My understanding is that in many instances when someone creates that “envelope” spray foaming the rafters and deck, by design you’re technically changing the design of the house. They won’t blow in insulation on top of your ceiling joists because now that the attic cannot be ventilated, it needs to be air conditioned/heated by your home HVAC system. Sealing it off with insulation just creates a stagnant, non ventilated space that will allow mold to grow. I’ve seen some newer houses with the foam insulation on the rafters/deck and they’re running a very oversized HVAC system to keep the attic cool too. Sometimes just as cool as the house.
There are vents all along there and inside of the roof leading to the second attic space and all are open not blocked.
I have a gable vent that you see on both ends of the house one on the north and one on the south,
Lots of air flow.
Having dark colored shingles can make matters worse. There are light colored shingles and there used to be reflective shingles they may still be an option.
There is a bit of science to air flow in attic spaces. Most home builders don't seem to understand it. What you need is a chimney effect drawing cooler air in at the bottom and exhausting it out the top. Loose insulation can get blown around and cover soffit vents.
If air is flowing like it should, there will be a lot of hot air coming out of that gable vent. A powered roof vent or two with a proper amount of soffit vents may be better than having gable vents.
Good luck with finding something to help with the problem.
There is a bit of science to air flow in attic spaces. Most home builders don't seem to understand it. What you need is a chimney effect drawing cooler air in at the bottom and exhausting it out the top. Loose insulation can get blown around and cover soffit vents.
If air is flowing like it should, there will be a lot of hot air coming out of that gable vent. A powered roof vent or two with a proper amount of soffit vents may be better than having gable vents.
Good luck with finding something to help with the problem.
I was impressed with this type insulation, but if it causes the decking to rot faster, it would be a nightmare to have to place the all the decking, and roof, and re-insulate, or even if the roofers had to replace a couple of sheets of decking which happens normaly after a few years? I hadn't thought of that.
That’s why they vent the attic space but if I did a metal roof I think i would go 2” blue foam, plywood, storm shield, metal.
That's a tough install for anything other than blown-in loose-fill insulation. They make radiant heat barriers (basically just a layer of aluminum foil, although they also make spray-on, paint-like products) to reflect radiated heat. The radiant barriers can be placed above the rafters, in between the rafters, attached to the bottom of the rafters, or above the joists - none of those would be easy with your limited access. "Fluffy" insulation (rock wool, fiberglass, cellulose, etc.) will only be effective for convective heat - and do little for radiated heat. And, you are correct, as the fluffy insulation becomes hot, and the trapped air becomes hot, that hot mass (air has mass and hot masses radiate heat) will radiate heat to your ceiling. My best off-the-cuff solution is to place all the fluffy insulation you can get in there while still maintaining a vent path so hot air can get out.
The passive "chimney effect" someone else mentioned is accomplished by having vents located at elevations that are as low (intake) and high (outflow) as possible. Again, difficult with your roof's shallow pitch. Passive ridge vents are an option (plastic spacers that elevate the shingle ridge cap to allow air egress at the ridge). Active venting could be accomplished with solar or otherwise powered fans at the peak, but they'd need adequate vents down low for air intake.
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I contacted a contractor a few years back regarding home insulation and it was going to be around $6,000 if I remember correctly. They spray the foam on the overhead in the attic (decking, (add $2,000 to remove all the blown in insulation that is already there), then they spray on the attic floor between the beams. What the guy was going to do was build walls around my central furnace (since it isn't one of those high energy savings models) and insulate the walls around the furnace.
I expect now that the prices have gone up probably to around $8,000. One contractor said that his electric bill in the heat of summer was around $30.00 per month with spray-in insulation. There is a difference between closed cell and open cell insulation also. I believe the closed cell is better and more costly.
And you cannot run additional wires in the walls once the spray is done, you're stuck with what you have.
I expect now that the prices have gone up probably to around $8,000. One contractor said that his electric bill in the heat of summer was around $30.00 per month with spray-in insulation. There is a difference between closed cell and open cell insulation also. I believe the closed cell is better and more costly.
And you cannot run additional wires in the walls once the spray is done, you're stuck with what you have.
We had one of these installed when our roof got installed.....made a big difference for us.
https://www.amazon.com/gp/product/B0002YWVJM/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0002YWVJM/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Roof decking seldom needs replaced anymore unless there have been leaks.
I bought this house in 1998 and I blew in 36 bags of cellulose and I have it deep up in the upper attic.
I blew some of that in the lower attic. Yea 2 attics.
And I have dense 4x2 fiberglass on top of that blow in.
Over knee deep.
That dense stuff was pulled from walls in a refrigerated warehouse.
I got trailer loads of the stuff and my houses and buddies houses got well insulated.
My uncle owned an insulation company and I blew in a lot of cellulose.
Dad owned a HVAC company and I roofed all my houses so I am very familiar with air flow and efficiency.
Just not so much up on the spray foam.
In 1 bathroom upstairs I put a reflective sun screen on top of the insulation and that spot runs way cooler.
But as dust settles in an attic that reflective gets covered up and becomes no better than paper.
I did install LowE windows in 1998 and behind the vinyl siding is 3/4" foam board on the north side and 1/2" everywhere else.
