Adding Insulation Under Floor


Old 11-22-17, 09:57 AM
Thread Starter
Join Date: Mar 2008
Posts: 30
Received 0 Votes on 0 Posts
Adding Insulation Under Floor

I recently had an addition built on helical piles and had the underfloor insulated with spray foam (see attached image). The insulation does an "ok" job but I feel the floor is still a bit on the cool side in the winter. The crawl space in the image is still a ventilated space even although I have added lattice around the perimeter.
Would there be any benefit to adding sheets of expanded polystyrene board to the underside of the joists shown in the attached image ? The EPS Sheathing has a R8 value .
Attached Images  
Sponsored Links
Old 11-22-17, 10:34 AM
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 23,699
Received 620 Votes on 573 Posts
Would it help. Yes. Would you be able to notice... maybe not.

How much insulation R value is in the floor already? Spray foam has an R value of about 6 per inch of thickness. If you already have 6 inches then you have an R 36 floor. Adding R8 insulation will help but is only about a 20% improvement to go from R36 to R44. 20% is significant but you might not notice it. If your floor had nothing it might have an R value of 2 and adding insulation with R8 would be a huge increase and very noticeable.
Old 11-22-17, 11:36 AM
Thread Starter
Join Date: Mar 2008
Posts: 30
Received 0 Votes on 0 Posts
Thanks Pilot. The insulation has an R value of 31. Would you say that the couple of inches of exposed joists could also be a cause of cold transfer ?
Old 11-22-17, 12:44 PM
Join Date: Oct 2008
Location: New England
Posts: 10,514
Received 37 Votes on 34 Posts
The r-value affects the heat loss, double the r-value and halve the heat loss. But doubling the r-value only makes a small difference in surface temp and surface temp is what you share with those floors. Citations available.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: