You are worried about condensation in the winter. Warm moist air in the dwelling goes into the wall cavity with a dew point at lets say 45 degrees f. Lets say its 15 degrees f outside. The sheathing is at 30 degrees f, that warm moist interior air hits the sheathing, goes thru dew point and look its water condensing on the sheathing. If you put foam on the inside you have to make sure its tight, tight , tight. So any penetrations in the foam is a cause for worry. Electrical outlets would probably be #1 worry. All cracks between foam would have to caulked or using canned foam if too big for caulk. I wouldn't trust tape but it might work if you use something besides cheap duct tape.

Would it be better to leave a small gap at the floor to spray foam it, or should you bring the foam panel to the floor instead? This is good info from everyone, thank you for the help. I'm not sure it will be worth the trouble, but It would certainly be nice to get the added R value. I've been spray foaming all my penetrations thus far, including the doors, gaps in the windows and lights. My wall outlets/lights as well.

My one thought on this would be if I do a 1 1/2 inch board, now my outlets are going to be about 2" recessed from the outside of the sheetrock, meaning I'm going to have to space/stand off my wall plates and outlets somehow, not really sure on that one. Anyone have experience with this?

Cheers,
Nick

 

[walrus]

You have already done electrical? They make box extensions if your wire is long enough. I'd bring the foam down tight to floor, caulk it but you could leave gap and use canned foam.

 

[Nickmm]

Yes, the Garage was partially finished, I'm just completing the work. I bought the house, and the garage was built but not insulated, except one wall which was insulated/sheetrocked. The wiring was half done as well, so in October I trenched and ran new 100amp service instead of the old 30amp stuff, and installed proper grounding, and ran 3 new runs for my heater, welder, and compressor, as well as a bunch of new lights and sockets for my garage door openers, etc.
Its a bigger project for me and not all in order, but I'm getting it done. Last few years working outside in -20 weather has me very interested in getting a well insulated building, and since its +40 in December for some reason, now is the time to do it. Hooray.

 

[walrus]

. Last few years working outside in -20 weather has me very interested in getting a well insulated building, and since its +40 in December for some reason, now is the time to do it. Hooray.

Funny when you work outside, the warmth of a nicely insulated building makes a ton of sense. Its the same reason I went with R54 ceiling and R33 walls in my shop. Some will tell you there is no payback but for me the comfort of being out there with no drafts and a warm floor has paid off in spades. Plus I'm not a slave to heating the place, it takes a pallet in the woodstove to heat the place up unless its really cold then I can stoke the stove up with the some red oak.

When I did an addition on my house I found a place that sold seconds foam. What it is, is foam that was made for doing flat roofs(not really flat the foam is made tapered). It was reasonably cheap, I cut and fit it into stud bays. Put it in layers until full 5.5 inches was full, canned foamed all the cracks as I installed the layers. put 1 inch over the face of the studs. Sealed it all tight so it became my vapor barrier. The addition is 16 by 24 and I never put heat in it. Heat comes thru doors(it has a full basement, which is also insulated in similar manner). Its works great, no complaints from cold blooded wife.

 

[tomroblee]

I built my rural SW Indiana home with 2x6 walls and attic trusses. The building is 36' wide with a 12/12 pitch roof which gave me an 18' wide upstairs with 8' ceilings. The vertical truss members (my upstairs walls) were 2x4. I wanted more insulation. I used fiberglass bat insulation between the 2x4 truss members (wall studs), then covered the walls with 2" foam, then drywall. It has worked well.

The only "issue" that I have noted is that there will be a small circle of condensation on the walls where there are drywall screws in extremely cold weather. The screws must create a thermal bridge between the heated room and the unheated (and ventilated) attic behind the walls.

 

[Nickmm]

Funny when you work outside, the warmth of a nicely insulated building makes a ton of sense. Its the same reason I went with R54 ceiling and R33 walls in my shop. Some will tell you there is no payback but for me the comfort of being out there with no drafts and a warm floor has paid off in spades. Plus I'm not a slave to heating the place, it takes a pallet in the woodstove to heat the place up unless its really cold then I can stoke the stove up with the some red oak.

When I did an addition on my house I found a place that sold seconds foam. What it is, is foam that was made for doing flat roofs(not really flat the foam is made tapered). It was reasonably cheap, I cut and fit it into stud bays. Put it in layers until full 5.5 inches was full, canned foamed all the cracks as I installed the layers. put 1 inch over the face of the studs. Sealed it all tight so it became my vapor barrier. The addition is 16 by 24 and I never put heat in it. Heat comes thru doors(it has a full basement, which is also insulated in similar manner). Its works great, no complaints from cold blooded wife.

I will look around for a building supplier selling seconds foam like that. Would be nice... its $17 a sheet at Menards so by my count, about $600 for the place, if not a tad more, and that's lowballing. I'm kinda not happy with that price considering batts cost less than half.

My next step after this will be improving on the garage doors. I have 3 doors on one wall, and they are only R10, and they don't really seal well around the edges, I need to make an overlap all around of some sort.

Interesting about the sheetrock screws, I wonder if there is a way around that. Maybe a different type of screw, painted screws, etc. Dunno.

Thanks for the tips everyone.