What's new
What's new

Shop insulation best practice for the 637th time...

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
2018 I got quotes around $12,000 for 3 or 4" (don't recall exactly which it was) of spray foam in 5500 sq ft of shop ceiling. Spent the last few weeks getting the same thing quoted for $27,000 for 3" or $33,000 for 4". The spray foam guys want to do 2" for $19k, but I don't think R14 is going to cut it. I plan to heat this area to about 60-65 degrees all winter with a couple heat pumps. I can't swallow that price so I'm pricing out DIYing it and coming in around $8500 for alternate materials.

My shop walls are R21 metal insulated panels. They are air tight, no thermal bridging. The building base structure is structural steel, but I built the roof deck with 2x6 joists spanning 10' on 24" centers topped with 3/4" plywood.

My big question is, would it be better to use R19 batt insulation between the joists with 2" polyiso foam with edges sealed attached to the bottom of the 2x6 joists, or to just use 4"+ polyiso foam attached to the joist bottoms and leave the area between the joists as a dead air void?

I'm going to attach steel siding with an air gap to cover the rigid foam to finish the ceiling.

Thoughts?
 
I like the idea of moving air between the roof deck and the insulation. Helps keep your roof cooler in the summer, and with proper airflow will vent condensation (if there is any) in the winter.

Need screened air intakes under the eaves, with a screened center ridge vent running the distance.
The blue/pink/green extruded foam is great insulation---stops airflow dead and moisture won't hurt it. Light and easy to install...expensive though. And the thicker sizes (2"+) have to be special ordered here in the semi-country.

Codes may require fire-resistant drywall covering, not sure if metal itself is adequate indoors?

Keep us posted, and share some pics!

ToolCat
 
One thing to consider is moisture accumulation due to condensation. Code where I am from memory says something along the lines of if using something like fiberglass you need to vent between the insulation and roof deck or if you use foam you need at least a certain R value (enough to prevent condensation for your climate) of sealed foam before you add any additional insulation if you were going to combine different types of insulation. I'm sure you can find online but if you can't let me know I can take a picture from my International Residence Code book.

My vote would be foam insulation board that has a reflective foil to act as a radiant barrier faced upwards with an air gap between it and the roof deck. Then either more foam or fiberglass, whichever is more economical. You probably already know but reclaimed foam insulation board is a thing.

I built my house with 16" deep wood TJIs and used 1/2"EPS foil faced foam to create an air-gap for the R-49 fiberglass insulation below and roof deck above and to act as a radiant barrier. While doing the installation I had just the foam on one side of the attic and nothing on the other side of the attic. There was a very notable temperature difference between the attics

I ordered fiberglass insulation for my shop a few years ago and and it went up like 6% in just a few months because I drug my feet ordering it because I didn't want it sitting around because the building was delayed. I ordered insulation for my house during the 2020 debacle from a big box store and like 3 weeks into a 6 week lead time they told me the supplier canceled the order and there was nothing they could do, meanwhile prices skyrocketed what was an already inflated price. I got lucky and my wife found someone that did insulation on the side and had enough to get it done in short notice at a fair price.
 
I think you guys have steered me away from disaster. Been reading lots about the difference between "hot deck" and "cold deck" roofs. The damp Oregon climate here would be a really bad combination with a hot deck roof.

I think my best approach is going to be forgetting fiberglass completely. applying 2x4's flat against the bottoms of the roof joists as furring strips to allow airflow from eves to peak, attaching 4"+ foam to the furring strips, sealing the foam edges 100% and attaching the steel siding below the foam.

My eves aren't vented, but I can make that happen.

I sure wish I wasn't dumb and broke when I built this building. I could have done things so much better, but it's what I got.
 
We rent with the 1x4's on the bottom of the rafters then 2 layers of polyiso foam, factory 2nds are quite economical by the truckload. We sorted them by thickness and taper then put then up so we had a uniform stacked thickness, half the sheets I bought were with OSB bonded, our heating costs are minimal and an air conditioner seems unnecessary as we were only over 75 a couple of days last summer. This pic of the ceiling is of a smaller building, but the materials and process were the same
IMG_6551.jpg
 
We rent with the 1x4's on the bottom of the rafters then 2 layers of polyiso foam, factory 2nds are quite economical by the truckload. We sorted them by thickness and taper then put then up so we had a uniform stacked thickness, half the sheets I bought were with OSB bonded, our heating costs are minimal and an air conditioner seems unnecessary as we were only over 75 a couple of days last summer. This pic of the ceiling is of a smaller building, but the materials and process were the same
View attachment 380973

I think I'm going to end up copying your work to a tee.

8'x4'x8' polyiso bunks are $800 each. I think it's the way to go.

What did you use to seal the edges of the foam to one another?
 
IF you have an air void, it needs to be directly under the roof deck.
What is the roofing material?
So , I have been a fan of rigid insulation with an air space under the roofing since I lived in a house with rigid foam and no airspace which would get hot at the end of the day as the sun load soaked through the roof.
I have begun to think that 'more insulation' is really the answer.
We should be constructing buildings for what energy prices are bound to be rather than what they are now
Buying the oil once[foam] rather than every year is always wiser.
If the foam is sealed, the airspace is superfluous. If you are using plywood rather than OSB the risks of moisture are small
One thing I did in my current house is I had leftover foam from the roof, so I bought the largest DIY spray kits available and cut undersize chunks of foam and stuck them in the wall cavities and finished it off with spray foam. THis works pretty well with the used foam available at a discount at various places.


bigger kits now available


They also make E84 [usually covering not required]


Find 'fan' nozzles as the cone nozzles are not that useful.
I have an R30 something roof, wish I had an R50 something roof with energy prices today.

It all comes down to what you can afford.
Pay for the oil now or pay for it later
 
The other thing I noticed in my last house is it was half cathedral 4 inch foam 1-1/2 decking, half truss roof, R39 then R50+ fiberglass
The fiberglass seemed to work better in the summer than the foam, the foam seemed to work better in the winter.
I have assumed it had to to with the greater Delta T in the winter causing more air movement in the fiberglass, but that is just an assumption
 
Fire break?
I had an exposed foam board ceiling. Head of the local fire dept had a shit fit.
Told me that there would be no way they would come inside if a fire as it would be raining on their heads.
That was many decades ago so do not know now.
Bob
 
I think I'm going to end up copying your work to a tee.

8'x4'x8' polyiso bunks are $800 each. I think it's the way to go.

What did you use to seal the edges of the foam to one another?
I didn't seal the sheet edges to each other, I just shoved them tight.

On the first layer, I shoved the sheets tight to one side and one end of the building then screwed them in place with a few deck screws, I used cans of spray foam for the last gaps at the edges by the walls. I put up the second layer at 90 degrees to the first with construction lags, then spray foamed the gaps on the edges by the walls. I then had 3" of spray foam sprayed in on top of the polyIso to seal it all up good and tight..
 








 
Back
Top