Rigid insulation

[njbadboy]

Hey everyone. I have a 30x32 barn that i would like to start insulating after 6+ years of building it! I have already framed walls with studs 16" OC and was planning on hanging metal linear panel on ceiling. I purchased all the insulation (r19 for walls and r30 for ceiling); however seeing a friends barn this past weekend I like what he did with his ceiling. He had used he rigid board insulation in between the truss and finished off with a white vinyl soffit.

I have priced new insulation out at $1,500, using a 1 1/2" board in between purlins and another 1 1/2" board on top, to give a 3" thickness, and I guess around a r19, which is a little steep. I found poly insulation on craigslist for about $650.

My question, IF I were to just use regular batt insulation instead, can it be placed directyl against the double bubble barrier under the metal roof or would there be a condensation issue?

The garage will not be heated until next winter, when I can trench for gas line, and will have in floor heat.

Also, is there adequate air gap between the double bubble barrier and metal for air flow in general?

Thanks in advance! Great site!

Jeff

 

[jvitez]

Lots of good questions. IMHO, you need to get more information and then you're plan forward will be much clearer.


1. What is the recommended ceiling R value for your location?

If you're in climate zone 4 you need R38-60, if zone 5 then R49-60. R30 is too low. I'd do at least R40.

2. Why are you planning on double bubble? It's not a recommended product in any code or building science program. I wouldn't use it in any structure.

3. The real question is whether you need a vapour retarder at all, and if so what spec. In your climate zone the recommendation is for a Class I or II vapour retarder, which is kraft-paper backed insulation or a layer of polyethylene.

4. Is your attic space ventilated? In photo 2 I see light coming from the "bird blocking" part of the truss edge, so I assume it's ventilated. If so, then you need to install soffit vent chutes and ensure the roof has mushroom or ridge vents. You need a 1:300 ratio, or 1 square foot of free ventilation for every 300 sq ft of attic. So for your 960 sq ft shop you need 3.2 sq ft of ventilation, 50% in the soffits and 50% in the roof.

5. If your attic is not insulated, or you don't want it to be, then closed cell polyurethane foam sprayed on the underside of the roof is the correct insulation method. You should then leave the attic space open.

6. If you have a ventilated attic and you install a metal ceiling, you can lay kraft faced fibreglass insulation directly against the metal. Or if you install a polyethylene air/vapour barrier with the metal under that, you can lay any type of insulation you want directly onto the poly. The cheapest is actually blown in cellulose or fibreglass.

7. You can certainly place rigid insulation between the purlins, then fibreglass batts or blown in material over the foam boards, buy why bother? Work and expense with really no gain. It it's extruded polystyrene it can act as a vapour retarder if 2" or thicker, but you'd need to tape each truss to air seal it with proper sheathing tape. Again, lots of work for little gain.

Before anyone else questions it, why have you bundled a whack of 12 gauge wire together? You need to de-rate them due to bundling. Or it that not NM wire?

 

[Hubmonkey]

He already has double bubble installed, not planning on it. See pics posted.

I have double bubble and awaiting the answer as I have the same question.. my walls I framed out and used r13 roll faced insulation.. need to do the roof and wondering the same if I can use vinyl roll insulation against the double bubble?

Hub

 

[jvitez]

I see what you mean, my mistake. I thought he meant double bubble attached to bottom of the trusses with metal panels under it as a ceiling. If I'm reading his questions correctly now, he wants to use metal liner as a ceiling but wants to insulate directly against the double bubble which under the metal roof.

OP: is your attic space ventilated?

 

[lakeroadster]

.... planning on hanging metal linear panel on ceiling. ....

My question, IF I were to just use regular batt insulation instead, can it be placed directyl against the double bubble barrier under the metal roof or would there be a condensation issue?..

Also, is there adequate air gap between the double bubble barrier and metal for air flow in general?

Thanks in advance! Great site!

Jeff

I'm confused

If you are hanging steel on the bottom chord, why not blow in cellulose on top of it? That would be the most cost effective.

If you don't like that, place the batts between the lower chords of the trusses... not up on the upper chords... or are you using this as a storage area?

Does the building have soffit vents or a ridge vent?

 

[shaggyant]

This thread is relevant to me. I have a 36x48x14 shop and it was built with trusses not suitable for bottom loading. This means I cannot install a conventional ceiling with blow in insulation. If I want to insulate I’ll have to do it hot roof style. The builders installed thin fiberglass insulation covered in white vinyl between the metal roofing and the purlins. I want to go over this with R30 stuffed in the 24” gap between purlins. What should I cover this with? Seems like doubling the vapor barrier and putting on both sides of the R30 might not be the best idea?

 

[Dragfluid]

OP, I see that you're in Jersey. If you want a warm shed, just look at my build thread and copy that.

If you're thinking about leaving the trusses open "to use them for storage", I'll give my standard answer to that, and that is, the **** you put up there today, will probably still be there 10 years from now without ever looking at it.

Shaggy, are you sure about the non loading of your trusses?

 

[shaggyant]

Shaggy, are you sure about the non loading of your trusses?

Absolutley, positively sure. I have the stamped engineering drawings of the trusses installed in the shop. They list a bottom chord dead load of only 1.00 pounds per square foot. I got this from the company who built the shop and I contacted the truss company to be sure. The upper chord is 5.00 pounds per square foot though so I can put insulation there.

FYI. There are only four trusses in the whole 36x48 building.

 

[lakeroadster]

........ The upper chord is 5.00 pounds per square foot though so I can put insulation there.

FYI. There are only four trusses in the whole 36x48 building.

So... where are you located?

 

[shaggyant]

So... where are you located?

Northern Idaho. Zone 6

Huh, I better put my location in my profile. Hold on....

Edit: ok updated

 

[njbadboy]

I am located in Southern New Jersey (Zone 7a). They call for R38, which I won't achieve with what I pan on doing.

I know the bottom cord can handle the metal liner panel, but I was thinking about changing my plans and installing insulation on the roof instead.

When the barn was built they put the double bubble for condensation purposes only.

The air flow from the eves flows through the roof and double bubble is enough, if I decide to insulate on the underside of the double bubble? If i go this route, is there an issue of using fiberglass batt on the underside of the double bubble?

The fist photo is what I would like to do. The second and third photos is of my current barn.

 

[Angelfire]

Besides the issue with regards to having to de-rate due to bundling, it looks like he may have it spanning without supports, beyond 4.5', which IIRC, is the maximum allowable span for NM-B.

One option to break up that bundle would be to use these:

https://www.lowes.com/pd/Gardner-Bender-20-Count-1-1-4-in-Plastic-Stacking-Cable-Staples/3410964

 

[Hubmonkey]

When the barn was built they put the double bubble for condensation purposes only.

The air flow from the eves flows through the roof and double bubble is enough, if I decide to insulate on the underside of the double bubble? If i go this route, is there an issue of using fiberglass batt on the underside of the double bubble?

Same Situation here except my whole Barn is wrapped in double bubble.

Hub