Insulating an already insulated shop

[absintheisfun]

Just bought a place with an epic 40x60 red steel shop. The previous owner had 2 inches of soft cell foam sprayed on all walls and ceiling. I want to expand on that as I finish the interior. I'm in Texas so it is hot and humid and I am unsure the best way. My plan is to get 2" rigid foam board and put it up with a small 1"-2" air gap to the foam. I'm then going to wall the interior of the shop with 1/2" or 3/4" osb. My question is vapor barrier....do I need it? Does the foam serve that purpose? Ultimately there will be a mini-split or two and many ceiling fans to keep it cool. Heat isn't as much of an issue, but I will address it if needed in the winter.

Those of you that know about vapor barrier needs, please chime in and let me know the best way.

Thanks

 

[billconner]

I'm guessing "soft cell" is what many call open cell?

I believe in Texas you don't want an interior vapor barrier and one on exterior is probably not required and would be hard now.

I think I'd add rock wool or fiberglass inside the existing spray foam - flash and batt - and definitely not leave an airspace.

 

[dcg9381]

My plan is to get 2" rigid foam board and put it up with a small 1"-2" air gap to the foam. I'm then going to wall the interior of the shop with 1/2" or 3/4" osb. My question is vapor barrier....do I need it? Does the foam serve that purpose? Ultimately there will be a mini-split or two and many ceiling fans to keep it cool. Heat isn't as much of an issue, but I will address it if needed in the winter.

I'm in Texas. I know of many, many steel structures where open cell is sprayed straight to the steel siding. I've done it this way and will continue to do it this way.

I believe you could add another layer of open cell foam if you want to spray it.

I'm a firm believer in "hybrid" insulation, that is a layer of foam (open or closed cell) and traditional insulation after that.

I would not leave an air gap. Air gap makes sense if you have a means to vent it at the top.

We've had no vapor barrier issues in Texas (3 structures so far) with open cell sprayed to siding / roof deck. That's the traditional way to do it in Texas.

 

[absintheisfun]

Yes, open cell (not soft cell)-my bad. The horizontal purlens are 8" wide (deep) so with 2-3" of open cell, it would be difficult not to have an air gap since the open cell foam is so uneven. I was going to screw the osb directly to the purlins which would leave 4-5" of dead space. Even if I went with a rolled fiberglass or matt type of insulation, there would still be some air void, right? It would be cheaper for the insulation, but much more expensive overall since i would have to build a frame to hold the insulation.

From what I've read, you can spray a 2nd layer of foam over closed cell, but not open cell...??

Just out of curiosity, what would happen with an airgap that isn't vented? Could I put in those screens that are on eves of houses to allow venting, but then wouldn't that diminish the insulation's ability to insulate?

Again, thank you for advice and ideas!

 

[545_days]

Which part of Texas? The Gulf Coast has very different conditions than the panhandle.

 

[545_days]

I'm in Texas. I know of many, many steel structures where open cell is sprayed straight to the steel siding. I've done it this way and will continue to do it this way.

I believe you could add another layer of open cell foam if you want to spray it.

I'm a firm believer in "hybrid" insulation, that is a layer of foam (open or closed cell) and traditional insulation after that.

I would not leave an air gap. Air gap makes sense if you have a means to vent it at the top.

We've had no vapor barrier issues in Texas (3 structures so far) with open cell sprayed to siding / roof deck. That's the traditional way to do it in Texas.

Which part of Texas?
(I forgot to reply to your post when I replied above.)

 

[absintheisfun]

Small town north of Dallas

 

[billconner]

The rigid foam is a vapor barrier, which I think we agree is not needed. If it were needed in your climate, it would be exterior. The problem with a gap between rigid and open cell is the possibility that the gap side of the rigid could be cool enough for exterior moist air to condense, though I doubt it. 2" should be enough to keep exterior side above dew point. I guess it depends how cold you'll keep it inside, and if cooked air will leak through rigid seams. Consider taping or otherwise sealing seams.

 

[dcg9381]

Which part of Texas?
(I forgot to reply to your post when I replied above.)

Central, but pretty much all of Texas does foam insulation in steel and stick/brick construction the same way.

Even if I went with a rolled fiberglass or matt type of insulation, there would still be some air void, right? It would be cheaper for the insulation, but much more expensive overall since i would have to build a frame to hold the insulation.

In modern home construction 2x4 / 2x6 walls, they "shave" the foam to fit the wall. Completely flat. Even if not completely flat, rolled insulation deals with different gaps pretty well. You could always use cellulose (if you're going to do a finished wall)

From what I've read, you can spray a 2nd layer of foam over closed cell, but not open cell...??

Consult a foam contractor, but I buy this... Closed cell is much harder, it'd provided a better backing. And closed cell is more dense, I wouldn't want to add (too much) over open cell which is going to have limited internal strength.

Just out of curiosity, what would happen with an airgap that isn't vented? Could I put in those screens that are on eves of houses to allow venting, but then wouldn't that diminish the insulation's ability to insulate?

You'd get hot air with no way for it to escape. Is it better than not adding additional insulation? I'd think so, I've seen some pretty poorly vented house attics with tradition "ceiling" insulation.

I'd NOT do unvented air cap in climates that have substantial condensation concerns.

 

[WisJim]

A vented airgap between 2 layers of insulation would put a layer of outside air (temperature and humidity of the outside air) between the layers of insulation, and would pretty much negate the value of the outside layer of insulation--in my opinion. Don't leave any gaps between layers of insulation. It sounds like you will have enough room for a layer of 6" fiberglass?