Yet Another "How Do I Insulate This Roof?" Post

[Willb]

Hello All - I finally got my shop built (I first posted about it back in early '21 when I first joined). It's 30x42 pole building with a 2nd story mezzanine/man cave that I will be finishing once I get some other things sorted. It has wood T1-11 siding over 2x8 girts and a composition roof. There is no venting installed currently.

The biggest issue I'm having now, and I've been doing a lot of searching and reading about this, is how the heck do I insulate a roof with purlins and no rafters? (see pictures) I've found all kinds of info on how to properly insulate and ventilate a roof with vertical rafters, but these horizontal purlins have me scratching my head and not much discussion about dealing with this in my type of building vs. metal buildings and roofs. I've already had one "rain" event inside the shop after a fairly warm day and a big cool down at night which caused lots of condensation on the roof sheathing and framing.

I'm in central Western Washington (Snohomish County) and we get cool, wet winters (an occasional snap of below freezing weather, but infrequent) and warm, fairly dry summers though it can be a little humid at times.

I got a screaming deal on a couple of hefty 240v wall-mount space heaters so will be using those to heat the work space. No plans for cooling the work space, just a large ceiling fan. For the man-cave I'll use a window-mount AC unit for cooling and a forced air heater.

For the work space, I have no plans to really finish the roof or walls so exposed insulation or vapor barrier or whatever is fine, but in the man-cave I want to eventually get drywall up there and have a nice, finished space.
I'm insulating the walls for now by installing 2" thick EPS with a foil laminate on one side (foil facing interior of building) between the girts.

What should I do? I really don't want to do spray foam if it can be avoided.

 

[Hank11]

I know you already said you did not want foam, but I would 100% go open cell foam the depth of the framing. Then any kind of ceiling that is easy and flame resistant attached to framing or strapping if needed to fit dimensions of your ceiling material.

 

[JohnX14]

You don't want spray foam, but it's your best choice right now. I've never seen a roof framed like that.

 

[BobnCO]

I like you have never been willing to pay for large areas of spray foam (but have done smaller areas with the two tank kit myself). But realistically you have limited options due to depth of wood “purlin/joists”. Only other due it yourself might be nail 1” x 4”s to the bottoms perpendicular and instal foam sheets as thick as you can afford/fasten. (Would allow some air movement between spaces.) You also have a challenge on the gable wall section between roofs that could also be done with foam sheets. Nice new shop!!

 

[Uncle murph]

Hello All - I finally got my shop built (I first posted about it back in early '21 when I first joined). It's 30x42 pole building with a 2nd story mezzanine/man cave that I will be finishing once I get some other things sorted. It has wood T1-11 siding over 2x8 girts and a composition roof. There is no venting installed currently.

The biggest issue I'm having now, and I've been doing a lot of searching and reading about this, is how the heck do I insulate a roof with purlins and no rafters? (see pictures) I've found all kinds of info on how to properly insulate and ventilate a roof with vertical rafters, but these horizontal purlins have me scratching my head and not much discussion about dealing with this in my type of building vs. metal buildings and roofs. I've already had one "rain" event inside the shop after a fairly warm day and a big cool down at night which caused lots of condensation on the roof sheathing and framing.

I'm in central Western Washington (Snohomish County) and we get cool, wet winters (an occasional snap of below freezing weather, but infrequent) and warm, fairly dry summers though it can be a little humid at times.

I got a screaming deal on a couple of hefty 240v wall-mount space heaters so will be using those to heat the work space. No plans for cooling the work space, just a large ceiling fan. For the man-cave I'll use a window-mount AC unit for cooling and a forced air heater.

For the work space, I have no plans to really finish the roof or walls so exposed insulation or vapor barrier or whatever is fine, but in the man-cave I want to eventually get drywall up there and have a nice, finished space.
I'm insulating the walls for now by installing 2" thick EPS with a foil laminate on one side (foil facing interior of building) between the girts.

What should I do? I really don't want to do spray foam if it can be avoided.

Neat design.

 

[JuncleJohn]

I have a similar dilemma. The only difference is I put 1/4” foil backed foam in between the steel roof and the OSB sheeting. However, I do have a 1-1/2” spacing under the roof sheeting all the way up on both gables. Therefore it vents through the overhang. But unfortunately, also ushers in the cold air.

I had thought about installing 5-1/2 inch fiberglass as that would leave a 1-1/2” gap between the sheeting and insulation. Just not sure if that would sufficient as it’s 42’ gable to gable.

I don’t understand why you would have an inside rain event as mine has been bone dry for 20+ years.

John

 

[Dig Doug]

Here in Ca. We build 100,000 + sf where houses ( concrete tilt ups ) w/ a flat roof structure like that

we insulate w/ a batt insulation and then add a SCRIM radiant barrier below
it gives a nice clean look
but
after time the scrim needs to be touched up and can come loose wind pops loose the scrim from the staples

 

[carlaisle]

What's the center to center on those?

 

[JuncleJohn]

Radiant barrier wouldn’t be enough in the Midwest. Temps range from 100 to -15 depending on the season.

John

 

[Willb]

I know you already said you did not want foam, but I would 100% go open cell foam the depth of the framing. Then any kind of ceiling that is easy and flame resistant attached to framing or strapping if needed to fit dimensions of your ceiling material.

That would be a lot of foam! Those are 2x12 purlins.

 

[Willb]

You don't want spray foam, but it's your best choice right now. I've never seen a roof framed like that.

Neither have I. I really wish I'd known more about insulating and whatnot when I started this project, I'd have pushed back hard on this design for the roof. I also would have pushed for a couple other design changes, but too late now.

 

[Willb]

I like you have never been willing to pay for large areas of spray foam (but have done smaller areas with the two tank kit myself). But realistically you have limited options due to depth of wood “purlin/joists”. Only other due it yourself might be nail 1” x 4”s to the bottoms perpendicular and instal foam sheets as thick as you can afford/fasten. (Would allow some air movement between spaces.) You also have a challenge on the gable wall section between roofs that could also be done with foam sheets. Nice new shop!!

Yeah, that's a possibility. Would have to install a ridge vent and soffit vents but that's certainly doable. We were talking about doing that in the man-cave section then covering with drywall, but don't see why I couldn't do the entire thing like that(just without the drywall).
The vertical walls, including the gable sections are getting the EPS foam board treatment. I need to go get some better pictures, but this shows what I'm doing in one small area.

 

[Willb]

Neat design.

Thank you! I just went through my pictures and realized I need to take a few more, better ones and I'll post those once I do.

 

[Willb]

I have a similar dilemma. The only difference is I put 1/4” foil backed foam in between the steel roof and the OSB sheeting. However, I do have a 1-1/2” spacing under the roof sheeting all the way up on both gables. Therefore it vents through the overhang. But unfortunately, also ushers in the cold air.

I had thought about installing 5-1/2 inch fiberglass as that would leave a 1-1/2” gap between the sheeting and insulation. Just not sure if that would sufficient as it’s 42’ gable to gable.

I don’t understand why you would have an inside rain event as mine has been bone dry for 20+ years.

John

Well, I would guess that ventilation you have plus the layer of insulation on top of the sheathing is really helping! My building is sealed up and no insulation in the roof at all. We have some big temperature swings at this time of year and the day that happened was warm and rainy then warmer and sunny in the afternoon then it dropped 25+ degrees at night down to around 50. All that warm, moist air inside the building caused massive condensation on the sheathing once the roof cooled down since there's no insulation between the shingles and sheathing. It also probably doesn't help the the concrete is still only about a month old and still sweating a little.

 

[Willb]

Here in Ca. We build 100,000 + sf where houses ( concrete tilt ups ) w/ a flat roof structure like that

we insulate w/ a batt insulation and then add a SCRIM radiant barrier below
it gives a nice clean look
but
after time the scrim needs to be touched up and can come loose wind pops loose the scrim from the staples

Thanks for the video! I gave it a once-through but will watch it again to see if I missed anything. I think they mention ventilation between the foil and the roof though, and I don't have any...???

 

[Willb]

What's the center to center on those?

Not sure, I'll have to go look at the prints but I think it's either 18" or 24".

 

[Willb]

Well, after the replies here and thinking about it, I'm wondering if this wouldn't work?

Questions:
Would ventilation be adequate to stave off mold/rot?
Would this help keep the roof from cooking in the sun?

Install 2x2 furring parallel to the purlins on 48" centers.
Add a ridge vent to each roof section.
Add soffit vents.
Install 4'x8'x3" sheets of foil faced EPS to the underside of the furring strips. It's the same stuff I'm using on the walls, just thicker.

This has the advantage of being relatively cheap (compared to spray foam, at least), easy to install and would make wiring the lights and fans a breeze as I could just staple the wires to the underside of the purlins.

 

[Hank11]

Well, after the replies here and thinking about it, I'm wondering if this wouldn't work?

Questions:
Would ventilation be adequate to stave off mold/rot?
Would this help keep the roof from cooking in the sun?

Install 2x2 furring parallel to the purlins on 48" centers.
Add a ridge vent to each roof section.
Add soffit vents.
Install 4'x8'x3" sheets of foil faced EPS to the underside of the furring strips. It's the same stuff I'm using on the walls, just thicker.

This has the advantage of being relatively cheap (compared to spray foam, at least), easy to install and would make wiring the lights and fans a breeze as I could just staple the wires to the underside of the purlins.

That will work but you won’t have much insulation value. Also, you have to either vent it or seal it up. Poorly vented is where the big problems come in.

Your idea will leave you with a hot man cave. I don‘t think you’ll like being up there in the summer. That area will need more insulation to be comfortable.

Open cell foam is cheaper than closed cell and its what you need for your job, imo. I’d price it to see.

 

[Dig Doug]

Thanks for the video! I gave it a once-through but will watch it again to see if I missed anything. I think they mention ventilation between the foil and the roof though, and I don't have any...???

We don’t vent the joist bay we would add a whirly bird or a vented skylight every so many sq feet.

 

[Willb]

That will work but you won’t have much insulation value. Also, you have to either vent it or seal it up. Poorly vented is where the big problems come in.

Your idea will leave you with a hot man cave. I don‘t think you’ll like being up there in the summer. That area will need more insulation to be comfortable.

Open cell foam is cheaper than closed cell and its what you need for your job, imo. I’d price it to see.

EDIT: I see your first response up above which explains your plan.

Three questions though

- if I fill the bays between the purlins with open cell foam and then install some kind of impermeable ceiling material to the bottom of the purlins, won't that create a moisture trap?

- If there is no ventilation between the ceiling and the roof sheathing, won't this cause the roof to really heat up and "cook" the composition shingles?

- Is fiberglass batt a reasonable substitute for open cell foam? Those are either 2x10 or 2x12 purlins and at that depth fiberglass batt would have an R-30 value which should be sufficient.

 

[JuncleJohn]

My purlins are 2x8’s which is why I was considering 5-1/2” fiberglass. I also thought about putting 1/2 or 3/4” foam over the fiberglass.

I am concerned that only about 1-1/2” of air space may not provide enough ventilation to prevent moisture buildup.

John

 

[Hank11]

https://huntsmanbuildingsolutions.com/en-US/sites/en_us/files/2023-06/23.00089%20Generic%20OpenCell%20Roofs%20Brochure_EN__Web_v1.pdf

Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck | Building America Solution Center

Guide describing a method for insulating an unvented attic by spraying foam insulation along the underside of the roof deck.

basc.pnnl.gov

The biggest question I see for you is whether or not your specific climate indicates a hot roof with open cell foam.

After that, I think your plans for maintaining heating and cooling and humidity in the space is the second question. You describe a fairly moderate climate in your location so it should work great. But it might not work great if you have a lot of humidity inside the building envelope. So if you’re gonna put a sauna in there or a car wash, you’ll probably have to think of something different. You might check around with some local installers and see what is the standard installation where you live.

 

[Willb]

https://huntsmanbuildingsolutions.com/en-US/sites/en_us/files/2023-06/23.00089%20Generic%20OpenCell%20Roofs%20Brochure_EN__Web_v1.pdf

Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck | Building America Solution Center

Guide describing a method for insulating an unvented attic by spraying foam insulation along the underside of the roof deck.

basc.pnnl.gov

The biggest question I see for you is whether or not your specific climate indicates a hot roof with open cell foam.

After that, I think your plans for maintaining heating and cooling and humidity in the space is the second question. You describe a fairly moderate climate in your location so it should work great. But it might not work great if you have a lot of humidity inside the building envelope. So if you’re gonna put a sauna in there or a car wash, you’ll probably have to think of something different. You might check around with some local installers and see what is the standard installation where you live.

Great links, thanks much!
No sauna or car wash but the bathroom will have an oversized shower in it so I can give my dogs baths in there, so there will be a "dog wash." LOL. Of course, the bathroom will be it's own sealed and vented room within the shop so shouldn't contribute any significant moisture to the rest of the building and we don't bathe the dogs that often anyway.
Thanks for your input!

 

[rsanter]

Best is spray foam.
if you really,don’t want that then rigid foam cut to fit then I would use sheet metal siding for the ceiling