Author Topic: Manufactured home attic cavity - insulate more or gable vent?  (Read 12543 times)

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Atokatim

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Manufactured home attic cavity - insulate more or gable vent?
« on: August 19, 2015, 12:56:06 PM »
I live in a 24 x 50 Manufactured home and am trying to improve the efficiency as much as possible as I don't plan on moving any time soon.  I just recently installed a 2.15KW sun tracking solar panel system and had TVA (Tennessee Valley Authority) come out and do a capacity check on my air conditioner which turned out fine. 

Since I live in a manufactured home, I can only do so much to polish this turd :P  My question is, should I just fill the roof cavity with more insulation ($700+) or should I install gable vents ($200)?  Which would be more efficient?  The current insulation in the roof cavity is probably bare minimum for code.  There are 2 static vents on the south side of the house about 1.5 to 2 feet down from the ridge and spaced about 25ft apart.  I did install a tin roof on slats over the shingles a couple years ago but that did not seem to help my electric bill any.

I know there are other things I can do like windows and belly insulation, but I want to stick with one thing at a time.

Mary B

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #1 on: August 19, 2015, 08:18:08 PM »
Insulate!

MattM

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #2 on: August 20, 2015, 06:21:25 AM »
Insulation now, think active ventilation later.  Much easier to actively move air in the limited space than to rebuild the roof vents.  You might consider a metal roof with an air gap on the next re-roof.  Prefabricated homes tend to be easier to put on metal roofing.  The air gap insulates the underlayment and keeps the sun heating off the rest of the house.  Aluminum roofing radiates the heat really well.  Copper does even better.  But if you have trees think steel.

Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #3 on: August 20, 2015, 07:23:17 AM »
I did go with a tin roof and I installed the sheets on slats that I screwed down over the shingles so there is about a 1/2" air gap between the shingles and the tin.  The insulation would be a pain in the butt since no insulation company wants to deal with my house since there is so little space to get into and blow in insulation.  One company said $600 to blow in the insulation as long as I do all the prep work.  A couple days later, the price changed to $900...

I am looking to do it myself by buying the insulation from lowes or home depot and renting the machine to do it myself.   I figure the cost for 1200 square feet of cavity will cost me about $500 for the material and I think I can rent the machine for free if I buy a certain amount of insulation at once.  I am by no means a professional, but I am sure I can do it myself.

Installing the gable vents would be the easiest and cheapest option for now but I am afraid if I install the gable vents, it could hurt in the winter time with my existing insulation being so minimal.  And...if I add more insulation, the airflow will be restricted across the cavity since I was told to pack the roof full of insulation to get the highest "R" value I can.

Lowes has blow in insulation that can cover 106 square feet per bag and it is $32 per bag.  I am guestimating I need about 15 bags to fill my roof cavity.  At the peak of my roof, I have about 18" to 24" gap between the sheet rock and the top plywood and it angles down from there to the over hang of my roof.  Can someone check my math?

klsmurf

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #4 on: August 20, 2015, 10:50:28 AM »
You did mention you have a couple of roof vents, but are there any soffit vents? 

How much insulation is in there now?

What is the height of the attic space?

Is there electric up there now?
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Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #5 on: August 20, 2015, 11:03:45 AM »
There are soffit vents but they are small (2" or so) and only 2 or 3 on each side of the house.  The whole eave has a perforated plastic cover but I don't know if that is actually doing anything.

The insulation up there now is some sort of white clumpy stuff and it is probably 6" to 8" deep from what I could see when I replaced the vents to install the metal roof.  I could not reach down and touch the insulation where I was so it has plenty of room to fill.

I would guess there is about 24" at the peak and probably 8 to 10" at the edge of my roof.

There are some wires up there but they are for the ceiling fans or light fixtures.  Most all the other wiring is run under the house since it is a manufactured home.

Mary B

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #6 on: August 20, 2015, 05:16:19 PM »
They make a plastic chute looking piece that should be stapled to the underside of the roof between each rafter if you are going to pack the area tight. It will remove moisture buildup so you don't get mold growing in the insu;ation

Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #7 on: August 20, 2015, 06:13:41 PM »
If I could install something like that I could.  I am not sure I explained how LITTLE room I have to work in and insulate.  The house is a double wide manufactured home.  In order to install plastic in my roof, I would have to remove the entire roof off the frame since there is very minimal room.  This is why I am wondering about a gable vs insulation.  The only part of the roof cavity I can move around in is the very center of the house where there is about 24" of room then it slopes smaller from there.

klsmurf

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #8 on: August 20, 2015, 07:02:15 PM »
It's tough with such a shallow pitched roof, but you just got to tell yourself.   At least it's not crawling in a muddy crawlspace!

A manufactured house should have the same code requirements, so your insulation is probably adequate.  In Tennessee you are probably more concerned with keeping the heat out more than cold.  Air circulation is must to help keep the temperature down in such a small area. You should check that there's no obstructions in your soffit vents.  Cooler air must come in from below for your roof vents or gable vents to work properly.  A couple passive gable vents will probably help some.  A power vent would definitely help, but that's electric and some noise.

Kevin
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MattM

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #9 on: August 20, 2015, 11:42:42 PM »
You have to ventilate that attic.  Does the top ridge have venting?  If so then your vented soffit is key to get adequate air flow.  You can always insulate during the next reroof with 1.5" or better four by eight sheets of insulation dropped over the old roofing.  (Screwed down with tabs every 16" of course. Just depends on your building codes, some jurisdictions don't allow two old roofs under a new one.).  If your current roof has gabled ends you can add a blower on one end for active cooling.  Double hipped roof can still be active cooled, but it's more work.  You want negative air pressure in the attic, so a blower should never blow in, always out.  Many people don't know that.

MattM

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #10 on: August 20, 2015, 11:57:28 PM »
When you say you roofed over the slats, did you run cross slats then fasten your panels to them?    It's not the best choice, being that you end up with stagnant air pockets.  Next time throw 1.5" insulation in between the slats.  The only air gap should be uniform.  If you cross slat, run verticals first with horizontal on top, you can still insulate.

Bruce S

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #11 on: August 21, 2015, 09:53:48 AM »
Atokatim;
Let's do a little bit of detective work here. I think we need a little bit more info.
First, you have a double-wide mobile home, you have a tin roof about 1/2inches above the original roof .
Now what kind of better efficiency are you looking for? Lower cooling bills? Lower heating bills?
IF it's lower cooling bills, then here's what I did in my brick home up is St Louis, MO that has had a noticeable improvement (improvement being that instead of a $150/month AC bill it was only $90) . IF you have access to those cute little 100 and 120mm DC power fans for free , get as many of them as possible ( I used 16 120mm in total) . Wire them in parallel , mount them at each of the attic gables to push the attic air out, make sure you have access to air from either outside or inside the house so the fans have air to work with. This will move the heated air out of the attic thus helping your cooling efficiency.
Another one would be to try doing the same thing for the tin roof. Tin once heated up by the sun will heat the roof below it , again causing heat to be transferred into the living space.
 
IF it heating you're looking to make more efficient , then let us know.
Hope this helps a little

Bruce S   
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MattM

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #12 on: August 21, 2015, 11:35:02 AM »
And that's just it, he said its a half inch gap under it.  If the dead space exists between the slats and that half inch is above that, then the air doesn't flow as well.  Rather it will mix rather than quickly move the warm air up and out the top ridge.  Any time you mix different air temperature you get condensation.  Moisture barriers and effective air flow is prudent.  I fear his slats created heat and moisture traps.

Bruce S

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #13 on: August 21, 2015, 01:47:10 PM »
MattM;
I agree, from the days when I worked around corrugated tins roofs; those buggers radiated heat in both directions :-(.

For attic longevity; moisture is a verrrrry bad thing.   

I do think we need to know a bit more of what he's hoping to achieve, cooling is one thing & heat is another.

I'll get a picture up here of my 6-fan test unit, I grabbed them out of old servers we were Deactivating. Even with 8 of them running they do not use as much energy as our attic fan does.
I do have them on separate switches and batteries , but still with a solid brick home , we can tell the difference !
 
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Bruce S

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #14 on: August 21, 2015, 02:22:18 PM »
9059-0
Here's my test unit.
I sized my fan setups so they fit the vents, then added extra stability by building a wooden frame around the outside along with multiple coats of paint.

Bruce S   
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Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #15 on: August 21, 2015, 11:02:03 PM »
wow, so many things to answer :)  Thank you all for helping!

I don't have any gable vents so the fans would not work unless I add gable vents.  I was thinking of using the computer power supply fans with a solar panel to run them.  A thermostat would only enable the fans to kick on when the attic was a certain temp.

I did not install insulation between the shingles and the tin which I probably should have done but it is a little late now.  It took me 1 week to get all the tin up and cut by myself.... very hot job :P

My other thought was to put the same fans inside the static vents and do the same thing as the gable vent but since I only have 2 static vents only on the south side of the house, I figured it may not work as well.

My goal is to get my bill as low as possible in the summer since it is easier to heat than it is to cool.  I have a fireplace that we use every winter but I am looking to get a wood burning stove to make the wood last a little longer.  We also have gas heat which is much cheaper than electric.

Bruce S

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #16 on: August 24, 2015, 12:56:09 PM »
In Short "YES" add old computer fans and run them from solar charged batteries.
I tried using solar to fan setup, but couldn't get the fans to startup on just PV. I had several 12V 7Ah batts given to me as cast-offs , so I used those. I thought about just putting a cap on it , but the batts were free, and when it's hot/muggy I like the ability to run the fans even after the suns has gone down.
Any way you can actively move the air out will help.
IF you have access to a bunch of those old fans, you could gang them for more air flow.

A kind word often goes unsaid BUT never goes unheard

Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #17 on: August 24, 2015, 01:13:00 PM »
I will see if I can swing by mt friends computer shop and pick up a bunch of old fans and see what I can assemble.  I am sure I could wire the fans to a low watt computer power supply and use a thermostat or diode circuit to kick the fans on only when the temperature is high enough in the attic cavity.  Batteries will have to be charged and SLA's are a bit tricky to keep connected to solar at all times. 

electrondady1

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #18 on: August 24, 2015, 02:02:48 PM »
pull the hot air out of your attic and push it through a thermal mass that can retain the heat until winter time . in winter  pull air through that mass into your house .

Atokatim

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #19 on: August 24, 2015, 02:25:27 PM »
A thermal mass?  How can it retain heat for that long?  Seems expensive to me.

DamonHD

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #20 on: August 24, 2015, 02:29:17 PM »
It's called a 'seasonal thermal store' and can be done, for some.  Here were my musings:

http://www.earth.org.uk/milk-tanker-thermal-store.html

10t of magnetite, wrapped in LOTS of insulation, would just about do it for me these days!

Rgds

Damon
« Last Edit: August 24, 2015, 02:39:11 PM by DamonHD »

electrondady1

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #21 on: August 25, 2015, 06:32:36 AM »
i've seen a few "earth ship" videos with the idea employed . they just used rocks in an insulated container. it's something i have been thinking about . the idea of heated floors/ heated crawl space is appealing .

wine_guy_3

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #22 on: October 25, 2016, 08:30:17 AM »
As several others here have said, air flow is critical;
prevents condensation, and helps heating and cooling.

Continuous ridge vent would be the best, but it's a little late for that (roof already on).
gable vents do not help a lot, they only help at the ends, not much in the middle.
Continuous soffit vent would be helpful, and not hard to install.
You really need more than 2 vents in the roof. Probably 5 or so based on the 50 foot length.

Hope this helps.

MattM

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Re: Manufactured home attic cavity - insulate more or gable vent?
« Reply #23 on: October 26, 2016, 05:04:38 AM »
Just to be clear, you only ventilate if their is a gap anywhere at the roof line.  The standard is to ventilate in most every case because unless you adhere insulation to your underside, there is going to be a gap of some type.  General rule of thumb is cold air is dry, hot air holds water.  Not always true, but mostly.  You will get moisture underneath if there is any gap during cooling.  But if you had no airgaps and we're air tight with insulation below the metal, no ventilation is necessary.