Insulate Roof Deck

[chigger81]

Hello Garage Journal,

I just registered. I have really appreciated this forum as a resource during my shop project and hope to share my learnings with others here as well when finished. I am looking for suggestions about insulation.

In my application, NOFP Solex (1/4" foam sheet backed on both sides with white plastic) has been installed on the roof deck underneath a g-rib metal roof. See attached pictures. 2x8 roof trusses are tied together with 2x4s to the exterior. The Solex product is stapled to the 2x4s.

The roof peak has been left open. The roof cap is vented. Soffits are solid, not vented. I guess some airflow is being achieved between the Solex and the metal, up through the corrugated channels.

I live in zone 4 (Missouri). We get hot humid summers and fairly cold winters.
My building is 30x40. I live in the country where i am subject to absolutely NO building codes or inspections. The walls are 10' tall. The attic space is 10' tall at the peak with 8/12 pitch.

I want to insulate the roof deck to allow for a conditioned attic space.

I have investigated rockwool, fiberglass, open cell foam, and closed cell foam insulation. Technically, probably the best idea I heard so far for the roof deck was from the manufacturer of Solex who said to use R30 rockwool in the rafters spaced out 1.5" from the Solex. However I don't plan to finish the rafters and I read that leaving the rockwool exposed is not recommended for health reasons.

I am leaning toward 3" closed cell spray foam on the roof deck and gable ends. The contractor I have spoken with thinks it should adhere well to the Solex. He thinks he can work at a pace that won't make it melt or sag. He admits that we could have some condensation on the outside of the solex between that material and the roof. He suggests we keep a 3" min of foam to create a full thermal break which would minimize condensation. I am thinking that if we keep the roof peak cavity open with cardboard prior to installation, it will continue to vent and we will get some airflow between the solex and metal up and out the top to help keep that interface dry. We will also block off the soffit cavity with foam board prior to spraying foam which will leave a void in that space.

The walls are made of 2x6's tied together on the exterior by 2x4's. I have Tyvek Home Wrap between the 2x4's and the metal I am leaning toward Kraft backed R21 fiberglass for the walls. I would put it in myself. I think the Home Wrap will allow any moisture to go outward and the paper will be sort of an air barrier on the inside.

I will finish the walls with mildew resistant drywall for the top 7' and a 3' wainscot along the bottom, made from leftover roof metal I have.

I will finalize this conditioning project with a Mr. Cool 36K DIY mini split with 1 head on the center of the back wall which I have already pre-wired the building for. I did a few calculators online and I think it should be about the right size. I also found some people online who have installed this unit on insulated 30x40's with great results.

I know that fiberglass will create some air leakage at the walls vs closed cell spray foaming the whole building. This leakage was actually recommended by more than one local professional who have each told me that the building could be "too tight" if completely spray foamed and it could become damp without constant conditioning. I may program the fan to kick on every once in a while, but I only plan to run the HVAC for temperature control periodically when I am using the building.

I have not found much information about the Solex material so it has me nervous about generating moisture in the wrong place at the roof. Also, since the right answer is a regional one, I have found it hard to local similar examples online. Please give me your thoughts on this idea.

Thank you!
chigger81

 

[billconner]

I don't know what climate zone you are in but assuming mixed or predominate heating:

I'm ok with wall but would choose unfaced batt's and poly air/vapor barrier.

I'm very uncomfortable with spray foam on the solex along with a a concern for no make up air from soffit vents, made tougher by probably not wanting to add to rafter depth. I'd be more comfortable saying remove solex and spray closed cell foam for a hot roof. Without soffit vents, I don't see another option. I think the sole will eventually deteriorate and then what is holding close cell. In either case I don't believe you should leave closed cell exposed for fire reasons.

Not much help.

 

[chigger81]

Bill, I think you were actually a lot of help. I really appreciate your ideas.

I am now pretty close to pulling the trigger on a list of materials. The attached insulation plan will cost $8000-$9000 in materials... It is about the same cost as the bid I have to do 2" of spray foam on the walls and 3" on the roof, but of course now I will be the labor over a few weekends. I think I am more comfortable with the attached idea though, than the spray foam and R-value will be better. I should have about R29 at the roof and 23 at the walls.

I just have 4 questions in my head about the attached to finalize:

1. I could substitute R30 rockwool batts into the roof space but they would fit snug to the 2x4s. I would cut off airflow vertically to the ridge. I thought about installing like a little home-made plastic triangle shape baffle of some kind on the bottom of each 2x4 in the center of each bay to allow air to flow upward even with the thick batts. It would mean I need to compress the batts over those triangles though, and I think that stuff is pretty ridged from what I can tell? Air might not draw that well. I would probably end up with a hump at each of those baffles. R23 with a big gap seems safer.

2. I could substitute 1.5" polyiso (R9.3). I may still do this, but it is going to be a chore installing this stuff on the ladder out in the "wings" of the attic where there is no floor. I have 2 vertical supports in each truss to cut around out there. I think cutting around these and taping will be easier with 1" (R6.0). I found 8 different products online that can be installed as exposed to the interior. The one that seems to be available to me is Johns Manville CI Max which is special order from Menards. This kind of product is a requirement for the plan. I am definitely not putting drywall up there. I am planning to tape the polyiso with some tape that I found where the test included tolerating a flame.

3. The soffits on my building are really leaky. They are just boxed in with sheet metal. See attached where daylight comes in (and a few wasps). I will leave that cavity wide open and connected to the air gap above the rockwool. I asked my builder about installing vents and he didn't see a need. I am curious of your opinion as to what vents to use and how many should be installed. Note that all of the bays are connected together. Maybe installing like 3 vents on each side of the building would be tolerable if recommended... My building is made out of 26 gauge so cutting those in above my head will be fun.

4. I think Rockwool is needed at the roof for water resistance. I don't want wet fiberglass pulling down my foam boards or something if my venting fails. However I could save $1300 if I substitute fiberglass batts in the walls instead of rockwool which might not add much risk. I could also start on the project sooner since rockwool is at 4-6 weeks leadtime right now in my area. However I will not heat and cool this building all of the time, so moisture could end up in the walls. I was thinking the extra money here could save me from a musty smell in the future. Not sure! Have never used rockwool before.

Thank you all for your help!
-chigger81

 

[billconner]

Sorry for drlay.

No real disagreement with this plan.

Beware that the rockwool or fiberglass exposed to "attic" (vented space under roof) will loose some effectiveness. Wind washing it's called. I dont know if it's 5% or 50% - wild guess in the 15-20% range. I probably would shrug my shoulders.

My experience with sheets of foam and attaching to overhead where gravity wants to pull them down, leads me to suggesting battens - the least expensive fitting 1x3s you can find. If trusses are 24" o.c., then screwed through foam to trusses. If 16" o.c., I'd probably go perpendicular to trusses at 24" o c. Or skip it and know if you have a problem, you can do it then. I guess you could try like 4" squares of 1/4" ply also, but the furring strips would be useful if you ever wanted to add drywall or ply or whatever.

So it seems like a good plan.