reflective board on bottom of rafters
I’ve looked thru the archives and I can’t find exactly what I’m after.
I’ve got a 50’s ranch house, hip roof, 5/12, asphalt shingles (black), 2×8 joists with original rock wool (about 3″) with fiberglass blown in over the RW even with the joists. The footprint is about 70’x30′ with a couple of bump outs. I’ve added ridge vent and replaced the soffits and added continuous soffit vents. The soffits were full of fiberglass, so I blocked and added wind tunnels.
What I would like to do is add another foot or so of cellulose. If I do that, the cellulose will be over the top of my tunnels. If I add reflective insulation board to the bottom of my rafters (shiny side up) I’m leaving the tunnels open to the space created by the board and the sheathing. Air can move from the soffits vents to the ridge vent (I’ve got some thinking to do on the hips). If I leave an opening in the reflective board at the ridge, the attic space can also breathe.
Pros? Cons? Suggestions?
Sorry about the size. I just got a new computer and I just realized I haven’t got a resizer installed yet. Oh, Rez, can I get a little help please.
Hey Mister Sushi, you forgot to cook my fish.
“Am I dead or alive? What’s this? Linoleum? I must be in hell.” -The Salton Sea
Replies
I belive that the Florida Solar Institue and DOE reports show that is only "cost effective" for homes in the deep south (FL, TX). But I don't know remember how they figured cost. Most likely it was based on commercial company coming in an installing the barriere.
So part of it depends on how you value your time.
But I would not use a board. Probably much easier to install the foil coated plastic systems.
If you go to the recent thread on radiant paints (Radiance?) you will find several links to those sites that I have posted. You can look around them. I know that the Fl site have tons of test and reports on radiant barriers.
But I would not use a board. Probably much easier to install the foil coated plastic systems.
In this case, it would be easier to install board, since it's tough to get down to the plate because of the low pitch and nasty fiberglass.
I looked at some of those links and didn't see what I was looking for. I'll check again. Thanks.Hey Mister Sushi, you forgot to cook my fish.
"Am I dead or alive? What's this? Linoleum? I must be in hell." -The Salton Sea
Go back and look at the directory of pubs or use the search on the FL site.
It this case, the rigid board is mostly just acting as an insulation dam, right?
Looks like a good design. Secondarily it's a radiant barrier to prevent ice dams, if you live in a cold climate.
Also, if you did have air leaks from inside the structure in the soffit area, the air would be directed out into the 'attic proper', where it would do less harm.
Look up irfanview.com and download ."If 'tis to be,'twil be done by me."
Should have an ice damming problem where he lives.But that does remind me that he want to leave some gaps or drill some holes in the boards to allow moisture that coves through ceiling to be able to get into the venting channels and out.
But that does remind me that he want to leave some gaps or drill some holes in the boards to allow moisture that coves through ceiling to be able to get into the venting channels and out.
I intend on stopping the board or foil a foot or so short of the ridge, so any moisture can exit thru the ridge vent. Hey Mister Sushi, you forgot to cook my fish.
"Am I dead or alive? What's this? Linoleum? I must be in hell." -The Salton Sea
Secondarily it's a radiant barrier to prevent ice dams, if you live in a cold climate.
I'm in Ky where it can get to 105+ or stay at -15 for extended periods. I've been in this house for 3 years and have not had ice dam problems, but we haven't had any serious snow in the last 3 years either.
My objective is to run my AC and furnace less and increase the comfort level while doing so. Also, the benefit of stopping my DW from b1tching cannot be measured on a payback chart. Hey Mister Sushi, you forgot to cook my fish.
"Am I dead or alive? What's this? Linoleum? I must be in hell." -The Salton Sea
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you only run the barrier to the top of the insulation, not to the ridge. the air coming in the soffit must push the hot air out of the attic. If you are blocking the air from entering the whole attic , it serve no purpose.
GraciasHey Mister Sushi, you forgot to cook my fish.
"Am I dead or alive? What's this? Linoleum? I must be in hell." -The Salton Sea
for once, I don't have an opinion. maybe too tired tonight.
But you might nopt need a re-sizer for that one. It is currently a BMP. just open it in your PC and what ever prog has it open, go file > save as > and chose JPG from the drop down file typs list. You will then have two files, one the large bitmap and the other a compressed Jpeg image.
The bitmap format is better for large detailed colour photos because it spoecifically saves information for each and every pixel -
"This one is white
this one is white
this one is white
this one is white
this one is white
this one is white
this one is white
this one is blue
this one is white
this one is white..."
but the jpeg format compresses without losing too much information by recording more like, "These first seven are white..." so the photo can be as large to look at, while the file size is considerably smaller.
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The only problem with having the shiney side up is that it will become covered with dust. This dust negates its reflective properties. On the other hand, if you have it shiney side down (counterintuitive I know), it will stay cleaner and it will prevent heat from being radiated down into the attic space. You must keep the new insulation from touching this surface though, otherwise it acts like the dust did and boloxes up the properties of the aluminumn reflective sheet. Another option is the "Bubble Wrap" type radient foil. You would staple this to the undersid of the rafters, or tack it to a sheet then wedge it up under the rafters. These sheets are actuall mde of multiple foil layers seperated by the bubbles. This basicly creates its own air gaps - though not qite as well as just bare air.
Rebuilding my home in Cypress, CA
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