open beam insulation


  #1  
Old 09-07-18, 04:29 AM
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open beam insulation

aloha everyone.

i have an open beam cieling that radiates heat from outside to inside the room. see pic.

i know that the retro fix is from the inside is not the correct or the best way to insulate. however it is what it is.

so my questions are; is closed cell foam sheets better than fiberglass rolls?
if foam is better what thickness would be best?
if fiberglass better what value recommended?

here on the leeward side of oahu the all time record low temp is 62degrees f. we maybe get a month of rain all year, the rest of the time it is hot. so ideally we want to keep the heat out and the cool in. ( the room is air conditioned).

any ideas that would help reduce this heat from above would be appreciated. thanks to all

77 degrees
partly cloudy
wind 5 knots north
humidity 75%
surf 8-12' from the east
 
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Old 09-07-18, 04:44 AM
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Hopefully your roof coating is white. If not, that should be your first step.

Second, your roof is probably not designed with any ventilation, so that is not ideal and is not something you can retrofit with that type of roof... it is what it is.

Third, foam with a reflective surface like Thermax would be best. 2" minimum... first you would put a 1x2 cleat on the side of each rafter. Then install the Thermax, which you will rip lengthwise on a table saw to fit tightly. Caulk the perimeter of each sheet with PL300 foam board adhesive to air seal it. Then add another layer. How much you add is up to you. Fill the space for the most r-value.

Finally, Thermax is a fire rated foam that can be left exposed... (other foams cannot) but it will not be pretty to look at, so you might need to consider covering the bottom of you rafters with drywall... a tongue and groove wood ceiling or something.
 
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Old 09-07-18, 06:18 AM
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XSleeper: Can you elaborate on what the cleats are for? I'm trying to visualize this.
 
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Old 09-07-18, 06:31 AM
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Radiant barriers need an airspace above them.
 
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Old 09-07-18, 08:16 AM
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I got it. So the cleats provide that air gap/space. Couldn't soffit vents and a ridge vent be added to ventilate this air space if it were made to be larger?
 
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Old 09-07-18, 10:25 AM
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You don't put ridge vent on a low slope hot mop roof.
 
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Old 09-08-18, 01:22 PM
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aloha everybody. thanks for your input but had a few questions;

the reflective surface, does it face up towards the roof or down towards the room?
should the air gap be same thickness as foam, ie 2" foam = 2" air gap?
should i vent the downside of air gap? roof pitch is 2' @12'.
and yes roof coating is white.
 
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Old 09-08-18, 08:14 PM
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Thermax is foil faced on both sides.

This document starting on page 26 gives some details but not all. https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.nrel.gov/docs/fy12osti/53851.pdf&ved=2ahUKEwiftv7i96zdAhXB1IMKHZOLCtsQFjAKegQIBRAB&usg=AOvVaw31Ex_mdlGpXv3CPK-43odr

You would probably get various opinions as to the rest of your questions, depending on who you ask... and not all of them would be correct. Many people do not give answers with consideration to your particular climate or construction type, so would be giving you advice based on what works in their climate... Alaska, Michigan, Canada, etc. which may not be accurate. You could surely contact Dow directly to see if they have installation procedures that fit your circumstances.

But in short, you want an air space but also do not want to sacrifice insulation r value for the sake of a larger air space. Personally I would say 3/4" minimum, unvented, because you need a dead air space. Roofs in your climate are not treated like the ventilated roofs in the rest of the US that most of us are familiar with. The type of roof that I am describing for you is known here as a "hot roof"... and is often needed in cathedral roof assemblies or in attics with narrow rafters that are being turned into living space.

The specifics are probably outside of most of our expertise. Although there is one nice video on YouTube I have found, entitled Thermal insulation short. It goes along with the earlier link nicely.

Bud9051 is a member here that may have suggestions for you. Another option might be to have the underside of the roof professionally spray foamed with closed cell polyisocyanurate insulation of sufficient thickness.
good luck!
 
 

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