questions about reflective roof foil
 

As part of my man cave makeover and to get ready for the mini split HVAC system my focus now turns to finishing the ceiling in the garage. Currently all open no sheet rock no insulation. Pretty standard 4/12 pitch 2 x 4 rafters. Has one gable vent on each end and a ridge vent that was installed when the new roof was installed. Also have the soffit vents so air circulation should not be a problem. So my plan was to buy commercially available attic foil reflective barrier in 2 ' and 4 ' width to make it a little easier to work around the rafter braces. I was going to " seal " with the foil right down to the plate that the rafters sit on. And then run all the way up to the ridge leaving 3-6 inches of open space at the ridge. So picture looking up at the rafters, solid coverage under the roof deck of the reflective foil from the sill plate all the way up to the ridge. Stapled to the bottom of the 2 x 4's you will have air movement from the soffit vents to the ridge vent via a " trough " of the 2' on center trusses does that make sense ? I want to seal it at the bottom to allow me to blow in fiberglass attic insulation and I don't want it to flow down into the soffit. Now here's the rub, most of these retailers of the attic foil suggest a 3 " air gap at the bottom and a 3-6 " at the top. Their theory is " most " of the hot air will travel up the rafter trough but " some " will spill out the 3 " gap. What will happen if I don't give that 3 " gap ? Won't the hot air on both sides of the foil still make it up to the ridge vent ? I guess I don't understand the need to allow some heat to spill out at the bottom ? My plan is to blow in insulation until the cavities are full right up to touching the foil at the perimeter because it will only be about 6-8 " thick there and then will be thicker as I come up towards the peak. And won't the two gable vents in addition to the ridge vent allow enough hot air escape path ?

Make sure that it is a Heavy Gauge foil, in order to be able to keep those government satellites in space from beaming mind control waves into your brain. Otherwise you will get strange looks when you wear a little tin foil hat, and we wouldn't want that now would we?

Why are you messing with the foil stuff when you're going to blow cellulose or fiberglass? Just buy some R19 unfaced batting insulation, some baffles for the soffit vents, stapled a baffle every other rafter space, pack the bottom with the unfaced insulation so it won't blow into the soffits when you blow the insulation and be done.

Heat rises so the soffit vents are allowing air to enter the attic space and the gable and ridge vent allows the hot air to escape (although not very well but it works) If you have punched or perforated soffit vents (you can tell by looking outside at your soffit, you'll see some smooth and some with noticeable holes or slots) whatever you do, don't just block off the spaces in the attic or you will have negated what they are there for.

Well Duh...so he won't have to mess with his regular tin foil hat anymore.

Using the reflective stuff on the inside of the roof in conjunction with R-19 insulation will make a profound, almost synergistic impact on your summer electric bills.

Before you spend a ton on insulation think about the marginal improvement of your application.

For example

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Insulation is great but I think air sealing is more important after you have basic insulation. Blown in cellulose is cheap so going overboard isnt a big $$ hit but it also wont show major improvement. Sure, it helps but if you want bang for your buck insulate to ~r19 and seal out the cold air.

The ONLY thing reflective bubble wrap/tin foil insulation will do for you is to seal up some gaps, and reflect the radiant heat you have already in the garage. It doesn't have an appreciable -R- value like pink bat fibergass insulation does. If used in conjunction with the bats, it can be good, but then again you could just buy the pink bats with a foil face.

foil keeps heat out of the living space
insulation keeps heat in the living space

Following Tobra's lead, I would like to add this: during my research on radiant barriers, I discovered that radiant barriers by itself has minimal, if any, impact on the R value; however, properly installed radiant barrier greatly improves the effectiveness (i.e., R-value) of the other insulation installed. What I can say through experience, is that on the 90 F+ days we had last Summer, the temperature remained comfortable in the upstairs until the late afternoon with no mini-split cooling being turned on. In the winter I can even see where the radiant barrier is installed by looking at the where the frost formed (and did not form) on the roof structure.

Sorry, I have no answer to your installation question.

This !!! Exactly what I have read on various sites. That's why I am entertaining the idea plus it's not that expensive and very DIY friendly.

Have you considered foam? Not only does it insulate but increases the structural integrity of the roof.

Can't justify the cost it is stupid expensive.

Not compared to comfort, and 20 years of HVAC bills.
Foam seals all the air gaps.
That's important in the middle of winter or height of summer.

Foam it Green Spray Foam Kits
About a buck/ft for closed cell at 1". Half that for open cell.
Go cellulose or batt over that.

Use the foil but whatever you do, don't block off your soffits. The soffit ventilation is a integral part of the attic ventilation. Put the baffles in as I mentioned and use R19 to fill the gap between the baffle and the top plate of the wall to keep the insulation you're blowing in from filling the soffets.

Radiant barrier is great stuff, but just staple it on the underside of the rafters. Cool air will come in from the soffit vents and hot air will be confined between the decking and radiant barrier, then leave through the ridge vent. I used it to cut the attic temperature at our old place from the mid-140s do just over 100 on a normal day.

That said, it makes a lot more sense if you were going to build a small attic underneath the rafters. Otherwise I'd use closed cell foam directly applied to the underside of the decking instead.

This is the one I'm thinking of buying.


Attic Foil Radiant Barrier - AtticFoil® Do-It-Yourself Radiant Barrier Foil Insulation

My intention was never to block off the soffit. If I extend the foil all the way to the top plate it would create a 2 ' wide trough ( rafters 2 ' on center ). This trough would start at the soffit and continue up to the ridge vent. But you gave me an idea. The rafters are made of 2 x 4's and where the horizontal and vertical boards meet is a perfect place to line up the foil. It would leave an air gap at the bottom. Once the sheet rock is in place as you suggest a piece of R19 fiberglass insulation could be slid into place. This would allow air to pass through and would block the blown in insulation from sliding down into the soffit area. A win-win. Probably 90-95 % of the hot air will flow up the trough between the roof deck and the foil. What little spills out through " the gap " will also flow out the ridge vent. Plus it gives the gap at the bottom that the vendor suggests. And it will save me tons of specialty cuts in the foil. Sure I would love spray foam but then the soffit vent would be covered and I don't want to do that. And I can do the foil and blown in insulation for much less than the foam. And have much higher R value. Being in the South I am more concerned with heat penetration vs. heat escape but with the foil and insulation I believe I will have both covered.

Winner, winner, chicken dinner.

Cut your pieces to length on the ground and bring them up rolled. Staple to the top, align one side, and work your way to the bottom.

You have this figured out, and you are going to love the results.

FWIW, radiant barriers need an air space to work correctly. They don't reflect heat if they are snug up against a surface.

You picked a good one - it's what we used. Also cutting them to length works best along with starting at the top. We took it as far down as practical - not all the way there and it worked out pretty well in the end.

I'm curious why you started at the top ? I don't think there is a right or wrong place to start just curious. If we are going to be totally anal ( maybe we shouldn't be ) it would seem to me that starting at the bottom and overlapping the seams on the outside of the previous row offers the least path for leakage. I'm probably over thinking this x1000 ! :D