Ceiling Insulation?

[EDDIE BUCK]

How effective would 4x8 styrifoam insulation nailed to the ceiling rafters cut down on the heat in a twenty by fourteen room addition off the back of my mh. The room has six windows but stays so hot a fan will not cool it and A/C is out of the question The walls are insulated well but the ceiling has none, just the rafters and the plywood the shingles are nailed to are all thats in the ceiling. Thanks Eddie

[Yankee1]

I will cut down plenty if you go 2 inch or better

[Ross]

Agreed it's how I made a 45 foot transport trailer usable in the summer.

[Beeman]

It's probably not fire retardant/building code approved but that's your choice. Insulation works best with air space above it that ventilates the trapped heat.

[agmantoo]

Eddie,
What is the roof configuration and what is the roof covering?

[raymilosh]

I had a similar situation and chose to use the 4 x 8 insulation board with a reflective silver side. I placed it with the silver facing up to reflect out summer radiant heat beating down from the roofing. I sealed the seams with the aluminum tape. It made a huge difference for cooling as well as heating. What I used was only half an inch thick. I imagine thicker would be better. Check out concrete supply places for the thicker insulation board. Way cheaper than at the big box store.

[EDDIE BUCK]

Originally Posted by agmantoo

Eddie,
What is the roof configuration and what is the roof covering?

Top down is shingles, felt, 5/8 plywood, 2x8 joists or rafters. All you can see on the inside is rafters and plywood.

[agmantoo]

Is there any ventilation from the eve across the underside of the plywood to where the rafters end? Using the insulation as described above and some ventilation would be most desirable. Depending on the pitch of the roof it is possible to get some natural ventilation provided there is an escape opening at the top of the roof slant. Ventilation and insulation are the answers to your problem.

[The Paw]

You can nail the rigid insulation to the bottom of the rafters and it will help a lot. There are two things to consider though:

1. If you get the blue board insulation, you are supposed to cover it with drywall or panelling for fire rating, as the stuff gives nasty fumes if it burns. (it also looks better).

2. Trapping the heat in your roof will shorten the life of your shingles. You are best to vent the space between the plywood and the insulation. If this is a lean-to/shed addition, this means trying to figure out a way to vent at the high end as well as the low end. It would be easier to frame an interior ceiling that is parallel to the floor, and hangs below the rafters a few inches. Then you just have to vent the ends of the resulting "attic" space.