Old Florida home, 2X4 truss roof, how to insulate t'ween trusses under sheathing

This home was completed in late 1965 and originally had a gravel roof I was able to keep painted white. Insurance required replacing roof so now it is 30-year asphalt shingle. Old A/C having a hard time keeping up with heat load so I'm thinking of installing 2 inch foil backed polystyrene between the "rafter" portion of the trusses. This foam would be installed flush with the bottom side of the "rafters" leaving an airspace of one and one half inches between the upper surface of the foam and the lower surface of the roof planks/sheathing.
The new roof has a ridge vent and there are vented soffits along both of the lower roof portions. This looks to me like it would provide adequate venting of the heat built up during our sunny days. I'm wondering if the 2 inches of foam, along with the foil as a radiant barrier, will do enough to reduce the heat load in the attic to allow the A/C ductwork to operate more efficiently thereby keeping the living space more comfortable. The ceiling has original blown-in fiberglass at three and a half inches. Not great but that's all the space we have to insulate. Also, I am tempted to install this foam with the radiant barrier up since there will be a 1 & 1/2 inch airspace above for it to work. Any thoughts would be welcome.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tuesday, June 2, 2015 at 10:44:05 PM UTC-4, William wrote:

Why I can't you get more than 3 1/2" of insulation between the living space and the attic? THAT is the main problem. What you're proposing would help, but the real and bigger problem is the lack of proper insulation where it belongs. The other issue is that if you do what you're proposing, it leaves the core of the attic unventilated, because air will flow from soffits to ridge inside the new insulation system. I guess you'd probably still have enough air flow in the rest of the attic that moisture wouldn't be a problem, but without knowing the details, IDK.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
replying to trader_4 , William wrote:

Thank you for your input trader4. The trusses are 2X4 so without raising the attic floor I don't see how I could get more than 3 1/2" of insulation between the living space and the attic. I have been supplementing the original blown-in insulation with bats where needed but not any more than 3 1/2 inches in thickness. As for ventilation, I intend to leave open space between the foam pieces at the ridge line (about 4 or 5 inches between them) so that the attic air space will be ventilated through the ridge vent. My main concern with the attic right now is to try to keep the sun-sourced heat buildup to a minimum during the daytime hours. I'm thinking that the 1 1/2 inch space between the top of the foam and the underside of the roofing planks will aid the radiant barrier at the top of the foam while also allowing the heat to escape through the ridge vent, along with whatever heat builds up inside the attic space during the day (through the ridge vent). The insulating properties of the foam, I am assuming, will slow the transfer of heat from that 1 1/2 inch air space above the foam to the attic air space below the foam. I just don't have any experience with this type of application so I don't know if 1) it will work at all, 2) it might work some or 3) the benefit will outweigh the cost. I am willing to do the work myself so labor cost will be minimal and the square foot area of the roof is only about 2,000 sq. ft. so I'm guessing the material cost will be easily handled as well.
I estimate the roof to be about a three-twelve pitch (maybe four-twelve) and the asphalt shingles have held up well for over three years (no leaking) and during that time there have been a few wind-blown rains during that time so I'm not particularly concerned about the water tightness of the roofing. The shingles are "white" but not as reflective as the roof coating I had applied to the gravel roof so the net effect has been that the A/C has had to work harder to try to keep the living space cool and it has not been able to keep up with it. We also have poor windows and a generally inefficient home in terms of insulation/emissivity. This attempt to slow the heat transfer through the roofing is the first step in getting this home to be more efficient. Thanks for your concern.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
William,

There is no problem covering the bottom chord of the truss. That is standard practice. Wood isn't the greatest insulator so it's actually beneficial to cover that bottom chord with insulation.
Just add baffles in each rafter bay so the ventilation isn't restricted, then add as much insulation as you want. If the attic is tall enough to allow ventilation, you can easily add 12" or more of insulation.
Blown in would probably be easiest, but you can also layer batt insulation too. Just make sure your batts do not have a vapor barrier attached, and ideally run each layer at right angles to cover the gaps of the previous layer (You're trying to minimize air flow between the batts).

Foam insulation is expensive and difficult to install. Batts or blown insulation will be easier and cost less.

3/12 is about the minimum slope for shingles, but it's doable if you have an ice and water membrane, or a double coverage of roofing felt.
Good luck,
Anthony Watson www.mountainsoftware.com www.watsondiy.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Friday, June 5, 2015 at 1:31:01 AM UTC-4, HerHusband wrote:

I've never seen 2x4 trusses used for the bottom of the attic/ceiling of the living space. But if that's what's there, then I agree with the above observation. Only problem is he said the attic has a floor and depending on how much of it has floor, how many nails were used, etc., it could be some trouble removing plywood to get the insulation in.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Trusses are almost always built with 2x4's, except for specialized trusses like attic trusses.

Did he mention an attic floor? I must have missed that.
If that's true, adding insulation will be problematic. Also, as you said, the bottom 2x4 chords won't carry any significant weight, certainly not for a living area in the attic.
Anthony Watson www.mountainsoftware.com www.watsondiy.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Friday, June 5, 2015 at 12:55:42 PM UTC-4, HerHusband wrote:

IDK what WTF exactly he has, because the description hasn't been clear. gravel roof? shingles? trusses? If you don't have an attic, then how are you doing what he proposes, unless you're tearing out the ceiling of the living space?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi William,

Can you describe the roof structure in more detail?
I always think of a gravel roof as a flat or low slope roof. If that's the case, asphalt shingles aren't really made for low slopes. Wind blown rain can easily back up under shingles on low slopes. I hope an ice and water membrane was installed under the shingles.

If you have room to get in and install the foam, you should be able to add more blown-in insulation instead. The standard solution is to install foam baffles in each rafter bay so you don't block the air flow when you add more insulation.
That will be easier, cheaper, and more effective than adding foam boards to the trusses.
Good luck,
Anthony Watson www.mountainsoftware.com www.watsondiy.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 3 Jun 2015 15:29:31 +0000 (UTC), HerHusband

Since this is another OLD thread brought back from the past, no one will explain the structure.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wednesday, June 3, 2015 at 11:30:49 AM UTC-4, HerHusband wrote:

Good catch. I missed that. I agree, going from a from a gravel roof on a house to shingles doesn't add up.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
replying to HerHusband , William wrote:

Anthony, thank you, as well, for your reply. Please see the reply to trader_4 above for details on roof structure. The original roof material looked like tar with embedded gravel and was about 1 inch thick. Gravel stop was used along the fascia so it looked to me like a gravel roof; apparently common in this area of Florida during the time this home was built (1960s). I'll take a look at your suggested method of blown-in with foam baffles. Thanks for your input.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 03 Jun 2015 02:44:02 +0000, William

Is that because the old roof was white and the new roof isn't?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.