Finishing a 2x4 ceiling using Foam Board

[Lrayvick]

Can I use R-tech 1/2" thick rigid foam insulation boards as a ceiling? I have read two posts on here about using foam board on ceilings.

Here is my situation. Conversion of a 60 year old 600 sqft barn to a cabin. The part of the roof/ceiling I am concerned with is a section of 10 foot span 2x4 24 inches on center 15 feet wide supporting a roof of 3/8 inch osb with lightweight polycarbonate roof panels on a 1 to 12 slope. This is a 60 year old structure which originally had an old (heavy) corrugated steel roof. There is a little of sag of about 1/2" in some of the 2x4's predating the new roof.

I would prefer not to have to add additional 2x4 rafters. I am thinking I will use rolled insulation then the foam boards. In a 2019 post NUTTSGT said 1/2" foam will sag under its own weight at 24" O.C. Also thinking I will paint the foil. Pretty isn't important.

Answering my own question I think I need to use 1".

 

[PCustoms]

Be aware of the flammability issues with exposed foam panels.

 

[tinmanwpk]

I agree with PCustoms as this was my very first thought.

 

[Lrayvick]

Be aware of the flammability issues with exposed foam panels.

Would that be an issue if the foil side was the exposed side? My big problem is what to put on the ceiling; not sure the 2x4's would handle even light weight drywall. I was even considering luan or stretched canvas as a ceiling cover.

 

[PCustoms]

Would that be an issue if the foil side was the exposed side? My big problem is what to put on the ceiling; not sure the 2x4's would handle even light weight drywall. I was even considering luan or stretched canvas as a ceiling cover.

I've done it for temporary walls, wouldn't recommend it. Not sure if the foil is an appropriate finish, I'm pretty sure there is a warning that it must be covered.

2x4 trusses should be able to accommodate drywall and insulation on a ceiling. Sounds like you have rafters and either ceiling joist or collar ties, you have to post a pic.

If you've got everything opened up adding some additional framing shouldn't be too expensive.

 

[KenC]

After seeing the aftermath of a bubble/foil sandwich ceiling insulation fire, no way would I have any of the foam products exposed. In that case a small cardboard fire in on corner of a big building spread in 2-3 minutes to cover the entire building. Cardboard boxes stored touching a metal electrical j-box with a bad connection inside.

 

[MoonRise]

I've done it for temporary walls, wouldn't recommend it. Not sure if the foil is an appropriate finish, I'm pretty sure there is a warning that it must be covered.

Not just 'covered' but covered by the required non-combustible layer, aka drywall.

Because the foam insulation boards are really-really-really flammable.

Barn converted to cabin means that CURRENT building code requirements for a dwelling unit must be used, not the 60 year old codes used for a barn/shed.

And 2x4 roof rafters seem undersized for a 20 ft span (10 ft on each side of the ridge/center), especially on 24 inch centers.

2x4 members used in a roof truss is a different structural method than rafters. A truss structure is NOT the same thing as rafters.

And 3/8" thick roof sheathing is not current code in many places anymore, because of people falling through the thin roof sheathing (roofers, workers, homeowner, firefighter, Santa and his sleigh and reindeer, etc). Many places the minimum required roof sheathing thickness is now 1/2" nominal plywood or 5/8" nominal OSB. YMMV on this one.

But then you say the span is 15 ft? Which is it? Span is side-to-side from the ridge/peak, the other direction is the length of the structure. Rafters go from the peak/center/ridge down to the wall plate, and are spaced down the length of the structure.

 

[PCustoms]

Maybe I've been too indirect.

Foam insulation panels on a ceiling is a stupid, very dangerous idea. Many people have died in fires where foam caused or directly contributed to the rapid inferno.

 

[WisJim]

Burning foam produces dangerous poisonous smoke and vapors, and hot or burning foam can run, fall and drip, spreading the fire.

 

[Zeke]

Aside from dangers, 1/2" foam spanning 24" won't hold itself long, much less insulation. Lightweight drywall and 5/8ths is standard for 24" O.C.

1/2 drywall will bow over time as well, but not as bad as foam.

 

[PCustoms]

Aside from dangers, 1/2" foam spanning 24" won't hold itself long, much less insulation. Lightweight drywall and 5/8ths is standard for 24" O.C.

1/2 drywall will bow over time as well, but not as bad as foam.

Around here we strap the 24" o.c. rafters and use 1/2" drywall, no sag.

 

[driftpin]

NFPA - Nightclubs and other assembly occupancies

Fires in assembly occupancies have shown to be some of the most deadly when the proper features, systems and construction materials were not present.

www.nfpa.org

The Station Nightclub Fire 2003

A fire occurred on the night of Feb. 20, 2003, in The Station nightclub at 211 Cowesett Avenue, West Warwick, Rhode Island.

www.nist.gov

I was attending a recertification class for my fire inspector's certificate, and one of the case studies was a simulation of the Station nightclub fire in W Warrick R.I. One of the primary issues was the use of pyrotechnic devices by the band. The nightclub was renovated without ensuring that the materials used were compliant with the current fire codes. One of those materials was polyurethane insulation, used for wall and ceiling coverings of the stage area where the band was. The proximate incendiaries used quickly caught the polyurethane insulation on fire, along with the paneling. Neither was rated for use in the occupancy. Because of a narrow labyrinth hallway for the primary front of the occupancy means of egress, stampeding patrons attempting to exit caused the means of egress to become blocked with fallen bodies. The heat, smoke, and fire gases, with the blocked means of egress, resulted in 100 deaths.

 

[PCustoms]

The Station Nightclub Fire 2003

A fire occurred on the night of Feb. 20, 2003, in The Station nightclub at 211 Cowesett Avenue, West Warwick, Rhode Island.

www.nist.gov

That incident is the exact reason I will never have exposed foam panels in a house/shop again. Temporary or not between the smoke toxicity and the fire spread there's too much risk of dying.

 

[jack stand]

So it's ok to use fireworks inside your drywall house?

 

[PCustoms]

So it's ok to use fireworks inside your drywall house?

Only on Friday and Saturday nights due to the noise ordinance.

 

[Lrayvick]

Around here we strap the 24" o.c. rafters and use 1/2" drywall, no sag.

What is "strap" the 24" o.c. rafters? My situation is essentially a 15 foot wide 10 foot span 2x4 shed roof. I may add a few rafters and some gussets and use lightweight drywall. Although I dont see how the foam could light easily from the foil side and there will be no electrical in the ceiling. And this will be an open 150 sqft room with two outside doors. Regardless thanks for the words of caution.

 

[duneslider]

That foil foam board is no where near rigid or strong enough to use as a ceiling layer. I still think 1/2" drywall would be cheaper and better. 1/2" drywall is just fine on ceilings.

 

[billconner]

Polyisocyanurate foam is not flammable, which is the concern.

 

[Zeke]

Strapping is the same as furring strips run perpendicular to the rafters and closer O.C. if you use 1/2".

If you search google and the GJ itself, you will find opinions on both sides regarding 1/2" on 24" O.C. Humidity plays a big part.

 

[PCustoms]

What is "strap" the 24" o.c. rafters? My situation is essentially a 15 foot wide 10 foot span 2x4 shed roof.

So your room is 10x15, and the rafters go across the 10' direction?

First, if they are horizontal, they are not rafters. Post a pic.

Second, 10' span for a 2x4 ceiling joist isn't terrible.

Here is strapping:



1x3 lightweight strips attached perpendicular to your joists. You can shim them to level the ceiling and adjust spacing to limit sag in drywall.

 

[billconner]

My situation is essentially a 15 foot wide 10 foot span 2x4 shed roof.

Are the 2x4s spanning 10' or 15'? it sounded like 15' but now I'm unsure.

Strapping, aka furring, means putting 1x3 e.g. across the rafters at 12 or 16" centers. Practically no weight and shortens span of ceiling material.

Metal liner panel with fibreglass or rockwool on it would be pretty simple. Or fur it and use polyiso, which comes with black foil most often in my experience.

 

[billconner]

The Station was polyurethane btw, which is much more flammable than XPS or EPS, not that either of those should be left exposed.

 

[juddspaintballs]

Around here we strap the 24" o.c. rafters and use 1/2" drywall, no sag.

Around here, we just set floor joists or trusses at 24" OC and slap 1/2" drywall up to them without an issue or sag. No strapping or anything.

 

[billconner]

Around here, we just set floor joists or trusses at 24" OC and slap 1/2" drywall up to them without an issue or sag. No strapping or anything.

Furring or strapping does seem to be a New England regular practice. That explains PCustoms. Guessing justpaintballs is not from New England.

 

[The Cobbler]

Around here, we just set floor joists or trusses at 24" OC and slap 1/2" drywall up to them without an issue or sag. No strapping or anything.

floor joists @ 24" OC ? wow

 

[Voi]

I've never seen it much less worked with it, but Rockwool makes a mineral wool insulation board that has a high fire rating.

No idea on what it weighs.

My main concern with it would be sealing it with something to encapsulate the fibers so they do not become airborne. Not sure if it could be skim coated with a lightweight synthetic stucco.

 

[Voi]

Quick search suggests that stucco over Rockwool Comfort Board is possible.

But there is also another mineral wool board product with various veneers, including stucco, I think.

Link didn't copy. Look up Sto Therm CI.

 

[NUTTSGT]

Polyisocyanurate foam is not flammable, which is the concern.

I wish I knew where the article was I read about the foil faced polyiso. I was quite impressed with the resistance it has.

Yes, 1/2" will sag under its own weight at 24".

I would use no less than 3/4" thick, nor would I paint it. Allow the foil to do it's job and reflect the heat back.

 

[juddspaintballs]

floor joists @ 24" OC ? wow

Yep. This is my house I'm currently building. Roughly 18' span on 14" TJI's, too.

 

[PCustoms]

Yep. This is my house I'm currently building. Roughly 18' span on 14" TJI's, too.

1 1/8" decking?

 

[rockettauto]

R-tech will sag and is flammable. Polyiso could be doable, if its only holding itself and not a pile of more insulation on top.

 

[juddspaintballs]

1 1/8" decking?

3/4”