Insulation/Ventilation plan of attack – 4a zone – expert opinion coveted! – GreenBuildingAdvisor

Hi All, 

I’m hoping I can attract Martin and other experts to this thread, really respect and value his and everyone’s opinions in this community.  My GC and Architect are really leaving this up to me, neither are really willing to engage, and I’m trying to make the best of a pretty difficult/tough situation (another story).  We are a few weeks away from when insulation decisions need to be made, so here goes, does this sound like total chaos, or should I be ok with this plan of attack.  

Caveats: I’m scared $#itless of spray foam, I do have it in my crawlspace/basement, and it worked out just fine after a lot of anxiety, but I have trust issues I think with some of the “pros” i’ve worked with, and have nightmares of a bad installation, being left with a situation where my new areas stink of off gassing foam forever, and my family and wallet hates me 🙂 .  

3 car garage being converted to family room, Mansard style roof that is “raised” so a roughly 10+ foot flat ceiling and sides that mirror the steep mansard shape.  HVAC ducts need to unfortunately go above ceiling joists, they will be R8 insulated, air handler in conditioned mudroom next door so no issues there, and roof rafters are only 2-3″ deep.  Soffits run continuously around the entire structure, no ridge vents or vents in the attic!  Veneer brick exterior, 1″ gap/cavity between interior framing, flashing and weep holes, 2×6 frames on inside.  

Plan:  Air seal ALL walls/ceiling etc. 

Walls: R21 faced HD fiberglass walls (paper faces in).

Attic/Ceiling: Install 3 box vents near ridge (ridge isn’t long enough).  I don’t have deep enough rafters for insulation there, so plan is to lay 2″ rigid foam on top of ceiling joists, have HVAC ducts and trunks sit on that.  Then sprayfoam 2-3″ of closed-cell spray foam just directly and around trunks and ducts to encapsulate it fully.  That gives me roughly R20-25 depending on how thick I go including the R8 duct wrap – I know I said I dont like spray foam but I can live with just encapsulating the ducts and keeping a vented attic for potential smells.   If I don’t spray foam I was going to have them build a box around ducts and trunk in rigid foam but I don’t know how efficient that is?  For ceiling  I was going to use R49 fiberglass between ceiling joists (faced paper down?) And then finally to help with some of the thermal bridging, continue that 2″ rigid foam being laid on top of ceiling joists that HVAC sit on and air seal it all, basically giving a gross R49 + R10 if I did XPS.  Is 2-3″ of rigid foam enough for ducts or should I lay loose fiberglass over all of the ducts on top?  Mansard style roof, so wanted to have the guys continue 2″ rigid foam down the sides/angle to outline the ceiling framing which also gives me additional R10, plus creates an air sealed type cavity to allow soffits to push air above this whole thing, allowing roof deck and all to ventilate.

New garage being built next to existing garage, 2nd floor will have cathedral like framing, but essentially no attic except for a shallow gap towards the center of the room as the ceilings flatten out about 9 feet high (29×20 interior space, so flat roof area is maybe 20×15 w/ 7′ side walls and pitched towards center. Rafters are 2×12, soffits, and again 3 roof vents will be installed towards ridge/top spaced out. 

Plan:  Air seal ALL walls/ceiling etc.

First floor –

Walls: R21 faced HD fiberglass walls (paper faces in).

Ceiling:R49 faced fiberglass, unless should I just use roxul/sound insulation?  rim joists, cut/cobble 2″ xps rigid foam.

2nd floor – 

Walls: R21 faced HD fiberglass walls (paper faces in).

Attic/Ceiling: I’d like to have my guys again use 1.5″ or 2″ rigid foam to create baffles all the way from soffits to ridge area directly against the roof rafters.  Here I think Martin has recommended EPS, but it looks like I can source just faced EPS, so is it better to just use XPS here as a backup plan?  R49 fiberglass in rafters below these rigid foam baffles/cavity, drywall.  the center of the attic/ceiling flattens out, so there is a gap between ceiling joists and roof/rafters, so if I’m doing rigid foam to create baffles/vent channel up towards the ridge i’d like to continue that around the top of the ceiling joists.  within the center of that space will be a framed out box for a 4way min split, and I’d have this 2″ rigid  cavity above and surrounding it.  

Apologies for the long post, I find that no one actually wants to plan this out with me or give me honestly any sound advice, my architect is probably closer to 100 and not interested in building science or best practices, he just says fiberglass insulation in ceiling joists and bare minimum details on the plan.  My GC says architect needs to give details on this, or “i’ll do whatever you tell me to do” instead of actually giving a sound plan.  If the overall feedback is you need to hire a specialist to come in and provide a plan, please let me know what type of specialist that is.

thanks guys! 

first pic existing 3 car garage being converted to living 

2nd is new 2nd floor of new garage- circled red polls are just temp