Have any of you ever installed 4 or 5 inch polyiso on top of your roof deck to create an unvented roof?
I can get 4″ polyiso with 1/2 cdx nail base for about $200 a square. I was contemplating installing that above our roof decks to allow for an unvented roof and getting r25 at the same time. I would then fill our rafter bays with cellulose to get a total of R-40 or more depending on rafter sizes.
Frankly my preference is spray foam, but I was evaluating some lower cost alternatives for those clients that do not want to spend the money on foam.
Thanks for the input.
Bruce
Replies
I'll bump this. I'd like to see some replies too.
A friend of mine did this on his house. He used 2x6 T&G decking over 4x8 rafters @ 48" OC. Laid 4" foam board over the decking, laid plywood over that, and then screwed the sandwich together with Timberlock screws that were just the right length. I didn't pay a lot of attention, so I'm not sure where those big screwheads are... recessed somehow into the plywood or bumping up under the shingles?
If you use foam with preinstalled plywood, what do you do to get your spacing between sheets? Seems like you'd need to take a can of foam up on the roof and run a bead between the panels before pushing them together.... leaving a gap between plywood edges. If you're talking about a plywood roof deck that won't be seen from beneath you could run long screws in wherever you need them.
Next time I go by my friend's place I will look to see how tall his fascia boards are. You need to hide that extra 4-1/2" + in height. If you have conventional rafters below you could be looking at some big fascia even if you rip the tails down.
You need to hide that extra 4-1/2" + in height.
I've seen that done by making the frieze taller or adding a frieze and using a 2x4 cornice ladder up the rakes. Only the top sheathing extended outside the walls. Similar detail at the eaves.http://grantlogan.net/
But you all knew that. I detailed it extensively in my blog.
I've never seen it done on a residential roof, but it is pretty common in commercial construction. Generally we install the nail base over a metal deck, but I did do one project where we went over non-com CDX sheathing. As long as your structural members are strong enough to handle the extra dead load, I don't see why you couldn't apply this to a reisdential application.
Shawn
Shawn
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Checker Contracting - SE Michigan
Thanks for the input.
Bruce
I did it with 2x8 t&g over the frame, then 4" polyiso, then 1x4 strapping fastened with the long headlock screws through the poly and into the frame. Then plywood nailed to the strapping. So it's vented through the strapping.
Thanks Tom.
Do you feel it was successful?
Bruce,
4" polyiso with 1/2" cdx for $200 a square?
Is this froma readily available supplier (where?) or is this a special find?
Mongo
ABC Supply-Iso factory is about 100 miles from here. I am trying to develop unvented strategies that minmize spray foam on roof deck. I like the iso for several reason-price, elimination of thermal bridge, and flexibility on roof designs (easy last minute changes).
Hiker,
SIP's do somewhat what you are asking.. but won't Iso foam absorb moisture? you'd need a whale of a vapor barrier.
Since it is used often on flat roofs, I do not percieve there would be any issues on a sloped roof.
I would consider SIPS, but our projects change constantly, ie add dormer, change slopes, move gables so a SIPS order would be challenging. Secondly, our project sites are tiny-no room for a forklift and cranes are not always an option either. Upcoming project, the driveway is almost 8' wide with project at the back of existing house.
Bruce
Hiker
It's so easy to cut panels to shape yourself that I'm surprised at your reluctance.. I did them all on my house. You pay for the whole panel when you order them why not get the whole thing and use as much of it when you can? All I ordered was enough panels to cover the areas I needed then I made all the cuts myself.. Zip with a skil saw and zap with a sawzall with a long blade.. maybe a few minutes more time than it takes to cut a sheet of plywood..
If I needed to cut splines in it or cut the edges away for blocking I simply used the melter they sold me..($110.00) Very fast to do and that way I could change my mind as oppertunities appeared. I added the 11th dormer that way.. I didn't realize what a wonderful view of the lake I had and it would have been a shame to build it as orginally planned.
I also moved the tower from the north west corner to the south east corner of the great room because I discovered a more interesting and creative way to build..
IN fact this whole house was pretty well improvised on the spot. I gave the city building inspector a set of sketches indicating what set back I would adhere to and my methods of construction and I was scruplious to meet or exceed all of that..
Site wise My yard is 50 feet wide and had mature trees thoughout it.. during construction I broke a couple of twigs but no branches. go to 85891.1 & 94941.1 for some pictures of my home and an idea of the site.
To put things into scale the house is 36 feet wide 75 feet long and a total of 5500 sq.ft. the peaks you see are just short of 30 feet high!
Just for information there is absolutely no way I could have built this place without the use of telehandlers.. and with them it was a breeze.. I demolished the old house piece by piece with the telehandler and you'd be amazed at how easy that is.. In fact I lived here during the whole process and never moved out.. I'd tear down one room and leave the walls up for the next segment..
If you note the roof is 27/12 for the west wing and 17 /12 for the main great room.. Do you know how easy it is to put the shakes on a 27/12 pitch roof? I did it myself in 4 days.. because I was on a 4'x16' work platform that raised up to exactly the right height with all the supplies right at my side.. I never carred a shingle! I pick one up, lean forward, set it in place and nail it..
Telehandlers are tools! if you learn how to use them properly they will save you 20% of the time it normallly takes with 20% less people doing it.. they are that productive..