GreenGuard XP38 / XFP38 Fanfold Underlayment

GreenGuard XP38-XFP38 Siding Underlayment

For use in Replacement Siding Operations to Provide a Level Surface Over Old Siding, Add Insulation and Weather Protection

For the best fanfold siding underlayment available, choose the XP Series. The extruded polystyrene (XPS) foam offers superior stiffness, so your siding job has the smoothest finish possible. The underlayment is extra durable, thanks to flexible plastic facers on both sides. Plus it’s perforated to allow moisture vapor to escape. The cut-and-fold hinge design ensures it lays flat. A perforated reflective facer that can enhance thermal performance is also available.

Product Information

Property Test Method Value
XP38 Siding
XFP38 Siding

Reflective Surface
Nominal Thickness (in)   3/8 3/8
Nominal Width (ft)   4 4
Nominal Length (ft)   50 50
Water Absorption
(Max. % by Volume)
ASTM C 272 2.0 2.0
Water Vapor
Permeance (perm)
(Desiccant Method)
1.7 1.7
Thermal Resistance, R-Value1
(ºF-ft²-h/Btu per in thick)
ASTM C 518
(@ 75ºF Mean
1.5 3.53
Fire & Smoke
Flame Spread2
Smoke Developed2
ASTM E 84/UL 723 25
10 (Class A)
  XP38 XFP38 - reflective
Board Dimensions 4' x 50' 4' x 50'
Nominal Thickness 3/8" 3/8"
Bundle/Pallet 44 44
Square Feet/Pallet 8,800 8,800
1 Specimens are aged and tested in accordance with the FTC Rule (16 CFR, Part 460) and ASTM C 578.
2 These numerical flame spread and smoke developed ratings are not intended to reflect hazards presented by these materials under actual fire conditions. All applicable building codes must be followed.
3 System R-value are calculated in accordance with the “ASHRAE Handbook of Fundamentals” using a ½” uniform parallel dead air space. The R-values of this products installed without an air space is 0.9.
GreenGuard XP38 Siding Underlayment_Product Sheet_US_CA_IMG

GreenGuard XP38 Siding Underlayment Product Sheet

GreenGuard Fanfold Underlayment Standard Installation Guide

GreenGuard Fanfold Underlayment Standard Installation Guide

CCRR 1022 1901

Code Compliance Research Report 1022 - Fanfold Underlayment

GreenGuard Siding Underlayment, Sheathing & Roofing Cover Board Safety Data Sheet

GreenGuard Siding Underlayment, Sheathing & Roofing Cover Board Safety Data Sheet

GreenGuard Reflective Sheathing & Siding Underlayment Safety Data Sheet

GreenGuard Reflective Sheathing & Siding Underlayment Safety Data Sheet

GreenGuard XP-Series Siding Underlayment Technical Bulletin #17

GreenGuard XP-Series Siding Underlayment Energy Chart

Do you need assistance selecting the right product?

Our teams are dedicated to helping make the process as easy as possible and ensure you get the right materials for your needs.

Contact Us

Kingspan Insulation North America

Head Office & Sales Office – North America

Kingspan Insulation LLC

2100 Riveredge Parkway, Suite 175

Atlanta, Georgia



Call: +1(800) 241 4402

We use cookies on our website To find out more about the cookies we use, or to change your cookie preferences i.e. to remove your consent to our use of certain categories of cookies, please visit our Cookie Policy & Control page.

Please click “Accept and close” to accept the use of cookies on our website. If you do not click “Accept and close” but continue to use this website: you thereby consent to the use of all Kingspan's cookies and third party cookies for the purposes of improving performance, improving functionality and audience measurement, in accordance with the terms laid out in our Cookie Policy & Control page; and we will assume that you have read and understood our Cookie Policy & Control page.

The Cookie Policy & Control and Website Privacy Notice were updated on 13 August 2018.