I have been thinking about the following as a way to insulate my roof to keep us cool. I doesn’t really get cold here in the desert southwest but it can get quite hot and I don’t want all that heat from the hot roof radiating down on us.
What do you think about this?
1. Roofing materials (felt covered by shinglesor standing seam metal or membrane or spray-on foam)
2. Plywood sheathing
3. Styrofoam for insulation
4. Bead board (for appearance from underneath)
5. Rafters (2X8)
The idea is to lay the bead board on top of the rafters to completely cover them. Then lay the full sheets of styrofoam. Then lay the sheathing and finish with the roofing materials.
If you walk on this roof, will the styrofoam compress and the nail heads poke up?
The original plan was to lay sheathing directly on the roof rafters, then in-fill between the rafters with foam covered by bead board.
I am also very concerned about the flamability of styrofoam. Is it okay to use as a building material, given its flamability?
Any thoughts on this?
Edited 11/1/2009 2:39 am ET by Mike_Mills
Replies
yes it will compress, yes you will get nail pops.... and it may not comply with your design wind loads for uplift
It should be pointed out that there are a number of techniques similar to one degree or another, though they generally involve putting the foam over regular roof sheathing (and sometimes retrofit over an existing roof). Never heard the foam/fire issue raised with these, curiously.
Another technique for hot country is a false roof with 2-4 inches of separation between it and the "real" roof below, and provisions for air to circulate through.
Use polyiso insulation instead of styrofoam. It's available with sheathing attached:
http://www.hpanels.com/pages/pdfs/Lit_Prod_Color/H-Shield-NB.pdf
copper p0rn
I really can't comment on your construction method but would like to say that there are rigid foam materials (other than Styrofoam) that have different compressive strength ratings and different smoke and flame spread ratings.
Also, as a general statement, regarding the flammability, flame spread and smoke developments of rigid foam insulation if the foam is fully covered by a fire barrier, the concern (both from a safety and inspection perspective) is greatly reduced. A fire barrier might be 1/2" drywall, plywood, etc.
Again, I have no experience with this type of roof construction but another approach might be: rafters, beadboard, firing strips the same thickness as the rigid foam installed on top of each rafter location, rigid foam filled in between, roof sheathing and roof covering.
Excellent!
the problem with foams is sheet edge.....the denser versions have good compressive strength....in the field... but edge nailing will over compress the foam...
then there is the problem of edge detailing... blocking and concealing the foam edge
if you are in an area that promotes carpenter ants you run the risk of giving a colony a very great place to excavate a nest in the foam
everything you want to do with the foam can be done by applying it to the bottom of the rafters instead of the topMike Hussein Smith Rhode Island : Design / Build / Repair / Restore
Yes, after reading the posts herein, I have dropped the idea. I had planned to do the insulation from the inside and that remains the plan.
I think the guys above are right about incorporating some ventilation. A side benefit is, if you have some air space just below the roof sheathing you can use radiant barrier sheathing which I have found to be one of the most cost effective energy upgrades - especially for the climate you described.
One thing I am learning is that the roof seems to have more complexity and more issues which need to be dealt with (design, materials, construction, etc) than the rest of the structure combined.
Yep. A roof is a system, with many backup layers.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
I have considered many different options for roofing the new sunroom. The one I keep coming back to, due to its relative simplicity and good performance, is the sray-on foam roofing.
This foam is white (low solar absorptance), has good thermal insulating properties (it's a foam), is leak-proof (it's seamless), is inexpensive and simple to install (spray directly over sheathing).
I really want a standing seam metal roof for aesthetic reasons (appearance) and but it is apparently a much more difficult proposition to make water tight on a low slope roof.
Lately, I have been toying with the idea of a spray-on foam roof for its water-tightness and its thermal insulating properties with a standing seam metal roof overlay for appearances.
Edited 11/2/2009 8:29 pm ET by Mike_Mills
The finished result was tec- shield in a roll. The A-frame had a ceiling that was approximately 6' down from the ridge so the appearance was not a factor. We installed a ridge vent for ventilation and will later install a attic fan to relieve the client of the heat issue in the summer. Thank you. By the way my web-site address is http://www.newcreationconstruction.us Brian
JLC recently did an article on doing exactly this.
One suggestion: Add a ventilated space under to top deck, and use a reflective coated radiant barrier Plywood or OSB (like TechShield). This will help keep heat out of that layer of foam insulation.
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Get some 2x6 (or 2x4, if you prefer) red pine roof decking. It has a T&G joint which is flat on one side and vee-jointed on the other so it makes a nice ceiling to look at.
Felt the roof deck as usual.
On top of that, lag-screw a vertical rack of 2x4 sleepers set on edge, and set 3" polyiso panels in between them. A pneumatic impact gun is the best tool for this; most cordless electric 'impactors' don't have the balls for that many big fasteners. Also: Calculate the length of lag screws carefully so they won't drill through the roof decking and be visible from the inside.
Now nail 1x3 strapping across the sleepers on 16- or 24-inch centers running up the roof, and attach the roofing to that.
You'll have a half inch of clearance under the strapping over the insulation; that'll give you a total of an inch and a quarter clear under your metal roofing. More importantly, because of the crossed grid you've built, that space will be vented in both directions--from both rake and eaves edges--to help evaculate accumulated heat.
No plywood, OSB' or other sheet goods required unless you want to use asphalt or wood shingles instead of metal.
There are photos of this construction on a very steep A-frame project I completed in 2003 in this thread.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
MM
there are roof insulations of different densities that are used in commercial applications. if you google john mansfield or owings corning you should be able to find an appropriate insulation. Some also come with a nailing surface attached.
The flamablity issue maynot be addressed in most locals because the Building codes are mostly concerned with fire inside the a building and the effects of toxic fumes to residents and fireman. that being said if you are in a area prone to forest fires this maybe a concern. Most roof flamablity issues are with the roffing material itself not the insulation or sheathing.
the vast majoity of flat commercial roofing is similar to the system you describe. ie roof membrane over insulation on structural sheathing.