Spray in foam insulation + minimum roof slope?

I've recently discovered the benefits of sprayed in foam, just in time (I hope) to use it when we finish renovations in our home in rural Maine. I'm having trouble with the roofing issue however and local roofers here don't know the answer as few of them do metal roofs at all (standing seam) and even fewer have experience with sprayed in foam.
We have a very mildly sloped roof, (mono sloped roof facing east). We had a metal roof put on 4 years ago and are very happy with it, snow slides off despite the fact that there is very little slope (less than 1 inch rise for every 1 foot).
We also have marginal roof rafter size, they are 6 inch dimensional rafters. We have a single wall in the middle of the house with about 14 feet of span on each side of the center load bearing wall. With the marginal roof rafter size, it is imperative that snow slide off the roof, if it was allowed to sit there and accumulate, it would surely cause the roof to collapse, especially when back to back blizzards occurr).
When we do the renovations in the Spring, we hope to leave the 6 inch roof rafters and spray in 4 inches of foam. This should drastically improve the heat loss, but I'm concerned that the improved insulation will cause the snow to build up and stay on the roof instead of sliding off as it does at present.
I can't find anyone who knows about this and I need to know whether we can salvage the existing roof rafters or whether we need to redo them with 10 inch rafters. Obviously, replacing the rafters will be horribly expensive compared to salvaging the existing rafters.
Do we need to dump the existing rafters and install a roof with a steeper pitch or is there a way to reuse the rafters that we have?
Thanks,
Art in Maine
mail to ky1k aatt pivot ddoott net
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You want to spray the roof deck ? Normaly attics are vented to outside temps to stop condensation. the attic floor is insulated. When your roof leaks you will not know where it is and will rot. Insulate the floor. If you are worried about supports just put more in screwing to the original. Only insulate the roof deck if you will use it as a heated living space.
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I think you're talking about the crawl space, but am not positive.
Please understand, the ceiling in the front of the house (East side) is 5 feet high, the ceiling in the backside is just over 7 feet high.
The rafters are laid from the front wall to the center load bearing wall, then another set is laid from there to the backside of the house.
The ceilings (inside) are not flat, there is no crawl space as you would have with a peaked roof or with a trussed roof. The ceilings (inside) follow the same slope as the roof does on the outside.
The 'roof deck' is above the second floor living space, we have our bedrooms on the second floor which is how we can tolerate the low ceiling height.
So, the only place to insulate is between the roof rafters and there is no ventilation above the roof surface because there isn't anything to ventilate:>:. There is no attic, there is no corner vent or vent at the peak of the roof.
We cannot evaluate whether condensation has compromised the roof rafters, but it is possible. Before we putt he metal roof on, the roof used to leak. So, some of those cavities have been moist in the past. Some of the ceiling was damaged, but it was patched, some drywall tape changed and repainted.
We also think that interior grade particle board was used as the covering for the roof rafters (instead of plywood or OSB). This is sub standard and doesn't add structural strength to the house as plywood would.
Perhaps the undersized roof rafters, the potential for moisture damage and the particle board covering tip the scale in favor of a complete replacement of the roof instead of trying to save or upgrade the existing sub standard roof rafters?
Art Art
On Mon, 8 Nov 2004 11:35:48 -0600, snipped-for-privacy@webtv.net (m Ransley) wrote:

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With a new metal roof you will be good for a long , long,time, So the roof is actualy your ceiling height, a heated area. how it it finished now, drywall? And why do you think it is under supported. Yes than foam would be good but 5.5 " foam is R 30.25? @ R. 5.5 per inch. Well under minimum code for my area which is zone 5. Maybe an Architect should be consulted for roof strength. Beams could be staggered to every other joist to give more depth and get more insulation, but I see roof hight is a concern . Also the silverfaced foamboard is R 7.2 per inch . 5.5 inches of that will gain you an Extra 5.5 R bringing you to the minimum codes for most Zone 5 areas. But I believe in alot more. So you will remove the drywall or ceiling to insulate? The snow will probably slide but call the roof manufacturer they will know best
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On Mon, 8 Nov 2004 12:44:53 -0600, snipped-for-privacy@webtv.net (m Ransley) wrote:

I'm not too worried about the ultimate R value, especially since I'm not using fiberglass insulation for the renovation.
Yes, the area under the roof is living space. It's a full floor where we have all our bedrooms. There is no attic, crawl space and the indoor ceiling height is 5 feet on the low end of the house and 7 feet at the high end. The slope on the ceiling is also the slope of the roof and there is nothing to ventillate.
If we can salvage the existing roof support structure, then we will remove the drywall from the ceiling and spray from the inside and install new drywall. If we have to do a whole new roof from scratch, then we will probably install a trussed roof with new covering and insulate it from the top side (before putting plywood down).
Ceiling height is an issue. But, the kids room are on the low ceiling height room, so they can live with low ceiling heights for quite a few more years.
If we have to tear everything out, we will redo it properly and everyone will have 8 foot ceilings.
Since our single slope roof is nearly flat, I think the snow load slides off because the existing fiberglass insulation is so poor and heat from underneath melts some snow which allows the snow to slide off.
All this goes away IF we reinsulate with foam, and the snow will tend to sit there because the heat loss will be much less. If we get 2 or 3 blizzards back to back (which happens sometimes), we could end up with more snow than the roof could handle since we have 6 inch rafters and a 14 foot span between the outside wall and the center load bearing wall.
My original question was regarding the snow slide on a metal roof with super insulation. If the snow will just sit there with a modest pitch, then we are headed for trouble. If the snow will slide off anyway, then we can save a big pile of money by just salvaging the existing 6 inch roof structure.
All comments appreciated.
Art
PS:
Just notice I forgot to answer your questions...
I believe the present roof is under-supported because it is 14 feet between the load bearing walls and the rafters are only 6 inch (1 5/8 by 5.5). Despite the very modest pitch, snow slides off the metal roof quickly after a storm.
The downstairs is nearly all open, and there is only one solid interior wall to transfer the weight of the center of the roof down to the basement floor. The distance from the center load bearing wall to the outside wall is 14 feet and the roof rafters are 2 by 6's.
I'm worried that the snow won't slide off so quickly if we super insulate as planned and that the snow load might cause the roof to collapse.
-----------------
Also, right now, the upstairs is dry walled (walls and ceilings) but not highly finished or polished. There would be little lost if we had to redo the entire interior up there as it was never properly finished anyway.
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R value is what makes insulation work, not the sales pitch someone gave you about houses heat 2-400 percent less with foam as you wronly state. I foamed the exterior of my house with R 14.4 foam and R 100 in attic my bills went down 25%. If you were correct I would be paying 400 % less. And nobody knows if Icynene will deteriorate like UFFI. Still R 7.2 foamboard is your best bet. So is following code minimums..
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Albert wrote:

I can't quite figure out what you are going to do, but it sounds like you may be asking for trouble. Is there any kind of attic or area between the living area and the roof deck? You really want that ventilated.
BTW snow that does not melt off the roof is good snow. It provides additional insulation and it means there will not be Ice dams because the snow is staying snow. The problems happen when the upper part of the roof gets warm and the lower edge does not.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Thanks Joseph,
Yes, I understand the ideal situation is for the snow to sit there for as long as possible and that it's important for more than one reason.
NO-there is no attic and nothing to ventilate. Check the other replies where I goave additional clarification and let me know if you need further explanation.
Art
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Albert wrote:

I saw your other replies and I think you are headed in the right direction. However I would seriously consider upgrading that roof structure. It sounds like it is under-engineered and may well be a serious problem in the future.
You will feel a lot better knowing you have a sold roof over your head.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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On Mon, 08 Nov 2004 11:17:59 -0500, Albert <> wrote:

Sprayed foam. Two caveats.
1. Since it is a chemical formulation it may outgas and cause respiratory problems. The chemicals may also affect your health if you have chemical sensitivities.
2. Foam effectively seals the structure it covers. Your studs, wood sheathing, etc. cannot breath and therefore cannot stabilize its moisture content with the ambient atmospheric conditions. New lumberyard wood retains some dampness. If that moisture cannot be vented because the sprayed foam seals it then wood rot occurs.
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OK, thanks.
The environmental effects, I've already researched. Both components of the foam are non-toxic before they are combined. If combined in the proper proportions, they give off heat, CO2 and the propellant gas. After curing, the foam is very inert with only trace outgassing due to temperature changes. After the initial cure, it's pretty safe, and can even be installed indoors without ventilation. The MSDS sheets are online for those who want to check themselves.
I hadn't thought of the wood issue! Yes, some wood is downright moist. We get alot of lumber from saw mills around here and they do not have kilns. The moisture content of the lumber can vary alot. I'll keep this in mind before sealing up the structure. If we do construction in the spring or summer, we can cover it from the outside to make house weather tight and then apply the foam in the early fall after the lumber has had a chance to dry out.
Thanks for pointing out the possible problem with moisture content of the lumber.
Art

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You should not be depending on the snow sliding off your roof to keep it from collapsing, no matter how steep a pitch it has. If your roof won't support the expected snow-load, then re-enforce it. You can do this by doubling up the rafters, or putting in a support beam (purlin) mid-span, or any number of other ways, only one of which involves taking the roof completely apart and re-building it from scratch.
--Goedjn
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I'm no expert, but I read a lot before redoing our roof, which is similar as out attic is finished, but we have, mostly, a greater slope.
One thing I read is that it is important to have unheated air circulate below the roof, so snow will not melt, then refreeze, causing ice dams. Thus, when planning to fill in the rafter cavities, you have to do something to preserve an airway between the ridge vent and the soffit vents. I read about something like a plastic or styrofoam product that looked something like a near flattened gutter. You mount that to the bottom of the decking, preserving an airway above the product, then fill the balance of the rafter cavity with insulation.
However, with your abnormal slope, I think you would be wise to bring in an expert to look at the situation and advise you on the best remedy.
Albert wrote:

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--text follows this line-- Note, I am not an engineer, insulation specialist, or anything... I am in no way qualified to give any structural advice, and don't know enough details from your situation to make an assesment if i were. I'm a geek who has done some theatre work and lots of reading. The following comments are just for consideration and possible picking apart from those who might be qualified to make those judgements.
That said, I have a couple comments.
I believe the roof insulation technique you are suggesting (insulating the entire cavity) is called a "hot roof". Google searches for that term and insualtion, etc might give you some more information. I've considered doing this in our attic, since we have no insulation and the same sloped roofs and ceiling setup on our third floor victorian, although, our peaks are trucated to ~7 feet, so we have a flat ceiling where it's high enough. I couldn't find out if it was appropriate for our climate, however. (Pittsburgh PA) I know it might void the warrantee on some asphault roofing materials, but i don't know how it would affect the metal roof. I think the way i'm going to go is to put insulation baffels down first, then spray in foam. The baffels will meet up in a plenum at the peak, and i'll deal with the heat from there. (possibly with solar thermal collection :)
On to structural: In theatre there is a type of platformed used called a stressed skin platform. It's used to make a very strong but thin platform by sandwiching two thin "skins" (plywood) over boards on their flats. (so glue/screw the 3.5" sides of a 1x4 to the plywood). The resulting platform is very thin, but has little deflection (sag) because the deflection force is trying to compress wood on one side, and strech it on another.
I believe there is a construction material made a similar way, using two sheets of plywood, and a foam core. The curing of the foam adhears the two sheets of plywood together and makes a fairly ridged board.
Perhaps removing the drywall from the ceiling, glue/screw-ing sheets of plywood to the rafters, and filling the void with insulation will cure several of your worries: 1: high insulation 2: new "stressed skin platform" roof has more strength than old roof.
If/when you get an engineer in to ask about this, perhaps you can give him/her that option to consider. It might make them look at solutions they didn't consider.
While we're on the subject... anyone use the DIY foam insulation product from fomofoam.com. What kind of results did you get? Any other DIY foam kits for sale?
--
be safe.
flip
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