Attic insulation selection


Old 07-15-03, 06:13 PM
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Attic needed on insulation....

Ok, we're converting our existing attic space into a master bedroom and bathroom. Here are the specs:
- subfloor will be 2x10's with blown in insulation thruout the floor.
- knee walls will be constructed about 4-5' high.
- Existing ceiling rafters are 2x4's at 24" on center.
- Ceiling height from finished floor to peak is about 9' high.
- Attic is currently vented with a couple of those whirly bird ridge fans.
- There is no ridge vent nor any soffit vents. House was constructed in 1924.
- House is well shaded with many trees so there is little or no direct sunlight in the summer.
- We're located in Dallas, TX. Total added space will be about 800 sq. feet.

So my questions are:
1) Can I get adequate insulation without furring out the 2x4 ceiling joists? I've only got about 3.5" to work with to stuff some insulation in there, which tells me I'm looking at a max value of R-13? We're also planning on installing a radiant barrier.
2) How do I ventilate the attic with no soffit or ridge vents? Or do I need to? Would installing a moisture barrier also do the trick?
3) Related to #2....Would a powered attic fan with some other kind of inlet in another part of the roof do the trick of ventilation?

I'm trying hard to avoid the extra time and money of installing soffit and ridge vents. But I will do so if that's the ONLY way to properly do the job and prevent moisture problems down the road. Need some advice here on how to proceed. And I guess it's worth mentioning that we hope to be able to do this job ourselves and not depend on a contractor.

Thanks for any help you can offer!

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Old 07-16-03, 07:38 AM
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to insulate the ceiling as is will only be and R13 if the area is only
3 1/2", even with the radiant barrier, the full Rvalue will only be
R-24. I would consider furring to get an R 19 in addition to the
RB. In Louisiana R19 is code for ceiling insulation, the minimum
reccomended Rvalue.
Keep in mind that this is the only time
you will be able to insulate this area without a major
undertaking, so this would be the time to insulate well for
the comfort of this area.
Also remember that RB is installed with the foil facing down, and needs a 1" airspace between the foil and
insulation (or any material) to perform correctly.
This may impact the way you were planning on installing the insulation and RB.
Also you must install RB in all roof areas, not just over new

Moisture barrier? If you are using faced batt insulation, the facing
will be your vapor retarder. Staple the facing to the inside of the
joists and you should be in good shape as long as the insulation
is not in contact with the roof decking. Insulation in contact
with roof decking can cause condensation issues.

While there are 2 schools of thought on ventilating attics
(vented vs unvented) I personally would ventilate. Not
enough data to set my mind at rest about unvented attics!
While it is a scary thing to climb on the roof with a skill saw
and install ridge venting, it is a good way to get continuous
ventilation at the peak of the roof and we all know - hot air
Do you have enough area for the ridge venting to be effective?
Would gable venting be feisable?

Stay away from power fans for venting, they use energy to do
something that occurs naturally.

Seems like one question leads to half a dozen doesn't it?

Just to address another point or two, you wrote that the kneewalls
would be 4-5'. When you pop up into your attic with this new area
Remember that you are putting a conditioned area in your 130 degere attic. Hot stuff!
On the exterior of your attic walls, install a foil board, with the
foil facing into the attic. Tape all seams with foil tape and keep
that hot attic air in the attic.

We test homes with a blower door in conjunction with photovotalics and find that the hot attic air enters walls at knee walls and makes these walls hot. Increasing the hvac load, and
reducing the comfort of these rooms.

In your bath area when you install the bath vent fan, make sure that you install the damper and vent it. The easiest way to do this
is to attach a metal dryer vent hose to the damper and either
vent it to the soffits, or in to your attic.
When you shower/bathe
the vent fam removes the moisture from the bathroom and vents
it where?
Into your insulation, not a good thing - moisture, food source, hot humid attic all the componets for mold.
By attaching the metal vent hose you can move the moisture out of the insulation and into an area where it will have time to disappate
and not cause any problems.

I am curious to know what you decide to do, so keep posting.
Old 07-16-03, 07:49 AM
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Thanks for your very detailed reply! You've given me a lot to think about. I've got a couple of questions though....

I understand that that you are recommending furring out to get a full R19 insulation value and that I need a 1" air gap between insulation and roof to get the benefit of the RB. Seems to make sense to me.....
You also mention the benefits of installing a ridge vent for continuous ventilation at the peak, but 'would gable venting be feasible?' Can you explain more on how gable venting works?

Also, if I were to install a ridge vent and keep the 1" gap between insulation and RB, do I need soffit vents? I guess I'm just confused as to where any 'fresh' air is being introduced into the attic. Or is that where the gable venting comes in?

Thanks for your continued help!

Old 07-16-03, 11:38 AM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,389

I think if you go to any of the www . gov you will find that RB is just a wast of time and there is no return on it over more insulation. Yes you have to have vents in the over hang or soffit and an air space from the roof to the insulation. And you do want to vent that bath fan outside not just in the attic anywhere. You said you had 9 ' to the ridge I would put a collar beam across the room at 8' on each truss. This would let the roof vent and you could put a fan in the gables or just vent both ends of the gable. The fan is like a 1/2 ton of AC for just pennies. We put a 2mil ploy on the room side of the insulaion even if it has paper on so you have a good tight V/B ED
Old 07-16-03, 12:29 PM
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If you want the highest R per inch look for EPS (Expanded Poly Styrene). It is the bead stuff you see coolers made from. It can be difficult to find an expensive, but will yield the highest R per inch and preserve your headroom.
I would also consider removing the floor insulation under the living space. All it will do is prevent heat from the lower floor from moving into the upper floor, which you are now turning into a living space. If you insulate under the rafters in the knee wall you can gain some storage space. I usually put a 12 inch cripple wall in the knee wall to run the insulation down to the lower ceiling. This also simplifies venting at the soffits. If you hold the EPS off the underside of the roof sheathing by 1-2 inches, provide soffit ventilation, and use a ridge vent or power roof vent, you will kill a number of things at the same time.
Old 07-17-03, 09:41 AM
Join Date: Aug 2002
Location: Indiana
Posts: 137
- subfloor will be 2x10's with blown in insulation thruout the floor.

Should there be insulation in the floor that is over the first floor living space? Would you want the heat to naturally rise and heat the upstairs?
Old 07-17-03, 11:43 AM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,389

Ok think about it. What is the first thing you hear in the heating & AC post here. My upstairs is to hot and the 1st floor is cold what can it do about it.You see it goes up all by its self. So if you can or its there now let it be . It can help some on this.Now on the other insulation EPS polystyrene in the other post. Why do they make cool out of it .Cause it is cheep cheep and has very little R value. It does make good dock floats and thats about all. If you need a foam board type insulation you want to use a polyurethane one .It will have like a beer color to it , and you will pay $$$for this one and it will have a good R value. ED

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