Question on roof insulation approach with mono slope
I’ve learned a lot with this group over the last year. Appreciate all the good direction as I’m designing and building my own house. I’m leaning towards PassiveHaus standards but without the certification process.
Attached are my current design plans. The wall detail will get me to around R-32, floor slab to be R-10 and I’m looking to get the roof to around R-50. I’m in climate zone 4A. Windows will be European triple pane.
The cost do a single roof nail base EPS is close to 60k. I’m trying to do drop lid ceilings throughout the inside with as many areas as we can so we can do blow-in instead. Way less cost. Would this be the right approach or should I take another direction? This also would lead to needing to put conditioned air into the “attic” areas depending on the insulation approach. The sections show a hatch for where we would do insulation in the 24″ trusses vs. the drop ceiling lids where we would do blow-in. Any suggestions would be appreciated. Thanks.
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Replies
User...999,
I'm maybe not understanding your question, but with 24" deep trusses, why are you not using a vented roof assembly, with either batts or blown in insulation? You don't need foam, to drop ceilings, or to create attic spaces.
A few suggestions:
- Increase your roof slopes to be at least 2/12. That makes both venting and the detailing of the metal roofing much more reliable.
- Design out the dead valley where your roof drains to the wall of your second floor. That's a risky detail.
- Unless you particularly want really thick fascias for aesthetic reasons, you don't need to carry the full depth of the trusses out on the overhangs.
- I would delete all the textures from your foundation and roof plans. It just make the drawings harder to read.
Thanks, MT for your response. I was planning on a tight enclosure with Zip system on the walls and roof sheathing. From what I've seen in the PassiveHaus approach, the truss and attic areas would be temperature-controlled to keep the HVAC and duct all within a similar conditioned air space as the house. By going to a vented truss approach, I'd have to move that barrier from the top sheathing to under the trusses themselves. Maybe I'm mistaken on this.
One thought I had was to do blown-in throughout all the trusses and achieve around R-60 in that cavity and then all the interior ceiling lids would just be open above with duct and mechanicals. I put some supplies above the ceiling lids to keep the space conditioned.
In regards to your suggestions:
1. the main metal roof is 2:12, I can make the top bonus room 2:12 as well.
2. that dead vallley would have a well designed cricket of similar to force the water away from the valley - totally agree on this needing to be done really well
3. Agreed - I need to think up a detail that is mounted on the outside of the house only
4. Good comment, thanks.
User...999,
Having the primary air-barrier at the sheathing on a roof means two things.
- You need to connect it with the walls, which necessitates a lot of gymnastics carrying it around the overhangs.
- You can't use a vented assembly, which limits the possible options to using roof top foam board, or spray foam (and a lot of either to maintain the safe ratios on an r-50 roof), as you can't use blown-in permeable insulation, or batts without either venting the roof, or using foam above.
Most common is as you say to move the roof air-barrier to the underside of the trusses. That still leaves any HVAC or small attics (areas above the ceiling lids) inside the conditioned space. If you are worried about the continuity of that air-barrier, you can strap the ceiling to create a service cavity - and you are then able to use a simple, effective vented assembly. That assembly, using blown in cellulose and an 1 1/2" airspace, can achieve r-50 with much shallower trusses or TJIs of 16" deep.
interesting to see you plans. the last page with the basement section seems troubling to me.
1. Generally poly-iso is not favorable for ground contact. I see EPS or XPS more commonly used.
2. usually a 10 mil vapor barrier directly underslab is a good idea. I'm not seeing it here. if the intention is to use the poly-iso as vapor barrier then you'll need to pay special attention to the seams, especially where horizontal meets the vertical.
3. the vertical insulation is a trade off -- energy efficiency versus fire safety. you might think about fire protection at the exposed edge of the vertical insulation where it looks flush with top of slab.
stamant,
The slab edge insulation should be covered by either overhanging the sill-plate, or tapering the top of the foam.
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I'm with Malcom on a simple vented roof.
All those reverse slope roofs towards a wall will be a leak nightmare. Looks like one section even shows a window in one. Those need to be re-though, even a flat roof is better than what you have.
As for ducting, with a partial 2 story structure you can put most of the ducting for main and 2nd into the floor joists space, the rest you can furr down the ceiling bellow the air barrier and run your ducting there. Those double height spaces it simply doesn't matter if the finished ceiling is 14' or 14'10". If you really want to sharpen your duct pencil, you can go with 6" rectangular duct (so only ~8" drop). Make sure to select high throw registers for anything in the ceiling as getting even 8' of throw is very hard. Sometimes a bit of floor heat by the windows is a simpler and much more comfortable solution.
Sorry for the delay and thanks for the feedback.
Vented vs. non-vented on a mono slope roof with trusses that have to be 24" because of their spans...what should I change with this detail? I understand the detail does not have everything to scale but I'm trying to get a roof that is both economical to build, sealed, and a higher R value for my 4A zone. The rendering of the house to date is attached.
Am I missing a 6" air gap from the underside of the sheathing to the top of the cellulose with a layer of rigid insulation on top fo the sheathing?
A top vented assembly is not the same as a vented assembly. It can be made to work with a highly permeable top layer (ie sarking membrane), 7/16" ZIP is not it.
You need the gap bellow the roof deck and you need to get air into there. Simples is to stop your wall insulation 2" from the roof deck and use that as the intake. The main air barrier now becomes your warm side Siga which you want to connect to your sheathing air barrier. Easiest way is a strip of Siga over your wall top plate before the rafters are set. You can than tape this on the outside to the sheathing and to the roof membrane on the inside.
This type of roof is harder to dense pack as you need a layer of insulation mesh to maintain the air gap. Lot of time, batts are the simplest option.
You also should still fix the roof that slopes into your wall. That is trouble waiting to happen.
We have done vented mono slope roof assemblies with both loose fill and dense pack insulation depending on the slope. Open web trusses are no one’s friend if batt insulation is in play. Our preferred assembly (see attached sketch) is to attach a 2x2 vent support to each side of the truss that runs from the eave to the ridge. We set the top of the vent support 2” down from the top of the truss chord and then lay in a panel of MSL ECO4 to create a vapour open and continuous ventilation path between each truss.
https://www.mslfibre.com/Produits/Fiche/13/SONOclimatECO4
The MSL is cost effective and when secured to the vent supports with roofing nail offers plenty of support from below to handle the pressure of dense packed insulation. We install a robust smart vapour retarder at the ceiling line which is also our air barrier. 2x3 strapping creates a functional service cavity between the vapour/air control and drywall.
When the slope is less than 6:12 we have simply had our insulation contractor use a loose fill approach which is effectively captured between our ceiling membrane and MSL. We haven’t found this to be unnecessarily challenging or complicated.
sommerbros,
I like that assembly - especially that you don't have to install the baffles from below, which is tedious work.
What are your thoughts on it versus using a sarking membrane covered by 2"x2"s?
https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
We have worked with both systems. A membrane installed over the trusses (in our case Solitex Mento Plus https://ca.475.supply/products/solitex-mento-plus) with a 2x3 crosshatched assembly on top is an excellent solution as well. It does however require the rake and eave to be part of crosshatched assembly which can create extra work. We considered both assemblies for the project I mentioned earlier and landed on the wood fibre baffles as a slightly more cost effective alternative.
sommersbros,
Thanks, that's very useful to know.