Insulating a Low-Slope Roof
I’m the developer/owner of a new construction 12 unit multi family building in Minneapolis, Climate Zone 6. We’re shooting for R49-55 using rigid insulation on a low slope roof above the wood web trusses.
My background is in architecture and construction, so I’m in the middle of the design team in making decisions and weighing input from both contractors, designers and energy modelers. I’m following the ‘pretty good house’ / stealth passive house approach where I’m not pursuing any one rating system or standard. Instead, taking the best parts for all, noting that because I’m building a multi-family building the size of an extra large size house conventional green wisdom has so far been often off when compared to either SFH and 50+ multi family building wisdom. It’s a tweener problem for sure. However, this insulation issue seems pretty straightforward.
I’ve read through all of Martin’s suggested reading on rigid insulation. Because a lot of the discussion on this board is about single family houses, I found this article on multi-family design in the northeast very helpful. I noticed though that polyiso was used on all the designs.
Given the articles about thermal drift with XPS/Polyiso and the polyiso drop in R value at lower temps, I’ve largely been considering EPS. My energy modeler is an EPS fan and modeled 13″ of it for an R value of around 54 depending on your decimal place choice of per inch rating. We can go as low as 49 or a bit more as was done in the model. We WUFI modeled more than R55 and it really didn’t move the meter performance wise. This confirms, in my opinion, the thoughts shared in the linked article: Air tightness in MF structure is outweighing sheer insulation value. In short, we don’t need to over do it on the roof.
With the new Owen Corning FOAMULAR NGX XPS out (waiting to see if local lumber yards can actually get a hold of it) that would be a bit better alternative to lowering the depth, with the sticker amount reaching R50 for 10″. Even if you consider a discounted long term insulation rate of 4.5 R per inch, that still keeps us close to where we need to be.
The biggest push back of using these materials vs Polyiso (only need 7-9″ of that depending on how you want to rate the per inch R value in cold temps) is the longer anchor bolts needed to secure the insulation on the roof. I’m trying to get some clarity from the actual installer guys on how much a 7-8″ fastener costs vs say 12″ or 14″ one, as my roof is less than 3000 sf and so the added cost of just the fasteners can’t really be that much even if you are anchoring every 12-16″. My hunch is that the more expensive (and thus less tall stack of) foam will easily equal/outweigh extra metal on just a materials cost basis.
I’m looking for some advice on
A) Is the fastener length issue really a problem, or just roofers grumbling about how they have to do something slightly different and really pay attention when drilling so they hit the truss?
B) Anyone gotten a hold of the FOAMULAR NGX XPS this building season? And if I can get it, is that lesser GW gas a good tradeoff for cutting the depth by 3″ and feeling good about the world at the end the day. I’m okay with the upcharge (finding out how much soon).
C) Is the EPS, due to it being cheaper, just the way to go, since once you drill a hole 8-10″, what’s really another inch or two?
D) Have folks actually been installing GPS now since it has been out for a few years and anything useful to note about price or installation with that? The 0.3-0.5 boost per inch isn’t really going to save much depth compared to how much we need.
I’m awaiting the latest per sheet costs from our supplier for each product to put a full cost analysis together.
Look forward to any in the field thoughts from folks who have weighed and tested on this material choice issue for their projects on low slope installations.
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