Air tightness testing for modular homes
I’m buying a 960 sq. ft. modular home this year. It will be shipped to my property in climate zone 7 in two equally sized pieces that will be stitched together on site. But most of the work on the home will be completed in the factory, and it will arrive with HVAC, electrical, and plumbing installed, drywall in place, etc.
Getting an airtight envelope is important to me, but I’m not sure how to test air tightness of window openings and various joints prior to finish when there will be a gaping 40-foot expanse where a wall should be. Basic construction form is metal stud walls with interior spray foam insulation/exterior rigid foam to provide continuous air barrier, SIP roof, and 3-pane windows. Any ideas for how we can work to assess air tightness under these circumstances at a point in time that allows for corrections? And I’d appreciate any references for clear and simple to understand instructions to provide to the builder to increase the likelihood they get the air sealing detailing as correct as possible.
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There are several details you will have control over, including the joint where the buildings join. This joint will be critical in terms of air tightness. Also, the air seal between the foundation and the prefabbed building will need to be completed. Does the package come with a roof or will you be having trusses installed on top? The junction of wall and ceiling air barrier is often poorly done and you could look at that. I would have the house blower door tested after the 2 pieces are joined, and then if the results are not to your liking you could use aerobarrier to make it better.
Roof will be SIP. I think I probably will do a blower door test after full assembly. I didn't know there was an opportunity for after finish air sealing.
Not directly related to your question.
" metal stud walls with interior spray foam insulation" I know this is a common wall for modular homes as it essentially glues the whole structure together. It is however not the best of thermal efficiency and not great use of SPF.
Because of the thermal bridging of the studs, the wall is about an R7 assembly even though the center of cavity insulation R value is way higher. Swap out steel for wood studs and you get up to R14 assembly.
I don't know how the insulation package works but your best bang for your dollar is to get less spray foam (1" to 1.5" is good enough) and more exterior rigid.
Thanks, I know thermal bridging is a big deal with metal studs. What assembly would you recommend to get me in the vicinity of R-40 for my walls?
What assembly works is up to the builder and your budget.
If they want to stick to metal studs, you would need about 7" of exterior GPS insulation or 5.5" of exterior polyiso.
If you instead swap out the steel studs to 2x4 wood studs with spray foam the amount of exterior insulation drops down to 4" GPS or 3.5" polyiso.
In either case the walls don't need to be insulated with spray foam, standard batt insulation works just as well and much cheaper.
Yes, they are wedded to steel. Doesn't the spray foam have some value for air sealing?
There are cheaper ways to air seal than spray foam. Maybe for the pre-fab shop, it is easier to deal with SPF than tape.
If they are fixed on steel, find out if they can get these:
https://steelform.us/products/deltastud
I've used them before for a project a while back specifically to bump up R value. Not as good as wood studs but better than regular CFS.
Margie a blower door test is only about 5% of the job you want/need you should be asking for blower door directed air sealing! A blower door test only tells you how leaky a building is at the moment of a single test.
Walta