中華民國內政部建築研究所中華民國內政部建築研究所

一、研究緣起
屋頂樓版之隔熱性能對位於頂層之室內熱得影響甚鉅，係由於水平面之日射輻射比垂直牆面來得多，日射熱之取得最終將反應於空調之耗能量。我國之建築節能設計規範以構造之熱傳透率(U值)作為評估屋頂隔熱性能之指標，雖屋頂之隔熱性能要求更甚於立面外牆。然而，U值僅能反應建築之熱傳透，單一的U值指標無法評估壁體或屋頂表面之熱反射、構造熱蓄熱以及雙層屋頂通風等效果。今日業界上常見之建築隔熱工法、材料眾多，然而其隔熱之原理亦各異，無法僅以U值來評估之，有必要進行各工法間對建築節能之效果比較，以獲得整體性之評估。
二、研究方法及過程
本研究以內政部建築研究所於台南歸仁所屬之實驗群用地，應用六座新建置之全尺度實驗屋，透過夏季熱季之長期實測，同步探討六種屋頂隔熱工法、材料間之空調節能與室內熱舒適性之改善效益，包括薄層屋頂綠化、密集型屋頂綠化、雙層通風屋頂、屋頂隔熱漆(或冷屋頂)、傳統PS隔熱版隔熱以及傳統五腳磚隔熱屋頂。本研究成果期能建立符合臺灣熱濕氣候區本土化之量化數據，以為業界進行屋頂隔熱策略選擇之參考。
三、重要發現
透過現場實驗屋實測實驗所得重要結論如下：
1.    雙層通風屋頂雖然透過中間空氣層而在整體頂構造熱傳透率上得以符合法規之規定，然而如構造中無隔熱材料，日間之屋頂下表面溫度將比傳統五腳磚或PS版隔熱屋頂來得高。因此建議採用雙層通風屋頂之構造，混凝土樓板仍需設置隔熱層為佳。
2.    實驗結果在有屋頂綠化之二組於日間皆呈現相對低溫，顯示具有良好的隔熱效果，但在夜間由於土壤層與蓄排水版間蓄水導致於日間蓄熱量亦高之故，使得夜間散熱不易而致屋頂下表面溫度較其他組別稍高。而冷屋頂構造，由於日間反射了大部分之日射量因此混凝土樓板之日射取得較少，在夜間降溫較快速而呈現最低之表面溫度。
3.    室內熱舒適之效益：不論是日間或夜間，雙層通風屋頂、冷屋頂、薄層綠化屋頂與密集型綠化屋頂在對室內空氣溫度或平均輻射溫度之控制上，皆較傳統之五腳磚隔熱屋頂與PS板隔熱屋頂表現來得優良。尤其在日間以綠屋頂最具改善室內熱舒適效果，然而也由於綠屋頂高蓄熱能力，在夜間反而呈現較高之室內溫度。
4.    在空調連續運轉下，相較於五腳磚隔熱屋頂，雙層通風屋頂、冷屋頂、PS版隔熱屋頂、薄層綠化屋頂與密集型綠化屋頂全日分別空調省能約33.1%、42.6%、5.2%、29.5%與34.3%。然而，應用冷屋頂技術尚須考量表面塗層會隨時間因氣候而劣化、灰塵汙染等情形而降低其節能效益，此議題因時間因素未能於本研究中探討，於引用本研究數據時應注意冷屋頂可能會有此劣化現象。
5.    本研究同時提出薄層屋頂綠化構造、密集型屋頂綠化構造、冷屋頂構造與雙層通風屋頂構造之各層建材熱阻計算表以供業界參考選用。
四、主要建議事項
根據本研究成果發現，提出下列具體建議，分別從立即可行建議及中長期建議加以列舉如下：
建議一
推廣宣導屋頂綠化構造：立即可行建議
主辦機關：台灣綠屋頂暨立體綠化協會、財團法人台灣建築中心
協辦機關：內政部建築研究所、台灣建築學會
本實證研究顯示，屋頂綠化有較佳之室內熱舒適改善與空調節能效果。以屋頂綠化而言，薄層屋頂綠化在15公分之覆土厚度下即有明顯之日間降溫效益，然而仍需視既有建築之屋頂層結構承載力搭配適合之土壤層厚度。
建議二
辦理屋頂綠化補助改善之示範案例：立即可行建議
主辦機關：內政部建築研究所
協辦機關：財團法人台灣建築中心
內政部建築研究所過去辦理之「建築節能與綠廳舍改善補助計畫」成效卓著，惟針對屋頂綠化改善之節能效益，過去多引用模擬或推估資料，缺乏直接之實測量化資料作為實證依據。本研究所完成之屋頂綠化空調節能實測，相較於五腳磚隔熱屋頂（U值約0.8），顯見屋頂綠化具有相當大之空調節能潛在效益，值得後續納入擴大辦理示範改善。
建議三
辦理屋頂綠化針對屋頂近表層戶外環境空氣降溫之實測驗證：中長期建議
主辦機關：內政部建築研究所
協辦機關：
本研究以符合法規低限之屋頂構造熱傳透觀點，透過二組實驗屋實測驗證屋頂綠化構造之初步節能效益。然而屋頂綠化對居住環境之影響尚有對都市熱島降溫之貢獻，過去國外許多城市常以屋頂綠化作為減緩都市熱島增溫之手段，然而國內針對各式不同型態之屋頂綠化對近屋頂表層之降溫量化效益則較為缺乏。綠化植栽種類(灌木或草本植物)、葉面密度、覆土厚度、以及屋頂綠化工法上蓄排水版之有無等皆會影響近表層之空氣溫度。實有需要透過不同屋頂綠化型態、種類之實驗組合下進行同步實驗，以定量化與找尋具較佳降溫效益之屋頂綠化組合，俾便未來政府作為降低都市熱島效應技術選用之參考。

1.    Introduction
The thermal insulation of a roof slab is of crucial influence on indoor heat gain because the horizontal solar radiation is relatively larger than the vertical wall. The solar heat gain will eventually contribute to the cooling load, hence increasing the cooling energy. The coefficient of conductivity (U-value) is an index used for the insulation performance of building roof compliance in Taiwan. However, solely U-value is incapable of evaluating some advanced roof insulation constructions such as reflective roof, ventilated double roof etc. To synthesis assessing the cooling energy saving benefit and the indoor thermal comfort improving capability, it is necessary to simultaneously conduct experiments among various roof constructions.

2.    Methods
Six full-scale experimental chambers located at Tainan, south Taiwan, were used for in-situ experiments of six different types of roof construction including thin-layered extensive green roof, thick-layered intensive green roof, cool roof, ventilated double roof, PS insulated roof and traditional concrete tile roof. The experiment was performed in hot season to simultaneously investigate the indoor thermal comfort as well as their cooling energy benefits. The purpose of this study is hope to quantify the above benefits in the context of local climate. The results of the study could be useful and applicable for construction industries.

3.    Significant findings
The following findings have been achieved through the in-situ experiments in this research:
(1)    Although the insulation performance of the construction of ventilated double roof is comply to the regulation, we found that the surface temperature of the interior side of the roof slab is higher than that observed in cases of concrete tile roof and PS-insulated roof because of the none existence of insulation layer of the ventilated double roof. Therefore, it is highly suggested that an additional layer of insulation material should be layered in the concrete slab for using such construction.
(2)    The sub-surface temperatures of the roof slab of both green roof cases are relatively lower than the other roof constructions, revealing good thermal insulation performance. However, due to their high capability of thermal storage of the soil as well as the module of water retention and drainage layer, the heat seemed difficult to dissipated outwards resulting slightly higher temperature than the other roof constructions. As for the cool roof, lower surface temperature is observed at night and decease rapidly because that the high reflectivity of the exterior layer of the cool roof reflects large amount of solar radiation resulting less solar heat absorb in the concrete.
(3)    The indoor thermal comfort benefits: The indoor temperatures and the mean radiant temperatures of double roof, cool roof, extensive and intensive green roof are both lower than traditional concrete tile roof and the PS insulated roof either in the daytime or at night. Especially for both green roof constructions, the diurnal indoor thermal comfort is satisfactory, however, due to their high thermal mass, slightly higher indoor temperature is observed as opposed to other cases.
(4)    The cooling energy saving benefits of double roof, cool roof, PS insulated roof, extensive green roof and intensive green roof against concrete tile roof under continuously operation of air-conditioning are 33.1%、42.6%、5.2%、29.5% and 34.3%. However, the weathering of cool roof coating is another factor that will influence its radiation reflective capability. Due to the time constraint, we are unable to quantify this effect in this study, therefore, the actual long-term energy saving benefit of cool roof may be gradually deteriorated over times.
(5)    Practical construction suggestions complying to current roof insulation regulation were proposed in this study for references and application purpose for the construction and architectural design industries.

4.    Recommendations
(1)    The research results reveal a promising benefits of green roof on cooling energy saving, therefore, propaganda of this passive design strategy is encouraged.
(2)    Based on the results of the quantified benefits of indoor thermal comfort and cooling energy saving, promotions and incentives for continuously adopting green roof as a means to improve roof thermal insulation performance through governmental projects is suggested.
(3)    The ambient temperature cooling effect is another contributing factor of a green roof. Many countries have considered green roof as a means to counteract urban heat island. However, seldom studies regarding the outdoor temperature reduction effect of various types of green roof had been conducted. It is suggested to establish quantified benefits of green roof by in-situ experiments using experimental test cells.