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insulating 4 season porch floor

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Jan 26, 2014, 10:42 AM
insulating 4 season porch floor
I am finding several wrong construction methods with my house. not only did I find my furnace is to small I also learned that my floor of my 4 season porch is not insulated correctly. I live in MN and especially this year we have had some very cold weather and the floor is freezing. I did the candle test with smoke and learned that the cold air from the floor is moving into the warmer areas of the house creating a drafty feel in the living room. I did some investigating and found that the insulation under is only r-19 and the kraft barrier is on the cold side. For some reason there is also some vents in the plywood that holds the insulation up. My understanding is the vapor barrier goes on the warm side. Also I have 2x10 floor joices so why put in a 5 1/5 inch insulation. I was thinking of spray foaming the underside but after getting a quote I cant afford to have it filled. this was my thought on how to do it but wondering if it would work

1. cut strips of 2inch pink foam for inbetween joices. glue in and can foam all edges.r-10 value at this point and air sealed

2. cut another layer of 2 inch pink foam and repeat step 1. now giving better air seal and r-20 value

3. take the vapor barrier off of existing r-19 and reinstall now ginving me r value of r-39 and filling entire cavity

4 put up 1 inch sheats of thermasheath or foil faced foam to bottom of floor joice. using metal tape on all seams to air seal again r- value roughly at r-45

5. finally reinstall 1/2 plywood with no venting in it.

Does this sound like a bad idea? I dont want to create any moisture problems. I am not sure what r-value I need to keep the floor warm. One thing i forgot to mention is the floor is roughly 4 feet off the ground so I do have plenty of room to work.

I am open to better ideas or concerns with what I want to do

thank you in advance

Jan 26, 2014, 11:12 AM
The average recommended under floor insulation for crawl spaces is R-19. However, I could easily see how this could be a higher R-number in MN.

At the very least, you should flip the existing R-19 batts over so that the paper is towards the living space. What it's doing now is trapping moisture between the kraft paper and the underside of the subfloor.

Adding solid foam insulation will certainly help,as will using expandable foam to seal any air gaps. But you need to be careful with mixing the two types of insulation. You could put in several layers of solid foam first up against the subfloor and then add fiberglass batts (unfaced) below. The solid foam is fairly impervious to moisture retention while the fiberglass batts are not, so you don't want to trap the fiberglass in a non-breathing area.

Considering cost, effort and net result this is what I would do:

Take the existing R-19 faced fiberglass and flip all of it over so that the paper is up against the subfloor. Add a second layer of R-13 unfaced below the R-19. This will give you R-32 in the floor - more than enough, even for MN. Skin over with thin plywood but add venting at each end of each joist cavity. A good way to do this would be to install two runs of 2-1/2" continuous soffit vents that run perpendicular to the joists.

You can actually get almost the same thermal results without using the plywood - it's purpose is to keep the critters out of the insulation. The venting needs to be there as you do not want a closed air space under the floor.

Some basic insulation tips to consider:

1. For most 'bang for the buck' took towards your attic space. You loose more heat there than in the walls or floor.

2. Down low to the floor, you will see more benefit to sealing behind baseboards with caulk than adding more layers of insulation underneath.

3. As walls have no access, the simple fix is to remove trim around windows and doors to add expandable foam to seal any cracks.

4. This one may sound strange but if you can get in to your attic and crawl spaces to seal any holes in the top or bottom wall plates it will make a difference. Even with drywall over an interior wall, your H/A system can pull through these areas making your house less efficient. Anywhere there is a wire or pipe is a potential problem area.

Jan 26, 2014, 11:38 AM
I understand about not trapping the moisture in the fiberglass insulation. If I dont install the bottom piece of foam I should be ok right? As far as doing what your saying with fiberglass insulation only and the vents. How do I get a good air seal? for instance today we have blizzard warnings and the wind/snow is blowing hard. So the vents let cold air in and fiberglass does not stop air movement so the floor would still be cold?? or am I thinking wrong? The venting you mentioned is actualy there now. maybe I need to go with the 4 inches of pink foam and then my r-19 and put my plywood back up with the vents? that way I maintain a airseal with some r value. This floor also get very warm in the summer...

thanks again
Jan 26, 2014, 11:42 AM
I also forgot to mention this porch is over the it gets all the elements. there is no crawlspace under. I think it was originally a deck and then turned into a porch.

Jan 26, 2014, 05:31 PM
Jason, yes, I figured that it was an overhanging style.

Like I said above - let the insulation do it's work. This set-up would be 100% fine with just fiberglass batts held up in-between the joists. The plywood is only there to shield it from the elements and mostly any living things that would want to live in there. If you install a plywood skin with vents you would leave the vents open all year round. There is no reason to seal it off and in fact, you want the air circulation. You will not feel a breeze through 10" or so of fiberglass batts. Breezes and swirling snow will not be a factor through the small space of the vents.

Also, saying this again: While there is no doubt that you will get a higher R-value by adding solid foam insulation, you will also get more than enough by adding a second layer of fiberglass. You have 2x10 joists, giving you 9-3/8" clear space. Again, take the existing R-19 and flip it to get the paper up against the subfloor. Then add an unfaced layer of R-13. You will get R-32 - more than enough even for your part of the world. Adding R-13 fiberglass batts has the duel advantage of being much easier and cheaper to install.