Spray and Board Foam on TOP of roofs?

D107

Most of the discussions I've seen on home insulation revolve around spraying open or closed-cell foam under the roof between the rafters. Now I've seen a few sites where after a roof tear-off, foam is sprayed (or foam boards are placed) over the clean tar paper to insulate from above. This white surface is apparently used as the roof itself, though I'd wonder if regular shingles could be nailed in over that.

Has anybody done this? Seems like you could spray above and below and get a very high R#. The white surface would apparently reflect away much of the summer heat, though as a roof surface I don't think it would look too good.

The link below is to a video on this subject; it ends with the claim that every year in the US $13 billion in lost cooling and heating energy escapes through holes or cracks in residential buildings.

http://www.youtube.com/watch?v=g37riSkyZzM

thanks,

David

realolman

i saw it once

i thought it looked so bad, it would cancel out any insulation value it might have.

it was over a slate roof on a nice brick house. I believe it was going bad, last time I looked.

I don't know what you'd do with it at that point... bad foam on a slate roof.

I wouldn't want it on my house.

bb_11

Flat roofs...

I have only seen spray on foam on flat roofs on the exterior. Usually with a membrane placed over them to protect against UV exposure.

I have never seen foam applied on a singled roof. Don't think I would try that.

It's amazing what a little insulation will do to the heat loos/gain of a building. Well worth the money over the long run. Especially on new construction.

bb

bb

[Deleted User]

it's routinely done

in slabs on flat commercial bldgs, and then covered with rubber roofing, and then often covered with stone. spray over shingles? i'd remove the shingles, slab it and go with whatever exterior that'd work for that particular installation

D107

thanks

I did mean nail shingles over the foam, not the other way around. Some building science unvented roof diagrams seem to show two layers of foam slabs nailed in above the sheathing--with seams taped and staggered installation--then shingles installed over that. and then foam also sprayed in attic ceiling between the joists. This would seem to help keep the summer roof temps from negatively impacting the life of the shingles.

Hopefully any spray foam in the attic ceiling could survive a roof tearoff and new shingle install intact.

Ted Robinson

Insulation under rafters - not so good

There are opinions that adding insulation under the roof sheathing will result in higher roofing shingle temperatures as the sun heats the roofing surface and there is less heat flow into the attic. The higher temperature long term exposure will shorten the useful life of the roofing shingle material.

[Deleted User]

then shingles installed over foam slabs

yeah, that'd work. but the potential for wind damage is in the added length of nails needed to securely hold the shingles down. sprayed between rafters, with proper eave to peak venting channels, would be best on a shingled roof

realolman

unless under the peak

of the roof is living space, you'd be better off to put your insulation on / in the attic floor, the roof vented at the ridge and at the soffit and have the roof deck cold.

I watched your link... very interesting.

D107

I was looking at doing an unvented attic

in order to make it a conditioned space. Building Science has extensive documentation on this, much of it free on their website. They claim that the loss of shingle life is relatively minimal compared to the savings and that with the standard practice of ridge/soffit and other vents you're still letting in humidity etc. Their main guy on this Joe Lstiburek says they've been tracking such unvented roofs for years with no major problems. See attached article.Be great if by the time I'm ready to do this they'll have perfected those shingles with solar receptors they have out now.

I'm trying to get my house as tight as possible before getting a new boiler; already have blown-in cellulose in the walls. If I get attic and basement rim joists done, I'll have done about as much as I can. Only regret is that I didn't do foam into the 1st floor rim joists before the cellulose. (Balloon framing--so now that rim joist is full of the cellulose which spreads laterally inwards since the rim joist is open to the interior of the house.

Thanks

David

clammy

Tight homes

I know i'll get some flack for this but i always wonder with all this spray foam stuff and tightening up of homes that after a short time every body will need air exchangers to remove stall air i also know that these HRV are not cheap nor would installing them and servicing them be for free either but as you tighten that home as a drum what about all the VOC that will be released not only from the foam spray installation but every other product that is in a home,and while that HRV is supposely covering all your fresh air requirement that's great but as i have seen people shop around for the lowest prices on everything including service.So after you don't get things serviced and all seems OK and you start to get sick from your home and the lack of fresh or vitale air who's responable the installer ,the manafacture i just wonder .i know a few guys who have tryed and where asked to do HRV for some very tight homes they just wasted there time and money when the HO got the estimates and it was found that they needed more then 2 systems i guess they thought these systems where very inexpensive not ,now there house have humidity and mold issues not to mention all the VOC from tri lames and tgi beams and all there VOC glues that are used .Just a rant and as for service just ask most mod con owners about yearly service most think either you don't need it or i never had to do this before i believe the later will be the death of either mod con or guys trying to sell them unless asked i,m back to CI boilers and standing pilot why cause even when the ecomony is good people are still cheap it's that simple it's that true just remenber before your home was air tight you did not have a HRV and all the asscioted BS that goes with it ,sorry for the rant clammy

D107

you're right clammy

and i know your opinions come from experience and great perception. I intend to do it right whatever I do; hopefully the HRV can be incorporated into the AC. I think we all should have some system like that anyhow. But you're right, in practice, people will do the foam and forget about the rest. that's why places like the Wall and Building Science are great since they teach us how to do it the right way, maybe now with some government tax incentive to do so. I agree about mod-cons too. if i had to i could live with a top of the line, properly sized boiler with outdoor reset, maybe with modulation and one ecm circ and zone valves.

Best,

David

Constantin

Hmmmm..

Building Science Corp did some testing on this in Las Vegas, under full sun in the summer. IIRC, the total on-deck temp difference between a unvented "hot" roof vs. the traditional vented variety was 17 degrees, and yet the hot roof was still within the roofing material specification limits. Also, the infiltration/exfiltration numbers greatly favored the hot roof structure.

They may still have that info on their site. Me, I chose a hot roof because it gave me a fully conditioned attic to install an air handler in w/o having to worry about insulation. Also, it keeps the critters out since they don't even have the beginnings of a hole to nibble on. Even if they make it through the outer layer, they have nowhere to go, hello Corbond.

realolman

the unvented attics pdf is interesting too

but, I'm not sure how much of what they're talking about has to do with "not venting", but "more insulation."

Why sbouldn't you do what they show with the foam and all on the ceiling joists instead of the roof rafters ? Why not a vapor barrier next to the living space? Seems to me I have heard some changing opinions about that as well.

It has come to my attention recently in an unpleasant manner, that you can install all the insulation you like, and somewhere toward the outside of the insulation envelope it will be darn cold.

Why wouldn't the humidified air they talk about condensing near the vents simply condense somewhere within the insulation envelope at the point ( and it will be there ) where it is cold enough?

I am not sure about the characteristics of foam. Does it allow water vapor to permeate and escape? If so, I'd say there will be condensation somewhere. If not, why not put it next to the living space, and keep the moisture inside where it is desirable during the winter months.

I know this. I used to have big icicles. I increased the insulation next to the living space and vented the soffit and ridge and I have no icicles now and a covering of snoww all year.

I am aware that the venting might be just blowing away the heat that has escaped through the insulation, but after 12" of fiberglass run in two opposite 6" layers, I can't imagine that would be much.

D107

the reason is to get that unconditioned attic space

I have an attic with a 10ft peak, maybe 30x30 floor space, so it's a good size. If i put the foam in the roof rafters and into the gable walls it will be reasonably habitable in both summer and winter, and help my AC air handler out considerably more than if I just had ridge/soffit and gable vents.

As far as condensation goes, I gather the foam seals things airtight so condensation is not said to be a problem. Open cell foam like polyicynene has some permeability but is not said to be a problem. The Corbond mentioned by Constantin is closed cell and has even more R value.

In new construction I've heard it said that you'd no longer need to insulate below the attic floor if you were doing the rafters. All the exfiltration would be stopped at the rafters so very little heat would migrate upwards in the first place.

With your multiple fiberglass layers you sound like you've improved your envelope greatly. But fiberglass doesn't seal, it just insulates. Foam does both, so that's the difference.

Of course after I finish sealing I'll definitely have to get a boiler with sealed combustion so I won't be creating the negative pressure inside.

Best,

David

[Deleted User]

my log home

has vaulted ceilings with 1-1/2"x6" t&g interior finish, and 4" foam sheet glued to 1/2" osb on top of that, and roofing paper and 40yr architectural shingles. and i can easily overheat it with my fireplace/furnace or radiant

realolman

I guess that's probably

my main question:

Do we install a vapor barrier in the ceiling or not?

martin

hot roof

We did basically a hot roof 21 years ago with stress skin panels on timberhframe structure. Can't tell you how much gas I've saved, only ever burned wood, about 7 pickup loads a year. Thats my total heat to keep it about 72. Yes my house is tight and I have an HRV for 20.5 of those years.I would rather challange mother nature on my turms with controlled ventilation than her's with lose construction

[Deleted User]

yeah, wood is my heat

of choice, 4+ cords this year, and i usually keep the joint @ 72+, with a heat pump for backup. i've bagged the radiant for now bec of lp prices

JJ_4

Roofing Consultant did not recommend

I also thought that the foam sounded like a good idea. For our 150,000 sf roof ("flat") replacement we hired a roofing consultant. He rejected the idea outright. Application is severely limited by temperature and wind, even with coating UV is a problem, and apparently the smell is incredible. He also does not like the typical EPDM rubber roof.

We ended up with what I thought was old fashioned...a builtup roof with layers of insulation, fabric, tar, and gravel. I thought of this as "old fashioned", but the materials have improved so much that this is warranted for 25 years vs. 15 for EPDM. I had smell complaints on the tar also, but the MSDS for this was much easier to explain to the employees than the foam would have been.

Constantin

The insulation is the barrier...

Corbond is so impermeable that it doubles as a vapor barrier. Icynene when it's thicker than 6 inches or so apparently also. Folk have been spray foaming penetrations through the attic floor for years, a reflection of the desire to limit exfiltration. The stack effect can be very powerful but putting a stopper at the top of the house helps limit it.

These permeable attic spaces may help improve indoor air quality with lots if daily air changes, but the price you pay (utilities, possible water damage, etc.) seems out of line with the benefits. Me, I'd rather be able to control where and when I get fresh air.

[Deleted User]

vapor barrier

if i'm correctly remembering what i saw; they shot the foam, trimmed it flush with the rafters, put on the v b, and then the drywall

mtfallsmikey

It has become

Very popular as a replacement for foam board/EPDM/aggregate roofs on commercial buildings. That was done 4 yrs. ago at the buildings I used to work at. Foam was sprayed on top of new foam board, and a UV resistant sealer applied over top of that.