Flame retardants have me stymied
Hello, I am renovating a house in Pleasantville Iowa( zone5) and thought to have the design all figured out until I saw an article about flame retardants on closed cell spray foam(TCPP) off gassing years after installation. My original roof design was:
-Black standing seam metal roof
-2×4” wood furrings since I was told black metal roof condensates more than other colors
-self adhered roof membrane
-1/2” plywood deck
– 6” rafters
-4” closed cell spray form on deck and at least 1” covering rafter edges.
I know that there are plenty of VOCs and chemicals on basically everything in a house however I dont know if I am willing to expose my little daughter to a known cancer causing chemical( TCPP). I have been trying to come up with alternatives but they all have issues:
1-4-1/2” of XPS above the deck and 5.5” rockwool between rafters, the problem is that XPS maximum rated temperature is 165F, not sure if I can protect it somehow.
2- foil faced polyiso above the roof and rockwool below however polyiso does not perform that well on cold climates
3-rockwool comfort board above the deck and rockwool below the deck, but at 8 pounds per cubic feet and lower R values I think it would make for a relly heavy roof.
Any ideas will be welcomed. Thanks!
Here is the link to the article:
https://passivehouseplus.ie/blogs/new-research-raises-spray-foam-health-questions
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Replies
Go for #2. The cold climate issues with polyiso are overblown in zone 5. Yes you loose a bit when very cold but that is only a small portion of the year. You can also look at fiber face roofing polyiso available from commercial roofing place which tends to be much cheaper than anything from a building supply place.
First I heard about issues with black roof. All metal roofs have some night time condensation so you have to deal with it no matter the color. This means a synthetic roofing underlayment under the metal panels. The panels can be installed on horizontal 1x4 strapping or a 2nd plywood deck above the rigid. Some seemed roof panels with clips can also be installed on steel bearing plates directly over the rigid.
The peel and stick under the rigid doesn't hurt as it gets the place under roof before the rigid and metal goes one and you can work on the inside while waiting on the roof.
P.S. Section of my roof at home is built exactly like this. It is also black.
Thanks for the quick response. Did you install foil face polyiso in taped off set layers? Do I have to install a thicker layer of polyiso to offset the cold weather performance? Also I was thinking of installing 2x4” furring strips on a 45 degree angle and if I understood correctly I will need a layer of synthetic underlayment between the furring and the metal or below the furring strips?
I used the fiber face iso. In my case it was a single layer, depending on how thick you go two staggered layers might be better.
I sized based on the LTRR of the iso I was using without any cold weather derating.
The synthetic underlayment goes under strapping (can go over but a pain to install) or over the 2nd CDX/OSB deck. I like to use a permeable underlayment here, combined with the fiber faced iso it allows for a small amount of drying capacity.
Unless you are in snow country and need top venting, I would install the strapping horizontally. Getting proper venting sorted it is a lot of work and only worth it if needed.
Make sure to mark out your rafters before the iso goes on. Hitting these through thick foam is always a challenge but at least you can take some of the guesswork out of it. I like to install a long screw near the ridge over each rafter and use it to snap a line over the underlayment along the rafter.
From what I have read polyiso goes from R-6 to R-2 when the temperature gets to 15F so that has me a little worried about performance since it does get cold here. The strapping on 45 degrees would help reduce ice dams and hopefully relp dry water is under it. As soon as I posted my question an idea came to mind as far as using both xps and polyiso on the roof, lets say 3” of xps below 1.5” of foil faced polyiso, have you heard of that? That would in theory solve both the polyiso bad performance on cold weather and xps bad performance under metal roofs, right?
Not sure where that is coming from. If you look at fig 3 here:
https://buildingscience.com/sites/default/files/migrate/pdf/BSCInfo_502r_Temperature_Dependence_R-Values.pdf
The range near 0F is not that much. If you want to stay on the safe side, use R5/inch.
Metal roofs will always have some snow melt and icing at the gutters, no amount of venting will get you around this problem.
Venting is only needed if you are in heavy snow country where you can have thick layer of snow sitting on the roof for long time, something around 60lb snow load if I remember correctly.
The usual "derate it for cold weather" R value used is R5 per inch. It's the mean (average) temperature through the material that matters, not the absolute outdoor temperature. For example, to get a mean temperature of 15 degrees in the polyiso, assuming you have your indoor temperature set to 68F, you'd need the outdoor temperature to be -38F. Even then, that "R2" is pretty extreme -- most of those really low R value estimates come from an article written some time ago showing performance of sevaral manufacturer's polyiso samples, and people pick the worst-performing sample. Things have improved since that article was written, and you'd probably never see absolute worst-case performance levels anyway since most of the manufacturer's samples performed better than the worst case sample.
Polyiso is commonly used on commercial roofs and has been for decades without any problems. The usual argument has just been what the long term R value is (the "LTTR" value), which usually ranges from a low estimate of around R5 per inch or so. Manufacturers used to claim overly high R values (R7 or so per inch) for their product, which may have been true when the product was new out of the factory, but ten years later it won't perform that well (XPS also has this issue, BTW). The more conservative R6/inch allows for this, so if you assume R5 per inch you're pretty safe.
Bill
That makes sense as far as the snow melting issue. The article I have was published by owens corning, it might be a little biased since they are the competition lol
http://www.owenscorning.com/NetworkShare/EIS/10019949-FOAMULAR-XPS-vs-Polyiso-Tech-Bulletin.pdf
Anyways thanks for all the information.
Interesting article, and I don't doubt the findings could be true--I can still smell the closed-cell foam in my mom's attic, ten years after installation, though it's faint--but it's not very robustly supported. I'd like to see more research on this topic.
Metal roof condensation is mainly a problem when the roofing is installed on purlins; the condensation occurs on the bottom side of the roofing. If there is no air under the roofing to hold moisture, there is nothing to condense. Installing metal roofing over purlins is more difficult than installing it over a solid roof deck, so if you don't need to vent the roof for other reasons--i.e., air-permeable insulation--I would keep it tight to the roof deck.
Of the options you list, I would probably go with #2 as well--just be sure that you have enough R-value on the exterior to keep the sheathing above the dewpoint temperature.
Michael
So in order to “eliminate” the air below the metal I assume I would have to put a new layer of decking above the foam like a SIP panel correct? If so wouldn’t I be introducing way more holes into foam, like screwing 6” on the edges and 12” on the field of the new decking ?
Leong,
I would be interested to hear from those that have laid the new layer of sheathing directly on the foam what screw spacing they use, but I doubt that you need to follow the type of structural nailing schedule you refer to (6 edge, 12 field). I'm guessing it's more similar to strapping and you would just do 2 foot centers. But either way, fastening the edges of a panel with long screws (joints landing over rafters) seems difficult to me. Im curious how folks approach that. Strapping almost seems easier and it's hard to imagine not using strapping on really thick foam (like 6+ inches), imo.
I would use the fastener scheduel for something like this:
https://www.hunterpanels.com/docman-categories/product-documents/hpanels/speciality-products/1388-h-shield-nb-application-guide/file
Screws, especially long ones, can get pretty expensive, I've found roofing deck screws (used for rigid insulation on flat roofs, sold at commercial roofing places in a big buckets) to be the best value.
So would you be trying to hit rafters here? The notes in that link say those panels must be secured over a 'structural deck' and must penetrate by 1 inch. That doesn't sound like any residential set up I am familiar with. More likely it's going to be 5/8" ply/osb. All the write ups on here about installing board foam above the roof talk about hitting rafters...
I don't understand how one could do that at the edges. You have all of 3/4" of rafter beneath the edge (assuming you got it perfectly lined up).
Hitting just the sheathing seems doable, but is that good enough with standard sheathing beneath?