Roof Insulation Options for Low Slope (EPS, XPS, Polyiso, Blown)
jkumon
| Posted in Energy Efficiency and Durability on
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 level 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.
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. Trying to figure out what the cost premium is.
With guidance from this article, I have considered some combination of batt or cellulose in the roof truss depth (which could be as much as 12-14″) to also bring down the amount I need on the roof deck.
So open to the hybrid idea that wouldn’t require all the insulation on the deck itself. It was just the easy way and less messy way long term if you need to open up the ceiling at any point.
I’m looking for some advice on
A) Should I just go approximately 50-50% with blown under/rigid over, knowing that the low slope will add a little extra in the areas that are thick around the edges.
B) Anyone gotten a hold of the FOAMULAR NGX XPS this building season? And if I can get it, is that lesser GHG a good tradeoff for cutting the depth by 3″ and feeling good about the world at the end the day.
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? My GC has noted that we need a thermal barrier next to the sheathing, so one layer of polyiso might be necessary as the first layer.
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.
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Replies
Not an expert, but some comments:
- First, as someone who's lived in a 12-unit apartment with poor to no insulation, THANK YOU for paying attention to this.
- I feel like the general sentiment on GBA is that foam insulation products, while useful in some applications, can generally be designed out of new construction (especially in roof assemblies) in favor of lower cost options. In particular, in new construction, a vented roof assembly with cellulose insulation seems to be heavily favored. A low-slope roof somewhat complicates that, but you could design in adequate ventilation space / cupolas / etc. to make it work.
-Regarding your concern about being "messy long term if you need to open up the ceiling" -- you could consider designing in a "service cavity" in the ceiling with the insulation on top of an air barrier layer and a little bit of space below between the air barrier layer and the finished ceiling, where all the MEP stuff is run.
- >Should I just go approximately 50-50% with blown under/rigid over: Be careful with this. In zone 6, the requirement is 50% (or better) exterior insulation -- and depending on your local climate it might be safer to go higher. Also, my understanding is that with this type of hybrid assembly, the interior insulation needs to be (and remain) in contact with the sheathing -- which means dense-pack rather than loose fill blown insulation.