Solid Foam insulation in the rafters instead of regular fiberglass??
#1
11-30-02, 09:32 AM
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Solid Foam insulation in the rafters instead of regular fiberglass??
I will soon be finishing off the attic in my older (built circa 1948) home. I only have 2x6 rough cut rafters up there now. I want to make this space into a master bedroom and master bath so usable head space is at a premimum.
I do have a ridge vent and undereve vents already installed, but there is no insulation up there. Originally I was thinking that I needed some of the foam W shaped rafter vents and then R-13. This would pretty much fill up the stud cavity.
So my question is this, can I use solid foam insulation instead?
http://www.dow.com/styrofoam/na/iso/value.htm Can I use this instead of regular loose fill fiberglass insulation? If so what do I need to know, or be aware of? I really like the idea of having an r-30 ceiling instead of an r-13 ceiling.
Your help is greatly appricated.
I do have a ridge vent and undereve vents already installed, but there is no insulation up there. Originally I was thinking that I needed some of the foam W shaped rafter vents and then R-13. This would pretty much fill up the stud cavity.
So my question is this, can I use solid foam insulation instead?
http://www.dow.com/styrofoam/na/iso/value.htm Can I use this instead of regular loose fill fiberglass insulation? If so what do I need to know, or be aware of? I really like the idea of having an r-30 ceiling instead of an r-13 ceiling.
Your help is greatly appricated.
#2
11-30-02, 10:13 AM
Yes, you could use the foam board but I would prefer you use compressed fiberglass boards. The fiberglass boards are better fire rated than the styrofoam boards. You would more than likely lose your entire attic if you had a fire up there with styrofaom boards.
The other concern is venting the roof. The easiest way to accomplish this is to install 1 inch slats against the roof on the rafters. Install the insulation boards against the slats and this will provide a clear passage for air to travel between the insulation and roof. I would also recommend you get fiberglass board insulation that had foil on both side of the panel and use foil tape to seal the seams between the boards. This will act as a radiant barrier and make the bedroom a lot cooler in the summer and there is some advantage with the foil on the inside during the winter too.
The other concern is venting the roof. The easiest way to accomplish this is to install 1 inch slats against the roof on the rafters. Install the insulation boards against the slats and this will provide a clear passage for air to travel between the insulation and roof. I would also recommend you get fiberglass board insulation that had foil on both side of the panel and use foil tape to seal the seams between the boards. This will act as a radiant barrier and make the bedroom a lot cooler in the summer and there is some advantage with the foil on the inside during the winter too.
#3
12-01-02, 08:50 AM
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I really like this idea, what is the R value of using compressed fiberglass boards? Also, how thick do they come? If I can fill most of the cavity that would just make my day. Also what sort of vapor barrier do I need to install with this application?
Man I love these boards
Man I love these boards
#4
12-01-02, 11:53 AM
They come in a variety of thicknesses. The foil is your vapor barrier and you will see a rating on the boards, such as SP-15. Actually you will see it on any type of vapor barrier.
If I were doing this job, I would use 1 inch thick boards. The reason for it when cutting the boards to fit, this way is more forgiving and I can install it in a way to overlap the gaps between the layers. It will actually make the installation more air tight. This is very important because there is very little or more air movement inside the compressed fiberglass. Because of this characteristic of board insulation, having multiple layers with vapor barriers will not have a negative affect.
The R-value is around 5 per inch and it should state it on the boards. When applying a layer install small nails or staples in the rafters to hold the layer in place.
If I were doing this job, I would use 1 inch thick boards. The reason for it when cutting the boards to fit, this way is more forgiving and I can install it in a way to overlap the gaps between the layers. It will actually make the installation more air tight. This is very important because there is very little or more air movement inside the compressed fiberglass. Because of this characteristic of board insulation, having multiple layers with vapor barriers will not have a negative affect.
The R-value is around 5 per inch and it should state it on the boards. When applying a layer install small nails or staples in the rafters to hold the layer in place.
#5
12-01-02, 01:39 PM
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So I should just stack this stuff into the rafter cavity with 4-5 layers? Do I need to tape each layer, or just the last one? I like this idea the most so far.
What will have to do for any electrical, just burrow out any cavities that I may need for boxes and wire runs?
What will have to do for any electrical, just burrow out any cavities that I may need for boxes and wire runs?
#6
12-01-02, 04:46 PM
Yes you should tape the seams for each layer. If you run electrical wires and boxes, I would recommend you cut out the insulation around them and after the installation use spray foam over them. This will reduce any air movement and should work well with the compressed fiberglass.
