Overheating risk in Passivhaus dwellings

Abstract

Highly insulated and airtight homes designed to reduce energy consumption are perceived as having a greater summer overheating risk than less insulated homes. If true, dwellings built to the well-known low-energy Passivhaus (PH) standard could be at greatest risk due to the use of superinsulation, especially as the climate warms. Existing studies are inconclusive and even contradictory, mainly due to small sample sizes. Hence, this paper presents the first large-scale overheating risk analysis of UK Passivhaus dwellings using high-resolution internal temperature data from 82 homes across the UK. Both the Passivhaus and the recently published Chartered Institution of Building Services Engineers TM59 criteria are analysed. Results show that the whole-dwelling Passivhaus standard, which uses a fixed temperature threshold, is met more frequently (83%) than when applied on a room-by-room basis (e.g. only 60% of bedrooms in houses meet the standard). TM59-1A, which uses an adaptive temperature threshold, is easier to meet with 100% of flats and 82% of houses in compliance. However, 55% of bedrooms assessed under TM59-1B fail, with little difference between flats and houses. This is a remarkable finding given that the summers under consideration were either typically mild or cooler than average, and that sleep impairment can significantly affect both physical and mental health. These results suggest that highly insulated dwellings such as Passivhaus should consider overheating in individual rooms, rather than at whole-dwelling level. Analysis should be undertaken throughout the year with particular attention to bedrooms, using either the good-practice PH-5% exceedance threshold which maps well to TM59-1B, or TM59-1B itself. Practical application: Overheating risk in new dwellings is an industry concern. Having the correct tools to predict this risk at design stage is important to help design comfortable and healthy dwellings for both today's climate and future, hotter climates. Comparing two different tools and their methodologies using in-use data is critical to gain confidence in their application at the design stage and to further understand overheating risk, including which dwelling types and rooms are more vulnerable to overheating.

Thermal Comfort in NZEB Collective Housing in Northern Spain

“…There is a varied body of research on overheating in residential buildings in Europe [5][6][7][8][9][10][11][12][13][14][15][16]. In [16], there is a wide sample of these studies focusing on buildings with Passivhaus certificate dating 2011 to 2015.…”

“…Overheating is a known problem in highly airtight and insulated buildings with numerous studies confirming it [5][6][7][8][9] and analysing energy efficiency and comfort in a variety of European climates. A common element in these studies is that occupants are often more satisfied with winter comfort than summer comfort.…”

Overheating risk of a typical Norwegian residential building retrofitted to higher energy standards under future climate conditions

“…Mitchell et al [9] did analysis on the overheating risk in passive dwellings. It was found that in highly insulated dwellings overheating should be considered for individual rooms rather than at whole-dwelling level.…”

Comfort within Budget: Assessing the Cost-Effectiveness of Envelope Improvements in Single-Family Affordable Housing

“…It was originally designed for cold climates, but a consolidated body of research has already investigated its suitability and applicability in temperate and warm climates [30,32]. However, even highly insulated and airtight Passivhaus buildings with active ventilation can face a summer overheating risk [33]. Therefore, optimisation of several design inputs for Passivhaus buildings, including external shading devices, thermal mass effect and glazing ratios are becoming increasingly relevant to prevent summer overheating even in mild climates [34].…”