Opinions sought on interesting roof insulation technique

[n74tg]

I came across this roof insulation pic here on the forum.  It's from Christina White's 20x36 1-1/2 story project. 



My garage roof is built this way (metal roof on 2x4 purlins) and the roof on the house I'm building COULD be built this way. 

I can see some advantages to doing a roof this way, ie no decking, and no roof felt.  But, what are the disadvantages.  Apart from if you ever had to remove a metal roof panel, besides being harder to remove, you would also have to reinsulate after.  What do you other builders see as problems/disadvantages.

Christina, I hope you didn't mind my borrowing your photo.


Edit: I'm reading the rest of Christina's project posts and there is some discussion of this method, but I'm still interested in other opinions too.

[glenn kangiser]

Okie BoB used it and he is totally happy.  Steel should be good for 100 years or more if cared for - I don't see any disadvantages and even changes shouldn't pose any excessive problems.

The only downside I see is it is expensive.

[n74tg]

Okay, how about this.  Put roofing felt (probably 30#)between purlins and metal roofing.  Then, if for some reason you had to remove a metal sheet, you could (maybe) without totally destroying the insulation.

Okie Bob, I'm particularly interested in your opinion.

[glenn kangiser]

That was why Christina couldn't use regular insulation, no felt.  The roofers she used said it wasn't required which could be true on a barn but - it is necessary on a house due to condensation - raining inside without the felt.  The foamed in insuation eliminates all chance of moisture from getting to the bottom of the steel and condensing to form water.

[Okie_Bob]

I sure don't claim any expertise in this area, only my own personal experience. So blame Glenn if you don't like my answers!

I built a garage with an apartment in one end so I'd have a place to stay while I built my house. I learned about Icynene insulation before
I started the garage but, made some mistakes regardless. I built the garage just like Christina Whites pics except, I used factory built trusses and 1 X 4 purlins, no tar paper, metal roof and then then Icynene. Again, at the time, I didn't know about enclosing your entire
attic with Icynene, no vents at all and wish I had done that. I did put a large vent fan in one gable, the one over the apartment. I don't feel I get the full benefit due to this. Other than that, I love the stuff and as Glenn said, the high cost is the only drawback.
I did learn on the garage believe I did it right in the house. Again, I used factory built trusses but, this time I decked it with 3/4" T&G plywood, 30# felt and then the 26ga metal. After I finished all my electrical and I had the Icynene blown in and completely sealed
the entire structure. I built on pier and beam and even had Icynene sprayed under the floor. I am totally convinced this is the best way to built. I went with 2 X 4 stud walls because I feel that even with less R value, the sealing quality of the Icynene makes up for it. I Also used Hardi Board siding on top of plywood sheathing.
I do believe that with all the other energy conservation efforts (18 SEER A/C, Low E insulated windows, etc) it will pay for itself in a few years. Here in E Texas it gets plenty hot and humid and my highest electric bill so far has been just over $100 for a house that is about 1500sf with a loft and that includes electric hot water, clothes dryer AND the very inefficient windown unit in the garage apartment. I had a small (700sf cabin, no garage, window a/c etc) prior to starting the garage and my electric bills would top $150 10 YEARS AGO!! That same cabin today would be unaffordable with todays electric rates.
HOpe that answered your questions, I should get paid by Icynene for all the free pub I give them! But, I do love the stuff and almost forgot, another reason is the sound abatment. I can sit out on my deck, covered extension of the main roof and the sound of acorns hitting the metal roof drive me nuts! I can't even hear them inside the house with the windows and doors closed.
That is probalby a lot more than you wanted to know from me but, if I did leave anything out, let me know, I'll gladly respond.
Okie Bob

[glenn kangiser]

I know nothing, Nothing......., BoB......

[n74tg]

Bob:
On the house, what made you choose to put plywood down under the metal roof?

It's a small world.  I grew up in Mexia.

thanks

[CWhite]

My 2 cents after what I went through. 
I live in the humid south, and didn't want tar paperbetween the tin and the purlins.  That moisture, however small could have a long range effect as far as shortening the life of my tin roof from the inside by causing rust where the contact points between the paper and tin.  The moisture would have built up between the two surfaces, and yes, exited down the slope, but why let it if you can avoid it?

The insulation in the picture has helped keep consistent temperatures inside, and there is a distinct absence of humidity upstairs as well as down.  I have only used my airconditioner split system about 4 times.  It is very efficient and only raised the power bill a bit.   Otherwise, the cross ventilation of end gable windows positioned in line with breezes, together with a ceiling fan have been all I need most of the time. 

Hind sight with the way my house was insulated is 100% positive.  I still don't understand all of the worry about a future catastrophe should the roof need major repairs from a tree falling through it or something similar.  The insulation is not rock hard.  It is squishy to the touch.  It can be cut with a bread knife after intallation or even now.  I would assume that there would be interior repairs needed as well, and the entire structure could be redone the way it was done originally.  The sheetrock would again, be the messiest part. 

The house is 1800 square feet including my attic area over the bedroom addition, and the power bill has yet to be over $75.00 for one month.  Electric hot water heater, refrig.  no dryer (clothes line)

I agree very much with Okie Bob.  The sound snuffer part is wonderful. 
Christina

[mrw]

I haven't used this system "yet" but I have seen several buildings that did. In my opinion it would solve several problems one is I don't see how you would ever get a leak around a screw hole since the insulation is basically sealing the underside PLUS you have now literally glued the steel roof to the rafters so I would think it would virtually impossible for a sheet of roof metal to come loose and blow off unless the entire roof rafter system went with it and in that case you would probably be rebuilding everything anyway. One building that I saw that used this just painted the foam after installation which didn't look too bad although this was in a commercial building not a residence. Anyway I like the idea

[glenn kangiser]

to the forum mrw.  I think it is a good way to go in many cases.

[kenhill]

I stapled the styrofoam vent sheets that you can buy at Lowes to the purlins.  That way if I do get condensation, it won't go into the fiberglass.  But now that I think about it, that means the condensation can run down to the sill!!!

[Okie_Bob]

Bob:
On the house, what made you choose to put plywood down under the metal roof?

It's a small world.  I grew up in Mexia.

thanks

n74tg, sorry it has taken awhile for me to respond. Being from Mexia you know what we face here in the summer in the way of humidity and temp too!! Believe you are in a better place up in Hot Springs.
I don't really have a good scientific reason for going with the plywood decking on the house. I felt it was just more substantial and gave a little added strength to the whole structure. Not sure I would do it again but, I'm glad I did...does that make any sense?
Did you leave anything in Mexia you want me to pick up for you? (I'm also wondering how many people on here have a clue how you pronounce Mexia!!!) It's MA HAY-A....
Okie Bob

[MountainDon]

(I'm also wondering how many people on here have a clue how you pronounce Mexia!!!) It's MA HAY-A....
Okie Bob

Precious few I bet. I learned many years ago after moving to NM. I worked a couple of Christmas seasons at the local JCPenney catalog phone order center. Most of the calls were directed to us from TX. I learned a lot about pronunciations and about how polite most of those people were.

[n74tg]

Thanks Okie_Bob for the offer.  I do have an ex wife there in Mexia, but PLEASE leave her there.  As George Strait said it "all my exes live in Texas", and that's just fine with me.