Shop insulation...kinda gloat...

[Mike Cruz]

Well, I've been mulling over the task of insulating the roof/ceiling of my shop (no ceiling, 10 feet to the bottom of the rafters, metal roof). It gets deathly hot in the summer, and the cold just slams right in during the winter.

With all the talk of shop heat lately, I knew that while I want to get a heater, I also knew that without insulating the roof, the efforts would be in vain. So, I've called up an insulation guy to get a quote. I was hoping he could get the materials cheaper than I could, but as it turns out, he can't get them as cheap as what I found.

We had intended to put up 4 x 8 sheets of foam insulation, 2" thick ($28 per sheet). Yeah, only a 13R factor, but air tight, and with an air gap between the board and the roof, and foil to reflect heat, yada yada yada, it actually equates to more.

But I found some 3" foam insulation boards on CL for $19 a sheet. They are rated at 19.5R (plus all the other benefits of foam boards). So I'll be saving $9 per sheet AND getting an extra inch! SWEET!!!!!!!!! And it is foiled on both sides, so it ought to reflect light back down into the shop quite nicely.

I go to pick it up tomorrow. Hope this doesn't jinx it by posting early...

[Jason Hanko]

I plan on doing much the same to my shop as I dont have sheetrock up on the ceiling joists, so you can see right up to the (currently un-insulated) rafters and roof decking. You can boost your R value even further by adding rolls of batted insulation on top of the foam board between the rafters (being sure to leave some space for airflow above). Heres my plan:

[Kris Bjarnason]

I am in the same situation. If you leave a gap for air, you need to have an entry and an exit for the air right (soffets in, ridge vent out)?

[Chris Damm]

I wouldn't leave any air gap. Why would you want to anyway? I cover all gaps with aluminum foil tape. I have been using the foil faced ridgid foam for decades. It really reflects heat back into the room.

[Mike Cruz]

Jason, yes, that would obviously be ideal...the more the merrier. My joists are 4 feet apart, so I'm going to be putting the boards up between the rafters. I see how your plan would add even more R value, though.

Kris, yes you need to leave an air gap if your roofers didn't lay down a layer of thin insulation/vapor barrier before laying down the roof. I'm not talking about the black tar paper. I have to admit, I'm dealing with a metal roof. So I can't speak 100% for a plywood roof with shingles, but I would imagine the same principles apply. Here is why you want an air gap AND soffit vents and a ridge vent:

When it is cold outside, and you heat up your shop (warm inside) you open the door for condensation on the underside of your roof. (Think of a cold pitcher of water outside in the summer, condensation on the warm side of the glass.) If you don't have a way for that air to vent away, you will get stagnant moisture on all the wood up there. So, you need to keep the air moving to prevent this. That is why you need to have the air gap.

I have had EVERYONE tell me this. From friends to roofers to help at the Borg to my insulation guy. Again, this may be only because I have a metal roof, but I doubt it.

Also, roofs get quite hot. So without the air gap to vent away the heat directly under the roof in the summertime, if you are air conditioning the shop, your insulation will have to absorb all the heat, making it less effective. So, the air gap further allows heat removal and increases the R value.

[Bob Winkler]

I wonder if high-performance insulations (spray foam, for example) get around this venting issue by preventing the heated moist air for getting to the roof deck underside in the first place. I the seal is perfect, the roof deck should be at outside air conditions.

Bob

[Don Jarvie]

I'm hitting mine with the vent molds to create air flow from the soffit to the ridge so I can stuff as much insulatation in the rafters.

I plan on making a few holes in the gable since I don't have a ridge vent and won't be re-roofing for a few years. I don't have a full catherdral ceiling because I needed to add 2x4s as cross beams.

You just need a way for the air to circulate.

[Brett Baldwin]

I wonder if high-performance insulations (spray foam, for example) get around this venting issue by preventing the heated moist air for getting to the roof deck underside in the first place. I the seal is perfect, the roof deck should be at outside air conditions.

Bob

You're right in the case of spray foam. It seals directly to the surface so there is no gap for moist air to form condensation.

[Mike Cruz]

Okay, just got it home! 35 sheets for $650 (he gave me a little break). Turns out, (well, he told me this over the phone, so I knew it going in) that these are factory seconds. Basically, instead of them being nice and coplaner (like they just went through a planer) they are a bit uneven and wavey...some of them at least. Some are a few inches short, yada yada. Anyway, I don't care. I just got 3" (actually most of it is 3 1/4") boards for $18.50 a sheet!

My next question:

Does anyone see a problem cutting this stuff to size on my TS? Obviously, I'll change out my blades and put in a crap one. But any reason NOT to? Cutting 3" material won't be easy otherwise. And I really don't want to do it by hand.

Thanks!

[David G Baker]

I have cut foam board on my table saw. Wear safety gear and protect your breathing. Don't let the foam spend much time touching the blade or it could melt some of it and if you plan on doing any gluing use a glue designed for foam board

[Scott Hildenbrand]

Only reason not to cut it on a TS is if you have a sawstop..

Otherwise it will be fine..

[Philip Johnson]

I have cut foam boards on the t/s no problem. Don't forget there is a 30 percent tax credit for insulation, get a few bucks back from the gov.

Phil

[Eduard Nemirovsky]

I did a spray foam insulation in my shop just recently. And they close tight the ridge vent. Both, them and inspector from county, tell me that I don't need a vent if I am using spray foam.

[Mike Cruz]

I guess I would have needed to have gotten a receipt for that though.. Thanks for the help, never the less.

[Mike Cruz]

I thought about going that route, too, but I didn't want the mess, and I can only imagine that if/when the roof ever needs replacing, that would make it a bear...whether it is for me or the next owner. Yeah, I know, who cares about the next owner, but in many cases with many of my houses I've had, I wish the previous owners had thought about me.