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Insulation for former screened porch --quantity? Vapor barrier?

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  #1  
Old 05-16-16, 08:36 PM
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Insulation for former screened porch --quantity? Vapor barrier?

Hi,

I have an older house that previously had a screened porch on the first level above the basement. Now, of course, that porch is fully enclosed to be in line with code. The room, though, has nothing below it--except 7 feet of air to the patio. The joists under the floor have pretty old insulation (we're talking decades) and I'm having it replaced, as it gets super cold in that room in the winters (I'm in Washington, DC). I am going with foam board as inevitably there will be humidity/moisture that can make its way into that space. My two questions are:

- how thick should the foam be (2 inches has an R value of 10, as I understand it ... will that do a decent job of keeping the cold out...or does it need to be doubled?)

- would i need a vapor barrier between the subflooring and the foam board?

Any other tips to make this situation work?

Appreciate your assistance and feedback,

H
 
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  #2  
Old 05-17-16, 06:21 AM
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One of the more important steps to keeping that space warm will be air sealing every possible leak from below. The rigid foam board will do 95% of that, but search out and seal that other 5%.

Is that space directly heated in winter?

There are many threads about living space above a garage or other unheated area and the complaints all deal with cold floors. Any heat in that room will be pushed to the ceiling leaving the cold air to pool on the floor. So, 2" is good, but 4" (staggered and taped seams) is better.

As for a vapor barrier, the rigid is a classed as a vapor retarder and should do the job.

Bud
 
  #3  
Old 05-17-16, 03:20 PM
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Hi Bud,

Thanks for taking the time to reply to my question with such a helpful response.

One quick follow up, if I may: by staggered and taped seams, I'm taking that to mean a vertical air gap between the two layers of foam board and the seams referring to the end-to-end connection between the boards. Is that correct? Elementary clarification, but just want to make sure.

Thanks again!!

H
 
  #4  
Old 05-17-16, 03:39 PM
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That's correct. The staggered and taped seams is kind of a standard statement for the installation of multiple layers.

One of the nice features of regular foam board (not foil or plastic covered) is that it has at least a very minimal level of permeability, some drying ability.

But, go crazy with the caulking to seal every seam.

Bud
 
  #5  
Old 05-30-16, 06:39 PM
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Thanks, Bud.

So, I had a guy come over to install it. He agreed to do it with one inch foam board stuff up to be touching the bottom of the sub-flooring. We then used foam to seal all air gaps. For the second layer, he bought 3/4 polyshield sheathing/underlayment. This layer will go closest to the outside. It will be covered with treated 3/4 inch wood screwed into the bottom of the joists. These will have furring strips to cover the gaps between the wood and then this would be caulked. It sounds like a lot, but my two questions are:

- is the Polyshield just the contractor going super cheap? It seems of little utility. Second, which side--shiny or flat--should face the outside? I believe it should be the shiny side that is pointing toward the outside (and not flipped so that it would be facing toward the sub-flooring).

Can anyone clarify for me? He's stopping by again tomorrow.
 
  #6  
Old 05-30-16, 08:00 PM
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IMO, the polyshield is a less expensive option and it provide less R-value.
You started by thinking maybe 2-2" layers for a total of r-20 and now you will be down to r-8.75 with those two layers. It will not work as well.

I'm having trouble with the layering. 1" up next to the bottom of the floor and then " across the bottoms of the joists??? Then 1" treated boards and then furring strips to cover the gaps with some caulking in there somewhere.

That combination is not in my book, not that I have a book. But it is difficult to predict how new combinations will perform.

Here are some comments:
1. The total r-value has dropped to low. Yes, it costs more, but if it fails to insulate this extremely difficult location all of this effort is lost.

2. The foil is a radiant barrier and if those cavities are well air sealed in all directions, then the foil should face up towards the large air space.

3. If the treated wood he is proposing is typical pressure treated material, every fastener that attaches to or through it must be specially approved fasteners.

4. The " boards with strapping o the seams, even with caulking will be a poor air seal. But, with the foam board above it, it might be tight.

Even if we add a couple of r-points for the radiant barrier facing an air gap you are still short on R-value. But often budgets rule so if this is the best you can do now it will be better than what you had.

As a suggestion, " plywood would be better than the " boards and should eliminate a lot of seams and no strapping, just caulk or tape them.

Bud
 
  #7  
Old 06-01-16, 08:59 PM
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Thank you again, Bud! This info was really helpful.
 
  #8  
Old 06-02-16, 03:33 AM
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Not wanting to dilute Bud's thunder, but wouldn't Roxul be a more viable alternative considering cost and initial R value? Inherent properties make it fireproof, waterproof, vermin proof, mold proof, along with increased R value over fiberglas.
 
  #9  
Old 06-02-16, 07:05 AM
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I'm with Larry - I don't think there's enough insulation in this current plan and something like Roxul would make a big difference.
 
  #10  
Old 06-02-16, 04:04 PM
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Thanks for suggestions on both options. It's a little too late this time around, as I went ahead and did a less than perfect solution with a contractor...factoring in the time and money I had available. I'm renting out the place, so it won't be my energy bills to pay .. but when I re-occupy, I'll add in better insulation.

Thanks again,
 
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