Attic insulation, flooring, plenum questions


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Old 05-23-10, 02:53 PM
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Attic insulation, flooring, plenum questions

I'm an electrician and weldor. I know little about insulation except that it insulates.

What I'm thinking about doing is adding more insulation in my attic, maybe even converting attic into more suitable storage space.

Currently my attic is blown insulation. What I'm considering doing is adding R-13 batts to the upper part of attic. Does this help or hurt?

Additionally, I'm thinking about flooring the attic (around ducts, pipes, and wires) to make it easier to use it for storage when I feel like maneuvering around the trusses. Said flooring (plywood or OSB) would be attached to top of joists with the blown insulation still underneath it. Any issues with this idea?

Further, I'm thinking that if I do this, maybe I can close off the attic ventilation holes and vents, and maybe make it a plenum for the return air of the air conditioner and furnace system.

So, what do the experts think? Here are the questions in summary:

1. Are you helped or hurt by adding insulation to top of attic when it already has insulation at bottom?

2. Any effect to overall insulation if I floor the attic?

3. Am I helped or hurt if I turn my attic into a return air plenum?

Thanks!
 
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Old 05-23-10, 05:11 PM
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Do not close those vents in the attic. They are very important. Using your an open attic as a return air plenum is going to suck a lot of dust into your HVAC system. You should avoid this idea.

I don't think your idea to add r-13 underneath the roof deck will help you very much. You should add more insulation to the floor area if you want to increase your R value up there. Are the floor joists filled to the top with blown insulation? If not, you could add more until it is. You can also put 2" Polyisocyanurate foam down and then your floor. That should add about and R12 to your ceiling.
 
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Old 05-23-10, 05:26 PM
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Once sealed off, I had planned to remove all the dust from the attic, just like any normal bedroom. I'm thinking it might be treated like a house without an attic. Or is that a no-no?

Before anyone asks, no I am not converting it into a living space. Just wondering if it's okay to make it easier to use for storage and if it's possible to increase insulation for the home at the same time.
 
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Old 05-23-10, 05:33 PM
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Originally Posted by drooplug View Post
...Are the floor joists filled to the top with blown insulation? If not, you could add more until it is....
Yes, the blown insulation does cover the 4" joists, and then some. I'd be moving it around and compressing it a little to add a floor.

On that thought, does insulation have the same value if it is compressed?

For example, R-13 fiberglass is 4" R-19 is 6" or something like that. If I were to compress the 6" insulation into a 4" wall (or ceiling) will it still be the higher R value?
 
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Old 05-23-10, 06:07 PM
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Originally Posted by MAC702 View Post
On that thought, does insulation have the same value if it is compressed?

For example, R-13 fiberglass is 4" R-19 is 6" or something like that. If I were to compress the 6" insulation into a 4" wall (or ceiling) will it still be the higher R value?
No. It will decrease the R value.
 
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Old 05-23-10, 06:20 PM
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Compressing 6" fiberglass into a 4" space will give at best the 4" rating and due to the compression, the fiberglass starts to look more like glass than fibers, so in the end the result may be even less than the R-13.

I have to admit, I'm not real familiar with LV insulation requirements, but I haven't heard of many places that can get by with R-13. Being in the trades, you should know a HVAC guy or two. Have them take a look and comment on your proposal.

Bud
 
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Old 05-23-10, 06:54 PM
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I'm more concerned with insulation value than with ease of storage. If just chucking more insulation in my attic, on top of what is there is better, then that's what I'll do, leaving the top of the attic uninsulated. Attics in Las Vegas get VERY hot during the summer, which is why I got to thinking.

By the way, several (maybe all) of the side vent holes have the insulation right up against it. Does that hurt them from doing their job? I'm assuming what make up air the attic needs would shoot through the insulation just fine. Maybe that's why there is so many...
 
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Old 05-23-10, 07:37 PM
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I LV you need all of the vent space you can get. Best to not have the insulation covering them.

Bud
 
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Old 05-23-10, 09:41 PM
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Maybe a good attic fan would be an even better investment?
 
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Old 05-29-10, 01:08 PM
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The first thing is there are a number of different types of insulation, most of them are useless.
Adding insulation to an attic is always a good idea.
However, some insulation is better than others.
The best available insulators are polyurethane foam, the best available, but expensive.
Polystyrene sheet almost as good, best as a DIY project, it takes time to fix.

Are you insulating against cold or heat from the sun?

With insulation against cold, the insulation needs to be as close to the air in the room as possible.

With insulation against heat from the sun, its best to start off with a white reflective roof.
Followed by a reflective radiation barrier just under the roof.
Followed by insulation, on the outside of the framing, insulation between the rafters and joists and insulation on the inside of the walls and the underneath of the ceiling.

Using polystyrene, you can fit it on the floor of the loft and stand things on it, without it spoiling.

The theory behind attic ventilation is that the wind will blow, create an area of low pressure to the lee of the home and will drag the air out from the attic.

The reality is that it usually pulls the heat from the home and makes you colder in winter, or the wind isn't blowing and water vapour builds up from the home and makes the attic wet and starts mould and wood rot.

1. Adding insulation to the top of an attic, when you are trying to keep warm indoors, is a waste of time.
Adding a white roof, a radiant reflective barrier and insulation inside the roof is good to help keep the heat of the sun out.
2. Adding insulation to the attic floor only works if, the floor is sealed all round the perimeter and heat cannot escape through or round the attic floor.
3.The crucial thing about domestic air, is that it picks up water vapour from cooking, washing and breathing this air needs to be kept away from any cold parts of the home as warm wet air, on meeting a cold surface, gives up its water vapour as condensation.
Condensation inside the attic or a wall will lead to mould, dry rot and then wet rot. Avoid, or use a dehumidifier to remove the water.
 
 

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