Flat roof insulation on ground floor apartment
Hello
对于另一个平坦的屋顶问题表示歉意，但我已经阅读了所有这些构建的情况，这大大限制了我能做的……。
I’ll be helping a friend refurbish a ground floor space of a three storey terraced building dating from 1890’s. The location is in London, UK, probably climate zone 4 by US standards. The ground floor is empty with two occupied apartments above it. This empty space will be refurbished into a two bedroom apartment. A portion of the ground floor ceiling sits below existing conditioned spaces (ie other apartments) but also directly onto the exterior where the current buildup is 2x12s, sloped furring strips, 3/4″ ply, then some form of felt/asphalt roofing.
我做了大量的平屋顶项目都做a “hot/warm roof” principle. However, the exterior is not owned by the owner of the flat, so it cannot be altered. The insulation must be on the interior. Spray foam here is hard to come by and expensive.
The existing soffits have been fitted with vent strips so I could in theory vent below the existing plywood, however I am aware of the limitations on flat roof venting, especially with air only coming from one direction (the rear of the building) in this case.
What are your expert thoughts on the best build up considering this is a low budget build not targeting high insulation (this old Victorian property has very little insulation anyway)?
My U Value code minimum is 0.18 (we don’t use R Value much here) which appears achievable with 4-5″ of xps from online calculators I’ve found.
My initial thoughts exterior to interior :
– 4″ layer of XPS fitted tight between joists and canned foamed around the perimeter. Do I leave an air gap below sheathing and attempt to vent this space as much as possible or not worth it?
- 用岩石型式绝缘填充托梁湾的剩余空间。
– Worth adding an air control OR an air and vapour control membrane (Pro Clima for example readily available here) under the joists?
– Drywall
或者
- 1英寸的XPS泡沫接缝胶带胶带，然后干墙充当热中断
I’ve already suggested to the client to avoid recessed ceiling lights in the ceiling areas that back onto the exterior.
一如既往地赞赏。
GBA Detail Library
由气候和房屋部分组织的一千个建筑细节的集合
Replies
Quinn,
Rigid foam on the interior (the so-called "cut-and-cobble" method) is risky for unvented roof assemblies. Cut-and-cobble in unvented roof assemblies is associated with moisture accumulation and rot.
Venting this roof assembly is impossible.
这会留下闭孔喷雾泡沫作为您唯一的选择。
Thanks Martin
What are your thoughts on cut and cobble rigid foam (4-6" thick directly to the underside of sheathing) then a thinner (2" say) layer of closed cell spray foam under the rigid and around all potential air leak areas to help ensure effective air sealing?
Kind regards
Quinn,
我没有足够的经验来确定这种方法的风险（尤其是在您的气候下）。您建议的方法听起来比剪切方法更好，但不像100％闭孔喷雾泡沫那样强大。
Hi Martin
Thanks I'll likely go that route unless I can source some reasonably priced spray foam.
To clarify if I may, it is better in this case to go completely unvented and insulate directly against the roof sheathing than to leave an air gap which is in turn poorly vented?
Kind regards
Quinn
删除d
Quinn,
Q. "Is it better in this case to go completely unvented and insulate directly against the roof sheathing than to leave an air gap which is in turn poorly vented?"
答：是的，只要您遵循创建未通风屋顶组件的规则。要了解创建未通风的低矮屋顶组件的所有安全方法，请参见本文：“绝缘低矮的住宅屋顶。”
Quinn,
It looks like you can buy diy spray foam kits in Great Britain. If you are dealing with a small area, this might be a way to do a flash and batt type installation without the costs associated with using a contractor.
感谢马丁。
史蒂夫 - 是的，确实我已经找到了这些工具包，这就是计划！
Caulk joints before you spray. Because after it's very hard.
The weather history in foggy dew never really cold Thames valley locations would make it far more comparable to US climate zone 4C (aka "Marine 4), not 4A or 4B. So if using the IRC prescriptives as a guide, it will relatively moisture-safe at the foam/fiber insulation boundary at just 20% of the insulation value, no need to boost it to fully 30%.
Is XPS sold in the UK blown with CO2 (as it is in much of Europe) or is it an HFC blend, as in the US/Canada?
The U-factor conversion to units used in the US would is 1 BTU/(h·ft²·°F) = 5.678263 W/(m²·K), so a U-factor of 0.18 in US terms would be 0.18/5.678263 = ~ 0.032 BTU/(h·ft²·°F), or a "whole-assembly R" of ~ 31.25.
这一点x than the IRC prescriptive for zone 4C of 0.026 BTU/(h·ft²·°F), or a metric U-value of ~U0.15 W/(m²·K). The IRC prescriptive U-factor includes calculating the thermal bridging & R values of the structural and cladding elements, not just the insulation layers.
https://up.codes/viewer/wyoming/irc-2015/chapter/11/re-energy-efficity#n1102.1.4
If the U-value in UK code is only applies to the thermal conductivity of the insulation without factoring in the thermal bridging of the beams & rafters it's even more lax. Given that energy is more expensive in the UK than in the US it's probably financially rational in most cases to apply the US standard.
How deep are the rafters? What are the RSI numbers for mineral wool at the standard thicknesses sold in the UK? (Couldn't find much data in a quick online search, but I'm sure that information is out there.)
Hi Dana
I believe XPS is CO2 blown here, but best I can find with a quick search is "our products use blowing agents with zero Ozone Depletion Potential and low Global Warming Potential
(GWP)."
The U value requirements are considered whole assembly and are more lax for a refurbishment project such as this than for new builds (newbuilds require 0.11 versus my need for 0.18).
For 25mm (ie 1") Rockwool, I'm seeing 0.034 Thermal Conductivity (W/mK) and 0.70 R-value (m²K/W).
The rafters are 2 x 12.
我同意4C似乎是正确的。我最初来自不列颠哥伦比亚省的温哥华（尽管实际上在冬季在伦敦的伦敦较温暖）。
RSI 0.70 = R 3.97 in US conventional terms, so it's a somewhat lower density rock wool than what's sold on this side of the pond, which is fine.
二氧化碳爆炸类型XPS将运行约R4.2（RSI 0.12） @ 25mm/1“。如果他们使用HFO1234YF或HFO1234ZE，则可能高于此。
Most DIY polyurethane kits here run about R6 (RSI 1.06) @ 25mm, R12(RSI 2.12) @ 50mm/2" Verify those numbers for any kit you end up using. The maximum sprayed depth per pass here would be 2"- thicker than that risks fire during the curing period immediately after spraying &/or excess shrinkage over time.
A 2" pass would give you about R12, enough dew point control for R48 of fluff in zone 4C or up to R40 in zone 4A (colder but shorter heating season than you have.)
With 9.25" of remaining depth at ~R4/inch (assuming the batts get compressed a bit , but not a lot) you'd have R37 of mineral wool, so you're good to go. (That would meet code for new construction in the US.) If compressing two 140mm batts (280mm total) into the ~235mm space the density would be a bit higher, delivering about R4.1/inch or R38, still not a problem.
If they're full dimension 2x12s you'd have 10" and ~R40 fluff, which would still squeak by for zone 4A and would be plenty for zone 4C.
So it looks like you'll be fine without interior side vapor retarders with just a 2" shot of foam. The vapor retardency of 2" of kit foam would meet Canadian code specification as a "vapour barrier", but at about half a US perm it still provides a slow, seasonal drying path for the roof deck- it's not a true barrier, which offers at least some resilience.
与切割的XPS相比，喷洒的泡沫泄漏空气并造成屋顶的问题的可能性要小得多。如果走这条路线，使用3英寸XP（不需要4英寸）将是谨慎的长期4英寸的CAN泡沫。
Dana - thanks very much for the time you've taken to offer such a thorough response. This is very helpful for both this project and my knowledge going forward.
Hello Dana (or someone else!).
Quick follow up to this if I may. You mention the benefit of a modest seasonal drying path using 2" kit foam. If I go the cut n cobble route using 3" foil-faced board polyiso, I'm a right to assume the foil-facing is a full vapour barrier thus there would be no drying potential? I believe due to manufacturing Polyiso here in the UK it is largely all foil faced.
If that is the case I should go for non-faced XPS? And going the 3" route as suggested would still give me about 0.5 perm so some drying potential?
Thanks again