# Insulate Ceiling Joists or Roof Rafters in Attic?



## Windows on Wash (Aug 30, 2011)

What does the Attic look like? Rafters or Trusses? Need to know that first.


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## chandler48 (Jun 5, 2017)

Insulating your roof rafters will negate the purpose of the air space in the attic. It should be as close to the outside temperature as possible. Insulate (or add to) the joists, and add ventilation, either via soffit/ridge vent or gable venting.


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## SLSTech (Jan 19, 2021)

The answer is it depends - I am a huge fan of hot roof assemblies (not done with batts) but in some cases regular attic ventilation with insulation is the best option. The biggest thing is air seal, air seal, air seal
Air Sealing: The Hot-Roof Option (thehtrc.com)
Air Sealing: Sealing those Attic Penetrations (thehtrc.com)


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## SPS-1 (Oct 21, 2008)

Right off the bat, if you insulate the rafters, you have perhaps 40% more area to block heat transfer. Exact number depends on the pitch of your roof --- I will let you do the math. So R30 insulation on the rafters will transmit 40% more heat than R30 on the attic floor. 
But I can't comment on how it effects your HVAC in the attic. Up here, we all have basements for our HVAC.


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## StrongEagle (Nov 1, 2019)

Windows on Wash said:


> What does the Attic look like? Rafters or Trusses? Need to know that first.


A picture (or two) is worth a thousand words. Looking each way from my attic stairs.


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## StrongEagle (Nov 1, 2019)

chandler48 said:


> Insulating your roof rafters will negate the purpose of the air space in the attic. It should be as close to the outside temperature as possible. Insulate (or add to) the joists, and add ventilation, either via soffit/ridge vent or gable venting.


I don't understand this. Plenty of green building sites say that you're air conditioning ducts and pipes should NOT be in the attic, rather, they should be in the same environment as the house.

I have soffit vents along the back of the house, ridge vents, and a gable vent on one side... see previously posted pictures.

In the Houston summer, there is no way that this is even close to outside air temperature... 120 or more with incredible radiant heat from the roof is not at all unusual.

My thinking is that radiant heat has to be slowed somewhere... either on the ceiling joists or the roof rafters... so why not the roof to try and control attic temperatures.

Thanks.


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## StrongEagle (Nov 1, 2019)

SLSTech said:


> The answer is it depends - I am a huge fan of hot roof assemblies (not done with batts) but in some cases regular attic ventilation with insulation is the best option. The biggest thing is air seal, air seal, air seal
> Air Sealing: The Hot-Roof Option (thehtrc.com)
> Air Sealing: Sealing those Attic Penetrations (thehtrc.com)


Thanks for the most informative websites. I doubt I can ever get a complete seal in a 65 year old house. But that does point out insulating options and makes the next step one of pricing... what does all this cost.


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## Nealtw (Jun 22, 2017)

StrongEagle said:


> I don't understand this. Plenty of green building sites say that you're air conditioning ducts and pipes should NOT be in the attic, rather, they should be in the same environment as the house.
> 
> I have soffit vents along the back of the house, ridge vents, and a gable vent on one side... see previously posted pictures.
> 
> ...


Do you have insulation under the floor up there?


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## StrongEagle (Nov 1, 2019)

Nealtw said:


> Do you have insulation under the floor up there?


Yes... those boards are just lying on the ceiling joists and were removed when blow in insulation was added. But, it's no thicker than the roof joists, and probably settled by now. If I were to redo this insulation, the boards would come out.


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## StrongEagle (Nov 1, 2019)

SPS-1 said:


> Right off the bat, if you insulate the rafters, you have perhaps 40% more area to block heat transfer. Exact number depends on the pitch of your roof --- I will let you do the math. So R30 insulation on the rafters will transmit 40% more heat than R30 on the attic floor.
> But I can't comment on how it effects your HVAC in the attic. Up here, we all have basements for our HVAC.


True dat. And the fact is, it would be nearly impossible to cover every square foot of the roof with insulation. It's 114 degrees right now, with partly sunny, under 90 conditions outside. And your questions bring up the nub of the issue... what costs versus what savings in AC duct losses and reduced heat into the ceilings of my living space. As a side note, it would be nice not to have 120 degree water in the kitchen when I turn on the cold water tap.


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## Nealtw (Jun 22, 2017)

StrongEagle said:


> Yes... those boards are just lying on the ceiling joists and were removed when blow in insulation was added. But, it's no thicker than the roof joists, and probably settled by now. If I were to redo this insulation, the boards would come out.


You have a lot working against you here. 
I would have the ducts redone so they were more like a basement where they run between joists and in straight lines so we can build boxes over them . They can bee insulated so they are in the envelope. 
More like this


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